Preparation Method of W-Microalloyed Medium-Entropy Nanostructured Alloys and Its Application in Fuel Cells

Abstract

A method for preparing W-microalloyed medium-entropy alloy nanomaterials and their application in fuel cells belongs to the field of fuel cell technology. First, carbon black-supported copper nanowires (Cu@C) are prepared. Then, using an electrochemical replacement-assisted polyol thermal reduction method, Pt, Co, and W are reduced and doped into the copper nanowires. Nanoalloying is driven by the high mixing entropy between the metals, and the alloy is subsequently acid-etched to obtain a W-microalloyed PtCuCo medium-entropy alloy catalyst. The W-microalloyed PtCuCo medium-entropy alloy is uniformly distributed on the carbon black support. The medium-entropy alloy morphology exhibits nanoworm-like structures with diameters of 2.5 to 6.5 nm, a face-centered cubic crystal structure, and a single alloy phase. The surface of the medium-entropy alloy exhibits numerous W-terminated step-like structures. This method is expected to replace commercial Pt / C catalysts and promote the large-scale practical application of proton exchange membrane fuel cells.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation of high-performance new energy nanoelectrode materials and fuel cell applications, and specifically to a preparation method and application of a high-melting-point metal microalloyed platinum-based medium-entropy alloy catalyst, and more particularly to a preparation method of a W microalloyed PtCuCo medium-entropy alloy catalyst and its application in fuel cells. Background Art

[0002] In recent years, with the rapid development of society, the extensive use of traditional fossil fuels has caused serious environmental pollution and energy shortages. Proton exchange membrane fuel cells (PEMFCs) have attracted widespread attention as a clean and sustainable new energy source. Hydrogen-oxygen fuel cells (PEMFCs) produce only water as a reaction product, without any pollution or carbon dioxide emissions, making them environmentally friendly and sustainable. Therefore, from the perspectives of energy conservation and ecological protection, hydrogen-oxygen fuel cells are a highly promising new energy technology. However, the large-scale commercial application of hydrogen-oxygen fuel cells is hampered by the very slow kinetics of the cathode oxygen reduction reaction (ORR). Therefore, efficient catalysts are needed to reduce the reaction energy barrier and accelerate the reaction. Currently, the most widely used commercial Pt / C catalyst is severely limited by the low reserves, high cost, and poor durability of metallic platinum. High-alloying of platinum with other elements is a feasible approach to significantly enhance its ORR catalytic performance and activity stability. Medium-entropy alloys, formed by alloying three to four elements, can produce strong lattice distortion and delayed diffusion effects, while also possessing structural stability driven by high mixing entropy. This invention uniquely utilizes the microalloying mechanism of high-melting-point metals such as tungsten to enrich trace amounts of W at specific sites such as grain boundaries and step surfaces in the alloy, leveraging its microstructural end-stabilization and performance regulation functions to produce, for the first time, a medium-entropy alloy catalyst with high activity and stability. The W-microalloyed PtCuCo medium-entropy alloy catalyst disclosed in this patent has a specific mass activity nine times that of commercial Pt / C. In a 30,000-cycle long-term stability performance test, its specific mass activity only decreased by 3%, and the peak power density of hydrogen-oxygen fuel cells reached 2.1Wcm -2 , the specific mass power density is as high as 21W mg Pt -1 , which is higher than commercial Pt / C. Therefore, the W microalloyed PtCuCo medium-entropy alloy catalyst prepared by the present invention is of great significance for promoting the large-scale commercial application of fuel cells. Summary of the Invention

[0003] The present invention aims to provide a method for preparing a high-melting-point metal microalloyed medium-entropy alloy catalyst with high electrocatalytic activity and stability, and its application in fuel cells. This preparation method is simple and rapid, enabling large-scale production of the catalyst. The PtCuCo medium-entropy alloy catalyst, microalloyed with the high-melting-point metal tungsten (W), exhibits a half-wave potential exceeding that of commercial Pt / C catalysts by 69 mV in catalyzing the oxygen reduction reaction in an acidic environment, and its specific mass activity reaches 1.14 A mg pt -1 , which is 9 times that of commercial Pt / C. In the long-term stability test, the half-wave potential dropped by only 12mV, and the specific mass activity decreased by only 3%. The peak power density of hydrogen and oxygen fuel cells reached 2.1W cm -2 , the specific mass power density is as high as 21W mg Pt -1 .

[0004] The present invention relates to a method for preparing a W-microalloyed PtCuCo medium-entropy alloy catalyst. The method comprises the following steps: firstly preparing carbon black-supported copper nanowires (Cu@C), then reducing Pt, Co and W and doping them into the copper nanowires by using an electrochemical replacement-assisted polyol thermal reduction method, nano-alloying driven by high mixing entropy (electrochemical / chemical energy) between the metals, and then acid etching the alloy to finally obtain the W-microalloyed PtCuCo medium-entropy alloy catalyst. The catalyst is a solid powder, and the W-microalloyed PtCuCo medium-entropy alloy is uniformly distributed on the carbon black carrier. The medium-entropy alloy morphology presents a nano-worm-like shape, and the diameter of the medium-entropy alloy nano-worm-like one-dimensional structure is 2.5 to 6.5 nm. The W-microalloyed PtCuCo medium-entropy alloy has a face-centered cubic crystal structure, is a single alloy phase, and has a large number of W-terminated surface step-like structures on its surface.

[0005] The atomic percentage of W in the W microalloyed PtCuCo mesoentropy alloy is 1% to 3%, the atomic percentage of Pt is 35% to 45%, the atomic percentage of Cu is 35% to 45%, the atomic percentage of Co is 9% to 27%, and the mass ratio of the W microalloyed PtCuCo mesoentropy alloy in the entire catalyst is 25% to 60%.

[0006] The preparation method of the above catalyst comprises the following steps:

[0007] (1) Preparation method of carbon black-supported copper nanowires (Cu@C) with a certain aspect ratio by anhydrous solid phase grinding method: first, a certain amount of water-insoluble cuprous chloride is used as a precursor salt, which is thoroughly mixed with carbon black by grinding; then a certain amount of anhydrous sodium borohydride is added as a reducing agent, and an anhydrous solid phase grinding reaction is carried out at room temperature; after a certain grinding time, carbon black-supported copper nanowires (Cu@C) are prepared; the ground product is soaked in ethanol until no gas is generated, then washed, filtered and dried, and the obtained solid powder is used as a sacrificial template material in subsequent synthesis steps;

[0008] (2) Preparation of W-microalloyed stabilized PtCuCo medium-entropy alloy nanomaterials. First, a certain amount of Cu@C black powder is dispersed in a small amount of polyol and ultrasonically dispersed to form a black slurry. At the same time, a certain amount of H2PtCl6 ethylene glycol solution, CoCl2 ethylene glycol solution, and W(CO)6 acetylacetone solution are added to the above-mentioned black slurry containing Cu@C. By controlling the concentration of a certain metal salt, the pH of the reaction solution is adjusted to 8-10. The electrochemical replacement reaction between the above-mentioned metal salts and Cu nanowires and the thermochemical reaction of the polyol are used to electrochemically and chemically reduce Pt, Co, and W under certain reaction temperature and reaction time conditions, and then dope them into the interior of the sacrificial template copper nanowires. The nano-alloying driven by the high mixing entropy (electrochemical / chemical energy) between the metals is used to realize the preparation of PtCuCo medium-entropy alloy nanomaterials containing trace amounts of W doping and stability.

[0009] (3) Regulation of the surface step-like structure of W-microalloyed stabilized medium-entropy alloy materials. In order to achieve a large amount of exposure of the specific electrocatalytic active surface of the W-stabilized medium-entropy alloy, the above alloy nanomaterials were etched in a special acid of a certain concentration at 60°C for a certain time to remove copper and part of the cobalt elements on the alloy surface, and finally a W-stabilized medium-entropy alloy material with a large number of surface step-like structures was obtained.

[0010] In a preferred embodiment of the present invention, the carbon black in step (1) refers to commercial carbon black particles, such as Vulcan XC-72, KJ-EC300J, KJ-EC600JD, and BP2000.

[0011] In a preferred embodiment of the present invention, the molar ratio of CuCl to NaBH4 in step (1) is 1:2 to 1:4, preferably 1:3. The aspect ratio of the carbon black-supported copper nanowires is 11 to 18.

[0012] In a preferred embodiment of the present invention, the reaction time of the anhydrous solid phase grinding of carbon black, CuCl and NaBH4 in step (1) is 0.5 to 1 h, and the grinding rate is 60 r / min-300 r / min.

[0013] In a preferred embodiment of the present invention, the polyol includes ethylene glycol (EG) and triethylene glycol (TEG), the thermal reduction temperature is 160-230° C., preferably 190° C., and the reaction time is 120-240 min, preferably 180 min.

[0014] In a preferred embodiment of the present invention, before mixing, the concentration of the Pt precursor metal salt in the metal salt dispersion is 1.5×10 -3 ~6×10 -3 mol L -1 , the concentration of Co precursor metal salt is 5×10 -4 ~2×10 -3 mol L -1 , the concentration of W precursor metal salt is 2.5×10 -5 ~1×10 -4 mol L -1 .

[0015] In a preferred embodiment of the present invention, the acid used for the acid treatment etching in step (3) includes H2SO4, HNO3, H3PO4, CH3COOH, and HClO4, wherein the molar concentration of the acid is 0.5 to 1.5 mol / L. When H2SO4 is used, H2O2 is added in a molar concentration of not more than 1 mol / L. 1M HClO4 is preferably used.

[0016] In a preferred embodiment of the present invention, the etching time in step (3) is preferably first ultrasonicated for 30 minutes and then stirred for 24 hours.

[0017] This invention uses a simple, environmentally friendly, highly operational, and mass-producible method to prepare microalloyed medium-entropy alloy nanomaterials for use in nanoelectrode material preparation and proton exchange membrane fuel cell catalysts. The catalyst material exhibits oxygen reduction performance far exceeding that of commercial Pt / C catalysts and exhibits excellent stability. The microalloyed medium-entropy alloy catalyst significantly improves oxygen reduction electrocatalytic activity and durability, achieving a peak power density of 2.1 W cm-2 in hydrogen-oxygen fuel cells. -2 , its specific mass power density is as high as 21W mg Pt -1 , much higher than commercial Pt / C (1.8W cm -2 ). BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) patterns of Cu@C obtained in Examples 1 to 3;

[0019] Figure 2 1 is a transmission electron microscope (TEM) image and an X-ray diffraction (XRD) pattern of the catalyst obtained in Example 1;

[0020] Figure 3 The linear voltammetric scanning curves of the catalyst obtained in Example 1 and the commercial Pt / C catalyst in acid medium and the linear voltammetric scanning curves before and after the long-term stability test;

[0021] Figure 4 This is a fuel cell single cell test curve of the catalyst obtained in Example 2 and a commercial Pt / C catalyst;

[0022] Figure 5 The linear voltammetric scanning curves of the catalysts obtained in Examples 2 and 3 and the commercial Pt / C catalyst in acid medium are shown. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below with reference to specific embodiments, but the protection scope of the present invention is not limited by the specific embodiments.

[0024] Example 1

[0025] W microalloyed PtCuCo medium entropy alloy catalyst prepared by acid etching using H2SO4+H2O2

[0026] (1) 100 mg of carbon black powder and 62.3 mg of CuCl were ground in a grinder at a grinding rate of 60 r / min for 30 min to uniformly load the CuCl on the carbon black. Subsequently, 47.6 mg of NaBH4 was added to the grinder and ground at a grinding rate of 60 r / min for another 30 min. The resulting black solid was placed in 50 ml of ethanol and ultrasonicated until no bubbles were generated. The solid was then filtered and washed with ethanol. The filtered product was then dried for 12 h to obtain Cu@C.

[0027] (2) Take 39.4 mL of ethylene glycol and add 36 mg of Cu@C to the above solution. Ultrasonicate for 60 min. Then, add 9.3 mL of 0.01 M chloroplatinic acid ethylene glycol solution, 0.31 mL of 0.1 M cobalt chloride ethylene glycol solution, and 1 mL of 2.8 mM tungsten hexacarbonyl acetylacetone solution. Add 1.2 mL of 1 M NaOH aqueous solution to adjust the solution pH to 10 to obtain a reaction suspension.

[0028] (3) The reaction suspension obtained in step (2) was placed in an electric heating reactor and stirred at a speed of 350

[0029] rpm, and the reaction temperature was set at 190°C for 3 hours. After the reaction was completed, a black slurry was obtained;

[0030] (4) The black slurry obtained in step (3) was filtered and washed with deionized water, and dried at 60° C. for 12 h to obtain a black carbon-supported alloy powder;

[0031] (5) The black powder obtained in step (4) was placed in 100 ml of 1 M H2SO4 + 1 ml of 30% H2O2, ultrasonicated for 30 min, and then stirred at 60°C for 4 h. After the reaction, the black suspension was filtered and washed with a large amount of deionized water until the filtrate was neutral. After drying at 60°C for 12 h, it was fully ground to obtain a W micro-alloyed PtCuCo medium-entropy alloy nanomaterial catalyst;

[0032] Example 2

[0033] W-microalloyed PtCuCo medium-entropy alloy catalyst prepared by acid etching with HClO4

[0034] (1) 100 mg of carbon black powder and 62.3 mg of CuCl were ground in a grinder at a grinding rate of 60 r / min for 30 min to uniformly load the CuCl on the carbon black. Subsequently, 47.6 mg of NaBH4 was added to the grinder and ground at a grinding rate of 60 r / min for another 30 min. The resulting black solid was placed in 50 ml of ethanol and ultrasonicated until no bubbles were generated. The solid was then filtered and washed with ethanol. The filtered product was then dried for 12 h to obtain Cu@C.

[0035] (2) Take 39.4 mL of ethylene glycol and add 36 mg of Cu@C to the above solution. Ultrasonicate for 60 min. Then, add 9.3 mL of 0.01 M chloroplatinic acid ethylene glycol solution, 0.31 mL of 0.1 M cobalt chloride ethylene glycol solution, and 1 mL of 2.8 mM hexacarbonyltungsten acetylacetone solution. Add 1.2 mL of 1 M NaOH aqueous solution to adjust the solution pH to 10 to obtain a reaction suspension.

[0036] (3) The reaction suspension obtained in step (2) was placed in an electric heating reactor and stirred at a speed of 350

[0037] rpm, and the reaction temperature was set at 190°C for 3 hours. After the reaction was completed, a black slurry was obtained;

[0038] (4) The black slurry obtained in step (3) was filtered and washed with deionized water, and dried at 60° C. for 12 h to obtain a black carbon-supported alloy powder;

[0039] (5) The black powder obtained in step (4) was placed in 100 ml of 1 M HClO4 and ultrasonicated for 30 min, followed by stirring at 60°C for 24 h. After the reaction, the black suspension was filtered and washed with a large amount of deionized water until the filtrate was neutral. After drying at 60°C for 12 h, it was fully ground to obtain a W microalloyed PtCuCo medium entropy alloy nanomaterial catalyst;

[0040] Example 3

[0041] W-microalloyed PtCuCo medium-entropy alloy catalyst prepared by acid etching with CH3COOH

[0042] (1)(1) Take 100 mg of carbon black powder and 62.3 mg of CuCl, grind them in a grinder at a grinding rate of 60 r / min for 30 min to uniformly load CuCl on the carbon black, then add 47.6 mg of NaBH4 to the grinder, continue grinding at a grinding rate of 60 r / min for 30 min, place the obtained black solid in 50 ml of ethanol, ultrasonicate until no bubbles are generated, filter, and wash with ethanol, then dry the filtered product for 12 h to obtain Cu@C

[0043] (2) Take 39.4 mL of ethylene glycol and add 36 mg of Cu@C to the above solution. Ultrasonicate for 60 min. Then, add 9.3 mL of 0.01 M chloroplatinic acid ethylene glycol solution, 0.31 mL of 0.1 M cobalt chloride ethylene glycol solution, and 1 mL of 2.8 mM hexacarbonyltungsten acetylacetone solution. Add 1.2 mL of 1 M NaOH aqueous solution to adjust the solution pH to 10 to obtain a reaction suspension.

[0044] (3) The reaction suspension obtained in step (2) was placed in an electric heating reactor and stirred at a speed of 350

[0045] rpm, and the reaction temperature was set at 190°C for 3 hours. After the reaction was completed, a black slurry was obtained;

[0046] (4) The black slurry obtained in step (3) was filtered and washed with deionized water, and dried at 60° C. for 12 h to obtain a black carbon-supported alloy powder;

[0047] (5) The black powder obtained in step (4) was placed in 100 ml of 0.5 M CH3COOH, ultrasonicated for 30 min, and then stirred at 60 ° C for 4 h. After the reaction, the black suspension was filtered and washed with a large amount of deionized water until the filtrate was neutral. After drying at 60 ° C for 12 h, it was fully ground to obtain a W micro-alloyed PtCuCo medium entropy alloy nanomaterial catalyst;

[0048] Figure 1The transmission electron microscope (TEM) and X-ray diffraction (XRD) patterns of Cu@C prepared in Examples 1 to 3 show that the Cu@C has a nanowire morphology and its main components are Cu, Cu2O, and C.

[0049] Figure 2 The transmission electron microscope (TEM) image and X-ray diffraction (XRD) pattern of the catalyst obtained in Example 1 show that the W microalloyed PtCuCo mesoentropy alloy nanomaterial is uniformly distributed on the carbon support in a worm-like shape, and is a highly alloyed single alloy phase with no phase separation phenomenon.

[0050] Figure 3 The linear voltammetric scan curves of the catalyst obtained in Example 1 and the commercial Pt / C catalyst in acid medium and before and after the long-term stability test are shown. The electrolyte solution used for the linear voltammetric scan is: O2-saturated 0.1M HClO4; the scan rate is 10mV / s; the scan voltage range is 0.15-1.1V (vs RHE); and the rotation speed is 1600rpm. The electrolyte solution used for the long-term stability test is: N2-saturated 0.1M HClO4; the scan rate is 100mV / s; the scan voltage range is 0.6-1.1V (vs RHE); and the number of cycles is 30,000. From the linear scan voltammetric curve after iR compensation, it can be seen that the half-wave potential of the prepared W microalloyed PtCuCo medium entropy alloy catalyst exceeds that of the commercial Pt / C catalyst by 69mV, and the specific mass activity reaches 1.14A mg pt -1 Long-term stability tests show that the prepared W microalloyed PtCuCo medium-entropy alloy catalyst has excellent stability. After 30k accelerated cycle test, the half-wave potential only dropped by 12mV and the specific mass activity only decreased by 3%.

[0051] Figure 4 This is the fuel cell test curve of the catalyst obtained in Example 2 and the commercial Pt / C catalyst, where the platinum loading on the cathode is 0.1 mg. Pt cm -2 , the Pt loading of the anode is 0.2 mg Pt cm -2 In a H2-O2 atmosphere with a relative pressure of 2 Bar, the peak power density of the prepared W microalloyed PtCuCo medium entropy alloy catalyst reached 2.1 W cm -2 , the specific mass power reaches 21W cm -2 .

[0052] Figure 5The figure compares the linear voltammetric scanning curves and cyclic voltammetric curves of the catalysts obtained in Examples 2 and 3 and the commercial Pt / C catalyst in acid medium. It can be seen from the linear scanning voltammetric curve after iR compensation that the W microalloyed PtCuCo medium entropy alloy catalyst prepared in this example has better oxygen reduction performance than the commercial Pt / C catalyst.

[0053] The above specific embodiments describe the basic principles and main features of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and any changes or modifications that are not conceived through creative work should be included in the scope of protection of the present invention. Without departing from the scope of the present invention, the present invention may be subject to various changes and modifications, and these changes and modifications will fall within the scope of protection claimed.

Claims

1. A method for preparing a W microalloyed medium-entropy alloy nanomaterial catalyst, characterized in that: The catalyst is a solid powder, wherein a W-microalloyed PtCuCo medium-entropy alloy is uniformly distributed on a carbon black carrier, wherein the medium-entropy alloy morphology is nano-worm-like, and the diameter of the medium-entropy alloy nano-worm-like one-dimensional structure is 2.5 to 6.5 nm; the W-microalloyed PtCuCo medium-entropy alloy has a crystal structure similar to face-centered cubic, is a single alloy phase, and has a large number of W-terminated surface step-like structures on its surface; the preparation method comprises the following steps: (1) Preparation method of carbon black-supported copper nanowires (Cu@C) with a certain aspect ratio by anhydrous solid phase grinding method: first, a certain amount of water-insoluble cuprous chloride is used as a precursor salt, which is thoroughly mixed with carbon black by grinding; then a certain amount of anhydrous sodium borohydride is added as a reducing agent, and an anhydrous solid phase grinding reaction is carried out at room temperature; after a certain grinding time, carbon black-supported copper nanowires (Cu@C) are prepared; the ground product is soaked in ethanol until no gas is generated, then washed, filtered and dried, and the obtained solid powder is used as a sacrificial template material in subsequent synthesis steps; (2) Preparation of PtCuCo medium-entropy alloy nanomaterials stabilized by W microalloying; first, a certain amount of Cu@C black powder is dispersed in a small amount of polyol and ultrasonically dispersed to form a black slurry; at the same time, a certain amount of H2PtCl6 ethylene glycol solution, CoCl2 ethylene glycol solution, and W(CO)6 acetylacetone solution are respectively added to the above black slurry containing Cu@C, and the pH of the reaction solution is adjusted to 8-10 by controlling the concentration of a certain metal salt; the electrochemical replacement reaction of the above metal salts with Cu nanowires and the thermochemical reaction of polyols are used to electrochemically and chemically reduce Pt, Co and W under certain reaction temperature and reaction time conditions, and dope them into the interior of the sacrificial template copper nanowires, and the nano-alloying driven by high mixing entropy between the metals is used to realize the preparation of PtCuCo medium-entropy alloy nanomaterials containing trace W doping stability; (3) Regulation of the surface step-like structure of the W-microalloyed stabilized medium-entropy alloy material; In order to achieve a large amount of exposure of the specific electrocatalytic active surface of the W-stabilized medium-entropy alloy, the above alloy nanomaterial is etched in a special acid of a certain concentration at 60°C for a certain time to remove the copper and part of the cobalt elements on the alloy surface, and finally obtain a W-stabilized medium-entropy alloy material with a large amount of surface step-like structure; The acid used for the acid treatment etching in step (3) includes H2SO4, HNO3, H3PO4, CH3COOH, and HClO4, wherein the molar concentration of the acid is 0.5 to 1.5 mol / L.

2. The method according to claim 1, characterized in that The atomic percentage of W in the W microalloyed PtCuCo mesoentropy alloy is 1% to 3%, the atomic percentage of Pt is 35% to 45%, the atomic percentage of Cu is 35% to 45%, the atomic percentage of Co is 9% to 27%, and the mass ratio of the W microalloyed PtCuCo mesoentropy alloy in the entire catalyst is 25% to 60%.

3. The method according to claim 1, characterized in that The carbon black in step (1) refers to commercial carbon black particles selected from Vulcan XC-72, KJ-EC300J, KJ-EC600JD, and BP2000.

4. The method according to claim 3, characterized in that The molar ratio of CuCl to NaBH4 in step (1) is 1:2 to 1:4; the aspect ratio of the carbon black-supported copper nanowires is 11 to 18; the anhydrous solid-phase grinding reaction time of carbon black, CuCl and NaBH4 in step (1) is 0.5 to 1 h, and the grinding rate is 60 r / min to 300 r / min.

5. The method according to claim 4, characterized in that The molar ratio of CuCl to NaBH4 in step (1) is 1:

3.

6. The method according to claim 1, characterized in that The polyol includes ethylene glycol (EG) and triethylene glycol (TEG), the thermal reduction temperature is 160-230° C., and the reaction time is 120-240 minutes.

7. The method according to claim 6, characterized in that The temperature of thermal reduction was 190°C and the reaction time was 180 min.

8. The method according to claim 1, characterized in that Before step (2) mixing, the concentration of the Pt precursor metal salt in the metal salt dispersion is 1.5×10 -3 ~6×10 -3 mol L -1 , the concentration of Co precursor metal salt is 5×10 -4 ~2×10 -3 mol L -1 , the concentration of W precursor metal salt is 2.5×10 -5 ~1×10 -4 mol L -1 .

9. The method according to claim 1, characterized in that When the acid in step (3) is H2SO4, H2O2 is compounded, and the molar concentration of H2O2 added is not higher than 1 mol / L.

10. The method according to claim 1, characterized in that In step (3), the acid used is 1M HClO4.

11. The method according to claim 1, characterized in that The etching time in step (3) is first ultrasonication for 30 minutes and then stirring for 24 hours.

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