Lignin-derived carbon-coated nickel particle electrolysis urea hydrogen production catalyst and preparation method thereof

Abstract

The present application relates to a kind of lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst and its preparation method, the preparation method includes the following steps: 1) lignin and nickel salt are sequentially dissolved in deionized water, and mixed solution is prepared;2) the nickel-based carrier after pretreatment is sequentially added to the mixed solution prepared in step 1) in reaction kettle and carries out hydrothermal reaction, after reaction, cooling to room temperature, the nickel-based carrier in reaction kettle is taken out and washed, dried to obtain preliminary sample;3) the preliminary sample prepared in step 2) is carried out high-temperature calcination under the mixed atmosphere of hydrogen / argon, and the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst is obtained.The carbon-coated structure in the catalyst prepared by the present application is the core of realizing high activity and long period stability: its physical barrier can reduce the oxidation corrosion of nickel active site, and porous carbon layer accelerates electrolyte mass transfer, so that the catalyst still maintains low potential and long life under high current density.

Description

Technical Field

[0001] This invention relates to the field of urea electrolysis catalyst technology, specifically to a lignin-derived carbon-coated nickel particle urea electrolysis hydrogen production catalyst and its preparation method. Background Technology

[0002] As my country enters a stage of high-quality development, the structural shortcomings of relying on traditional fossil fuels and the rigid constraints of ecological and environmental governance have created dual practical demands. Against this backdrop, developing efficient and renewable clean energy sources to address the challenges of synergistic energy and environmental protection has become a core direction and urgent task for supporting green transformation and strengthening development resilience. Compared to traditional fossil fuels, hydrogen energy not only has high energy density but also near-zero pollution. Furthermore, compared to renewable energy sources such as wind, solar, and hydropower, hydrogen energy offers stronger energy storage stability, greater safety advantages, and more diverse application scenarios.

[0003] Currently, hydrogen production through water electrolysis is one of the core technologies for achieving green hydrogen production. During electrolysis, the hydrogen evolution reaction (HER) occurs at the cathode, while the oxygen evolution reaction (OER) occurs at the anode. However, the theoretical potential of OER in traditional water electrolysis systems is relatively high (1.23 V vs. HER) and is a slow-kinetic four-electron transfer process, directly leading to high energy consumption and low energy conversion efficiency. Compared to OER, the theoretical potential of urea oxidation reaction (UOR) is only 0.37 V (vs. HER). This potential reduction significantly lowers the cell voltage of the electrolyzer, thereby greatly saving energy consumption in the electrolysis process. Furthermore, urea, a typical pollutant in industrial wastewater, domestic sewage, and human urine, can be used as a substrate in the UOR process to simultaneously produce high-purity hydrogen and harmlessly degrade urea-containing wastewater, achieving the dual benefits of "hydrogen production and pollution control." Furthermore, the mixing of the reaction products N2 and CO2 from UOR with the reaction product H2 from HER significantly narrows the explosion limit range and greatly improves the system's safety. This characteristic can promote the technological adoption of membrane-free electrolyzers and further expand their industrial applications. Therefore, developing high-performance HER / UOR bifunctional catalysts has significant research and application value.

[0004] In the field of HER / UOR bifunctional catalysts, noble metal-based catalysts (such as Pt / C and RuO2) have long held a dominant position. However, their large-scale application is significantly constrained by inherent defects such as resource scarcity, high cost, and insufficient reaction selectivity. In contrast, transition metal-based catalysts such as nickel-based catalysts have become the core alternative to noble metal-based catalysts due to their abundant resources, low cost, and intrinsic catalytic activity for HER / UOR reactions. However, existing nickel-based UOR catalysts still suffer from shortcomings such as insufficient exposure of active sites, poor conductivity, and poor stability, which limit their practical application efficiency. Meanwhile, lignin derived from biomass, as a major byproduct of the papermaking industry and biomass refining processes, has the natural advantages of low cost and wide availability. Its rich benzene ring skeleton and multifunctional group characteristics can be transformed into a carbon substrate with high specific surface area and good conductivity after heat treatment, making it a promising electrocatalyst support material.

[0005] Therefore, it is essential to develop a high-performance UOR catalytic material that combines low cost, high catalytic activity, and long-term stability. Summary of the Invention

[0006] Therefore, this invention provides a high-performance lignin-derived carbon-coated nickel particle electrolysis urea hydrogen production reaction catalyst with low cost, high catalytic activity and long-term stability, and its preparation method.

[0007] To achieve the above objectives, the inventors provide a method for preparing a lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst, which includes the following steps:

[0008] 1) Dissolve lignin and nickel salt in deionized water sequentially to obtain a mixed solution; the mass ratio of nickel salt to lignin is 2-18:1.

[0009] 2) The pretreated nickel-based support and the mixed solution obtained in step 1) are sequentially added to a reaction vessel. A hydrothermal reaction is carried out at 120-240℃ for 3-18 hours. After the reaction, the mixture is cooled to room temperature, and the nickel-based support is removed from the reaction vessel for washing and drying to obtain a preliminary sample. The ratio of the mass of the nickel salt added to the surface area of ​​the pretreated nickel-based support is 0.08-0.56:1 g / cm³. 2 ;

[0010] 3) Place the preliminary sample obtained in step 2) in a calcination furnace, introduce a mixed gas of hydrogen and argon, and heat it to 400-1200℃ at a heating rate of 3-7℃ / min. Calcinate at high temperature for 2-9 hours to obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0011] This invention employs the aforementioned scheme, using lignin from agricultural and forestry waste as a carbon source and low-cost transition metal nickel salts as a metal source. First, lignin and nickel salts are converted into nickel-based hydroxides or oxides and lignin-based chelates via a hydrothermal reaction. Then, they are reduced to nickel particles through high-temperature calcination, ultimately yielding a lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst. This lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst combines low cost, high catalytic activity, and long-term stability.

[0012] The lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared by this invention has the following technical advantages:

[0013] (1) Good catalytic activity: In this invention, the lignin-derived carbon substrate has a high specific surface area, which can achieve uniform dispersion of nickel particles and fully expose catalytic active sites; at the same time, the carbon substrate itself has good electrical conductivity, which can accelerate electron transfer in the reaction process, optimize the reaction kinetics of urea molecules on nickel active sites, reduce the reaction energy barrier of key steps in urea oxidation reaction, and thus significantly improve catalytic activity.

[0014] (2) Long-term stability: In the heterostructure prepared in this invention, the coating layer formed by lignin-derived carbon can build a physical barrier to effectively inhibit the agglomeration and growth of nickel particles during the reaction. At the same time, a stable interfacial bond is formed between the carbon layer and the nickel particles, which can reduce the oxidative dissolution of nickel active components. In addition, the lignin-derived carbon itself has good chemical inertness and can withstand the alkaline electrolysis environment of urea oxidation reaction, thereby ensuring the long-term stable operation of the catalyst.

[0015] (3) Synergistic adaptation effect of carbon coating structure: A deep synergistic effect driven by carbon coating structure is formed between lignin-derived carbon and nickel particles, which is reflected in the synergistic adaptation relationship between structural adaptation and interface bonding. From the structural level, the carbon layer formed by the derivation and carbonization of lignin-derived carbon forms a matching synergy with the size and morphology of nickel particles. The carbon layer can adhere to the surface of nickel particles to complete carbon coating. It will not over-coat and cover the active area of ​​nickel, nor will it cause poor dispersion of nickel particles due to insufficient carbon coating. The structural adaptation between carbon layer and nickel particles is realized. From the interface level, an interface bonding driven synergy is formed between carbon coating layer and nickel particles. The carbon layer is bonded to the surface of nickel particles through chemical bonding. It is not a simple physical load or independent component superposition, but forms an interface region with the characteristics of both carbon layer and nickel particles, realizing the fusion and unification of the interface properties of the two.

[0016] (4) Industrial application potential: The preparation process of this invention is simple and controllable, without the need for complex equipment and harsh reaction conditions. Moreover, the raw material lignin is a large-scale industrial by-product, which is widely available and inexpensive, greatly reducing the cost of large-scale catalyst preparation. Combined with its excellent catalytic activity, long-term stability and unique synergistic effect, it has significant performance and cost advantages in the field of urea oxidation coupled with hydrogen production, with broad application prospects and strong market competitiveness.

[0017] The preparation process of this invention is simple and controllable, achieving both low cost and excellent catalytic activity. In a mixed electrolyte system of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea, the prepared catalyst exhibits good bifunctional HER and UOR catalytic performance: in terms of HER performance, the corresponding current densities are -10 / -500 / -1000 mA cm⁻¹. -2 At these times, their potentials are -113.66 / -329.34 / -391.22 mV, respectively; in terms of UOR performance, the corresponding current densities are 10 / 500 / 1000 mA cm⁻¹. -2 At these times, their potentials are 1.29 / 1.37 / 1.43 V, respectively. Because the catalyst prepared by this invention combines low cost, high catalytic activity, and long-term stability, it possesses outstanding technical advantages and market competitiveness in the field of urea oxidation coupled with hydrogen production.

[0018] Further, the lignin in step 1) is one of the following: sulfate lignin, sulfite lignin, hydrolyzed lignin, basic lignin, organic solvent lignin, enzymatic hydrolyzed lignin, dealkalized lignin, dilute acid lignin, concentrated acid lignin, carboxymethylated lignin, sulfonated lignin, aminated lignin, phosphorylated lignin, carboxylate-modified lignin, acetylated lignin, oxidized lignin, hydroxypropylated lignin, and hydroxyethylated lignin;

[0019] The nickel salt in step 1) is one of the following: nickel nitrate, nickel acetate, nickel formate, nickel malonate, nickel phthalate, nickel chloride, nickel citrate, nickel sulfate, nickel carbonate, nickel aminosulfonate, nickel tartrate, nickel maleate, nickel gluconate, nickel lactate, nickel oxalate, nickel bromide, nickel iodide, and nickel hypophosphite.

[0020] Furthermore, the nickel-based carrier in step 2) is one of the following: nickel wire, nickel foil, nickel mesh, nickel fiber felt, nickel foam, or nickel sheet.

[0021] Furthermore, the nickel mesh is one of the following: angled nickel mesh, honeycomb nickel mesh, rectangular nickel mesh, spherical nickel mesh, square nickel mesh, pentagonal ring nickel mesh, triangular ring nickel mesh, folded interlaced nickel mesh, curved corrugated nickel mesh, tetrahedral nickel mesh, star-shaped ring nickel mesh, straight cylindrical nickel mesh, and octagonal ring nickel mesh;

[0022] The nickel wire is one of the following: ultrafine nickel wire, coarse nickel wire, fine nickel wire, straight nickel wire, or coiled nickel wire;

[0023] The nickel foil is one of the following: straight nickel foil, thin nickel foil, thick nickel foil, porous nickel foil, embossed nickel foil, or laminated nickel foil;

[0024] The nickel fiber felt is one of the following: porous nickel fiber felt, ultra-thin nickel fiber felt, thickened nickel fiber felt, composite layer nickel fiber felt, or oriented nickel fiber felt;

[0025] The nickel foam is one of the following: standard porous nickel foam, ultrathin high-density nickel foam, gradient porous nickel foam, directional through-cell nickel foam, and composite layer nickel foam;

[0026] The nickel sheet is one of the following: flat nickel sheet, ultra-thin nickel sheet, porous nickel sheet, or laminated composite nickel sheet.

[0027] Furthermore, the pretreatment method for the nickel-based support is as follows: the untreated nickel-based support is rinsed sequentially with anhydrous ethanol, 0.2-3.5 mol / L hydrochloric acid, and deionized water, with each rinse lasting 1-4 times and 10-30 minutes each time.

[0028] Furthermore, in step 1), after the lignin and nickel salt are dissolved in deionized water in sequence, they are dispersed by ultrasonic stirring or by shaking until all components are completely dissolved to obtain a mixed solution.

[0029] Furthermore, in step 2), after the pretreated nickel-based support and the mixed solution obtained in step 1) are added to the reactor for hydrothermal reaction, the resulting preliminary sample forms nickel hydroxide and oxide and is loaded onto the nickel-based support. The nickel hydroxide and oxide are one or a mixture of the following: NiOOH, Ni(OH)2, NiO, Ni2O3, Ni3O4.

[0030] Furthermore, in step 3), the volume fraction of hydrogen in the mixed gas is 2-15%.

[0031] This invention also discloses a lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst, which is prepared by the above-described preparation method.

[0032] The above technical solution has the following advantages, unlike existing technologies:

[0033] 1. This invention uses lignin, an agricultural and forestry waste, as a carbon source, which not only realizes its resource utilization and high value, but also conforms to the green and low-carbon industrial development direction. At the same time, it uses low-cost transition metal nickel salt as a metal source, and prepares a lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst by hydrothermal chelation and high-temperature reduction of lignin and nickel salt. The coating structure of lignin-derived carbon can regulate the electronic structure of nickel active sites through interfacial bonding, inhibit nickel particle agglomeration, and construct porous mass transfer channels, which directly helps to improve catalytic performance.

[0034] 2. The preparation method of this invention is simple and controllable, requires no precious metal additives, and is easy to scale up for mass production. The catalyst prepared by this invention exhibits high catalytic activity, long-term operational stability, and low cost in the urea oxidation reaction. The carbon coating structure is the core of achieving high activity and long-term stability: its physical barrier reduces the oxidative corrosion of nickel active sites, while the porous carbon layer accelerates electrolyte mass transfer, allowing the catalyst to maintain a low potential and long lifespan even at high current densities. Due to these characteristics, the catalyst prepared by this invention has outstanding application competitiveness in fields such as urea electrolysis for hydrogen production and urea pollution degradation. Attached Figure Description

[0035] Figure 1 The XRD pattern of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in Example 1 of this invention;

[0036] Figure 2 This is a TEM image of the lignin-derived carbon-coated nickel particle catalyst for the electrolysis of urea to produce hydrogen, prepared in Example 1 of this invention.

[0037] Figure 3 Linear sweep voltammetry (LSV) curves of hydrogen evolution in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea for the catalyst prepared in Example 1 of this invention and 20 wt.% of commercial Pt / C catalyst;

[0038] Figure 4 Tafel slope data for the catalyst prepared in Example 1 of this invention and 20 wt.% of a commercial Pt / C catalyst in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea;

[0039] Figure 5 LSV curve data of urea oxidation in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea for the catalyst prepared in Example 1 of this invention and 40 wt.% commercial RuO2 / C catalyst;

[0040] Figure 6Tafel slope data for the catalyst prepared in Example 1 of this invention and 40 wt.% of a commercial RuO2 / C catalyst in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea. Detailed Implementation

[0041] To explain in detail the technical content, structural features, objectives, and effects of the technical solution, the following description is provided in conjunction with specific embodiments and accompanying drawings.

[0042] Example 1

[0043] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0044] (1) Take an untreated rectangular nickel mesh (surface area of ​​10 cm²) 2 Rinse with anhydrous ethanol, 1.5 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation twice, each time for 20 minutes;

[0045] (2) Take 0.3 g of alkaline lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 1.72 g of nickel nitrate, dissolve them in 30 mL of deionized water, and disperse them by ultrasonication for 20 min until completely dissolved to obtain a homogeneous mixed solution.

[0046] (3) Place the rectangular nickel mesh processed in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 180℃, the reaction time is 6 hours, after the reaction is completed, cool to room temperature, take out the nickel-based support in the reaction vessel, wash and vacuum dry for 24 hours, and obtain the preliminary sample.

[0047] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 5% of the volume fraction of the mixed gas), heat it to 600°C at a rate of 5°C / min, and keep it at the temperature for 2 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0048] The lignin-derived carbon-coated nickel particle catalyst for urea electrolysis prepared in this embodiment was characterized by X-ray diffraction (XRD), and the obtained XRD pattern is shown below. Figure 1 As shown.

[0049] pass Figure 1XRD pattern analysis shows that the characteristic diffraction peaks of the catalyst in this embodiment are completely matched with the standard XRD card of metallic nickel (PDF#040850), confirming that the nickel component in the catalyst is a crystalline metallic Ni phase. At the same time, no sharp carbon-based characteristic diffraction peaks are observed in the pattern. Combined with the preparation process characteristics of lignin-derived carbon, it can be determined that the carbon layer mainly exists in the form of an amorphous carbon phase.

[0050] After XRD phase characterization, the catalyst was further characterized by transmission electron microscopy (TEM), and the obtained TEM images are shown below. Figure 2 As shown. (Through) Figure 2 The series of characterizations clearly revealed the core microscopic characteristics of the catalyst in this embodiment: First, the nanoparticles in the catalyst are uniformly dispersed and have no obvious agglomeration, confirming that the lignin-derived carbon substrate can achieve uniform loading of nickel particles; Second, the interface between the carbon layer and the nickel particles is tightly bonded, directly verifying the successful construction of the "carbon-coated nickel particle" composite system; Third, the crystalline structure of the nickel particles is consistent with the XRD characterization results, and there is interfacial strain in its region, indicating that there is a strong interfacial interaction between carbon and nickel.

[0051] The combined XRD and TEM characterization results above fully demonstrate that this embodiment has successfully prepared a lignin-derived carbon-coated nickel particle catalyst for the electrolysis of urea to produce hydrogen.

[0052] Example 2

[0053] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0054] (1) Take the untreated angled nickel mesh (surface area of ​​4 cm²) 2 The sample was rinsed sequentially with anhydrous ethanol, 1.0 mol / L hydrochloric acid, and deionized water for 30 minutes each time.

[0055] (2) Take 0.4 g of sulfate lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 1.98 g of nickel acetate, dissolve them in 30 mL of deionized water, and disperse them on a shaker for 20 min until completely dissolved to obtain a homogeneous mixed solution;

[0056] (3) Place the angled nickel mesh processed in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 200℃, the reaction time is 5 hours, after the reaction is completed, cool to room temperature, take out the nickel-based carrier in the reaction vessel for washing, vacuum drying for 24 hours, and obtain the preliminary sample.

[0057] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 10% of the volume fraction of the mixed gas), heat it to 700°C at a rate of 6°C / min, and keep it at the temperature for 3 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0058] Example 3

[0059] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0060] (1) Take untreated straight nickel foil (surface area of ​​1.25 cm²) 2 Rinse with anhydrous ethanol, 2.0 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation twice, each time for 15 minutes;

[0061] (2) Take 0.35 g of hydrolyzed lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 0.7 g of nickel malonate, dissolve them in 30 mL of deionized water, and disperse them by ultrasonic stirring for 30 min until completely dissolved to obtain a homogeneous mixed solution.

[0062] (3) Place the straight nickel foil treated in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 240℃, the reaction time is 3 hours, after the reaction is completed, cool to room temperature, take out the nickel-based carrier in the reaction vessel for washing, vacuum drying for 24 hours, and obtain the preliminary sample.

[0063] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 15% of the volume fraction of the mixed gas), heat it to 400°C at a rate of 3°C / min, and keep it at the temperature for 9 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0064] Example 4

[0065] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0066] (1) Take an untreated spherical nickel mesh (surface area of ​​11.75 cm²) 2 Rinse with anhydrous ethanol, 2.5 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation 3 times, each time for 10 minutes;

[0067] (2) Take 0.3 g of hydrolyzed lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 0.94 g of nickel malonate, dissolve them in 30 mL of deionized water, and disperse them by ultrasonic stirring for 20 min until completely dissolved to obtain a homogeneous mixed solution;

[0068] (3) Place the spherical nickel mesh treated in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 120℃, the reaction time is 18 hours, after the reaction is completed, cool to room temperature, take out the nickel-based support in the reaction vessel, wash and vacuum dry for 24 hours, and obtain the preliminary sample.

[0069] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 2% of the volume fraction of the mixed gas), heat it to 1200°C at a rate of 7°C / min, and keep it at the temperature for 2 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0070] Example 5

[0071] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0072] (1) Untreated ultrafine nickel wire (surface area of ​​19 cm²) 2 Rinse with anhydrous ethanol, 3.0 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation twice, each time for 20 minutes;

[0073] (2) Take 0.35 g of hydrolyzed lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 3.88 g of nickel chloride, dissolve them in 30 mL of deionized water, and disperse them on a shaker for 20 min until completely dissolved to obtain a homogeneous mixed solution;

[0074] (3) The ultrafine nickel wire treated in step (1) was placed in a 50 mL reactor and the mixed solution obtained in step (2) was added for hydrothermal reaction. The hydrothermal reaction temperature was 160℃ and the reaction time was 8 hours. After the reaction was completed, the nickel-based carrier in the reactor was taken out, washed, and vacuum dried for 24 hours to obtain the preliminary sample.

[0075] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 6% of the volume fraction of the mixed gas), heat it to 800°C at a rate of 4°C / min, and keep it at the temperature for 5 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0076] Example 6

[0077] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0078] (1) Untreated porous nickel fiber felt (surface area 30 cm²) 2 Rinse with anhydrous ethanol, 3.5 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation 3 times, each time for 10 minutes;

[0079] (2) Take 0.4 g of organic solvent lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 6.11 g of nickel citrate, dissolve them in 30 mL of deionized water, and disperse them on a shaker for 20 min until completely dissolved to obtain a homogeneous mixed solution.

[0080] (3) The porous nickel fiber felt treated in step (1) was placed in a 50 mL reactor, and the mixed solution obtained in step (2) was added for hydrothermal reaction. The hydrothermal reaction temperature was 190℃ and the reaction time was 7 hours. After the reaction was completed, the mixture was cooled to room temperature. The nickel-based carrier in the reactor was taken out, washed, and vacuum dried for 24 hours to obtain the preliminary sample.

[0081] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 8% of the volume fraction of the mixed gas), heat it to 900°C at a rate of 6°C / min, and keep it at the temperature for 4 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0082] Example 7

[0083] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0084] (1) Untreated curved corrugated nickel mesh (surface area 38 cm²) 2 Rinse with anhydrous ethanol, 0.5 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation 3 times, each time for 10 minutes;

[0085] (2) Take 0.38 g of enzymatically hydrolyzed lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 6.84 g of nickel citrate, dissolve them in 30 mL of deionized water, and disperse them by ultrasonic stirring for 10 min until completely dissolved to obtain a homogeneous mixed solution.

[0086] (3) Place the curved corrugated nickel mesh processed in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 150℃, the reaction time is 10 hours, after the reaction is completed, cool to room temperature, take out the nickel-based support in the reaction vessel, wash and vacuum dry for 24 hours, and obtain the preliminary sample.

[0087] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 9% of the volume fraction of the mixed gas), heat it to 1000°C at a rate of 5°C / min, and keep it at the temperature for 6 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0088] Example 8

[0089] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0090] (1) Untreated ultrathin high-density nickel foam (surface area of ​​59 cm²) 2 Rinse with anhydrous ethanol, 1.3 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation twice, each time for 20 minutes;

[0091] (2) Take 0.42 g of sulfonated lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 6.97 g of nickel tartrate, dissolve them in 30 mL of deionized water, and disperse them on a shaker for 20 min until completely dissolved to obtain a homogeneous mixed solution.

[0092] (3) The ultra-thin high-density nickel foam treated in step (1) was placed in a 50 mL reactor, and the mixed solution obtained in step (2) was added for hydrothermal reaction. The hydrothermal reaction temperature was 210℃ and the reaction time was 5 hours. After the reaction was completed, the nickel-based carrier in the reactor was taken out, washed, and vacuum dried for 24 hours to obtain the preliminary sample.

[0093] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 12% of the volume fraction of the mixed gas), heat it to 1100°C at a rate of 6°C / min, and keep it at the temperature for 4 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0094] Example 9

[0095] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0096] (1) Take the untreated cylindrical nickel mesh (surface area 67 cm²) 2 Rinse with anhydrous ethanol, 1.8 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation once, each time for 15 minutes;

[0097] (2) Take 0.36 g of aminated lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 5.91 g of nickel bromide, dissolve them in 30 mL of deionized water, and disperse them on a shaker for 15 min until completely dissolved to obtain a homogeneous mixed solution.

[0098] (3) Place the straight cylindrical nickel mesh processed in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 220℃, the reaction time is 6 hours, after the reaction is completed, cool to room temperature, take out the nickel-based carrier in the reaction vessel, wash, vacuum dry for 24 hours, and obtain the preliminary sample.

[0099] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 15% of the volume fraction of the mixed gas), heat it to 600°C at a rate of 5°C / min, and keep it at the temperature for 6 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0100] Example 10

[0101] A lignin-derived carbon-coated nickel particle catalyst for urea electrolysis to produce hydrogen is prepared as follows:

[0102] (1) Take the untreated flat nickel sheet (surface area of ​​9 cm²) 2 Rinse once each with anhydrous ethanol, 2.2 mol / L hydrochloric acid, and deionized water for 25 minutes each time.

[0103] (2) Take 0.39 g of oxidized lignin (a commercially available product, purchased from Shanghai Maclean Biochemical Technology Co., Ltd., brand name: L832292-100g) and 1.83 g of nickel oxalate, dissolve them in 30 mL of deionized water, and disperse them by ultrasonic stirring for 20 min until completely dissolved to obtain a homogeneous mixed solution.

[0104] (3) Place the flat nickel sheet processed in step (1) into a 50 mL reaction vessel, add the mixed solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 150℃, the reaction time is 8 hours, after the reaction is completed, cool to room temperature, take out the nickel-based carrier in the reaction vessel, wash, vacuum dry for 24 hours, and obtain the preliminary sample.

[0105] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 8% of the volume fraction of the mixed gas), heat it to 900°C at a rate of 4°C / min, and keep it at the temperature for 8 hours for high-temperature calcination, and finally obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

[0106] Comparative Examples

[0107] A nickel particle electrolysis urea hydrogen production catalyst, the preparation method of which is as follows:

[0108] (1) Take an untreated rectangular nickel mesh (surface area of ​​10 cm²) 2 Rinse with anhydrous ethanol, 1.5 mol / L hydrochloric acid and deionized water in sequence, repeating the above rinsing operation twice, each time for 20 minutes;

[0109] (2) Take 1.72 g of nickel nitrate, dissolve it in 30 mL of deionized water, and disperse it by ultrasonication for 20 min until it is completely dissolved to obtain a homogeneous solution;

[0110] (3) Place the rectangular nickel mesh processed in step (1) into a 50 mL reaction vessel, add the solution obtained in step (2) for hydrothermal reaction, the hydrothermal reaction temperature is 180℃, the reaction time is 6 hours, after the reaction is completed, cool to room temperature, take out the nickel-based carrier in the reaction vessel, wash, vacuum dry for 24 hours, and obtain the preliminary sample.

[0111] (4) Place the preliminary sample obtained in step (3) in a tube furnace, introduce a mixed gas of hydrogen and argon (hydrogen accounts for 5% of the volume fraction of the mixed gas), heat it to 600°C at a rate of 5°C / min, and keep it at the temperature for 2 hours for high-temperature calcination, and finally obtain the nickel particle electrolytic urea hydrogen production catalyst.

[0112] In Examples 1-10 above, the preliminary sample obtained in step (3) forms nickel hydroxide and oxide and is loaded onto a nickel-based support.

[0113] The lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalysts prepared in Examples 1-10 and the nickel particle electrolytic urea hydrogen production catalysts prepared in the comparative examples were subjected to HER and UOR tests in a 1.0 mol / L potassium hydroxide and 0.5 mol / L urea solution. The specific procedures are as follows:

[0114] (1) The lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in Examples 1-10 or the nickel particle electrolytic urea hydrogen production catalyst prepared in the comparative examples were used as working electrodes, and electrochemical tests were performed using an electrochemical workstation.

[0115] (2) The test conditions are as follows: A carbon rod is used as the counter electrode, a mercury / mercury oxide electrode is used as the reference electrode, and the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in Examples 1-10 or the nickel particle electrolytic urea hydrogen production catalyst prepared in the comparative examples are used as the working electrode to form 11 sets of three-electrode test systems. Then, the electrochemical performance of HER and UOR is detected in 1.0 mol / L potassium hydroxide and 0.5 mol / L urea solution. The results are shown in Table 1 and Table 2.

[0116] Table 1. Electrochemical performance of HER in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea.

[0117]

[0118] Table 2. Electrochemical performance of UOR in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea.

[0119]

[0120] Analysis of the HER electrochemical performance data in Table 1 shows that, in Examples 1-10 of this invention, under ambient temperature and pressure conditions, the various lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalysts prepared in this invention exhibit excellent HER catalytic performance in a mixed electrolyte system of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea. -2 At that time, the absolute value of the potential was lowest at 113.66 mV in Example 1 and highest at 131.87 mV in Example 8; when the current density was increased to -500 mA cm⁻¹ -2 At that time, the absolute value of the potential was lowest at 329.34 mV in Example 1 and highest at 341.25 mV in Example 3; while at -1000 mA cm⁻¹ -2At the given current density, the absolute value of the potential ranged from a minimum of 391.22 mV in Example 1 to a maximum of 401.49 mV in Example 7; from a reaction kinetics perspective, the minimum Tafel slope of this catalyst was 90.04 mV dec in Example 1. -1 The maximum value was 112.81 mV dec in Example 6. -1 The above data indicate that the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in this invention exhibits excellent HER performance in a mixed electrolyte system of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea. Considering the potential performance and reaction kinetics characteristics at various current densities, Example 1 shows the best performance at all current densities and is the preferred embodiment of this invention.

[0121] Analysis of the UOR electrochemical performance data in Table 2 shows that, in Examples 1-10 of this invention, in the same mixed electrolyte system of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea, the catalyst prepared by this invention also exhibits excellent UOR catalytic performance: when the current density is 10 mA cm⁻¹ -2 At that time, the lowest potential was 1.29 V in Examples 1 and 5, and the highest was 1.32 V in Example 9; when the current density was increased to 500 mA cm⁻¹ -2 At that time, the lowest potential was 1.37 V in Example 1 and the highest was 1.41 V in Example 7; while at 1000 mA cm⁻¹ -2 At the given current density, the lowest potential was 1.43 V in Example 1 and the highest was 1.51 V in Example 10; from a reaction kinetics perspective, the catalyst prepared in this invention exhibited a minimum Tafel slope of 33.54 mV dec in Example 1. -1 The maximum value was 72.89 mV dec in Example 8. -1 The above data indicate that the lignin-derived carbon-coated nickel particle catalyst for urea electrolysis prepared in this invention also exhibits good UOR performance in a mixed electrolyte system of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea. Considering the potential performance and reaction kinetics characteristics at various current densities, Example 1 shows the best performance across the entire current density range and is the preferred embodiment of this invention.

[0122] Analysis of the HER and UOR electrochemical performance data of Examples 1-10 and the comparative examples in Tables 1 and 2 shows that, under ambient temperature and pressure conditions in a mixed electrolyte system of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea, the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in this invention exhibits significantly lower HER and UOR potentials than the nickel-based control sample without carbon coating across the entire current density range, demonstrating superior electrochemical performance. From a reaction kinetics perspective, the Tafel slopes of both HER and UOR of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in this invention are also much smaller than those of the non-carbon-coated structure, indicating a more efficient reaction kinetic process. These data fully demonstrate that the lignin-derived carbon coating structure can significantly improve the electrochemical activity and reaction efficiency of the catalyst by constructing porous mass transfer channels, enhancing charge transport capacity, and accelerating reaction kinetics.

[0123] The lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst prepared in Example 1 and 20 wt.% of a commercial Pt / C catalyst were placed in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea, respectively, and hydrogen evolution linear sweep voltammetry (LSV) and Tafel slope tests were performed. The corresponding LSV curve data are shown in […]. Figure 3 Tafel slope detection curve data can be found Figure 4 .

[0124] LSV curves can reflect the electrochemical performance of a catalyst: at the same current density, the smaller the absolute value of the potential, the better the performance of the catalyst for the corresponding reaction. The linear sweep voltammetry curves of hydrogen evolution of the catalyst prepared in Example 1 of this invention and 20 wt.% of a commercial Pt / C catalyst in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea are shown below. Figure 3 .comprehensive Figure 3 As shown in Table 1, the concentration of potassium hydroxide in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea is -10 mA cm⁻¹. -2 At a current density of [value missing], the potential of the catalyst prepared in Example 1 of this invention is close to that of a commercial Pt / C catalyst, indicating that the HER performance of the catalyst prepared in this invention is comparable to that of commercial benchmark materials.

[0125] Furthermore, reaction kinetics is one of the core indicators for evaluating the practical value of a catalyst, and the Tafel slope (which can be converted from LSV data; the smaller the value, the faster the reaction kinetics) can reflect this characteristic. The Tafel slope curves of the catalyst prepared in Example 1 of this invention and 20 wt.% of a commercial Pt / C catalyst in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea are shown below. Figure 4 .comprehensive Figure 4 As shown in Table 1, compared to commercial Pt / C catalysts, which serve as high-performance benchmarks for HER, the catalyst prepared in this embodiment has a slightly larger Tafel slope and slightly slower HER reaction kinetics. However, this kinetic level is still within the practical performance range of non-precious metal nickel-based catalysts. More importantly, the catalyst prepared in Example 1 of this invention uses lignin-derived carbon, an industrial byproduct, as a support. The raw material cost is less than 1 / 20 of that of commercial Pt / C, enabling low-cost large-scale preparation and making it more suitable for low-cost applications in industrial-grade urea electrolysis for hydrogen production.

[0126] After analyzing the HER performance, the UOR performance of the catalyst prepared in this invention as a bifunctional catalyst was further investigated: the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production reaction catalyst prepared in Example 1 of this invention and 40 wt.% of commercial RuO2 / C catalyst were placed in a mixed solution of 1.0 mol / L potassium hydroxide and 0.5 mol / L urea, respectively, and urea oxidation linear sweep voltammetry and Tafel slope detection were performed. The corresponding urea oxidation linear sweep voltammetry curve data are shown in […]. Figure 5 The Tafel slope detection data curve is shown below. Figure 6 .

[0127] comprehensive Figure 5 As shown in Table 2, the UOR potential of the catalyst prepared in this embodiment is lower than that of the commercial RuO2 / C catalyst across the entire current density range, indicating that its UOR performance has a significant advantage over the commercial benchmark material.

[0128] Further verification and integration from the perspective of reaction kinetics. Figure 6 As shown in Table 2, the catalyst prepared in this embodiment has a smaller Tafel slope compared to commercial RuO2 / C catalysts, indicating that its UOR reaction kinetics have a significant advantage.

[0129] This invention uses lignin as a carbon-based precursor, which is derivatized and carbonized before being coupled with nickel particles to construct a composite catalytic system of lignin-derived carbon coated with nickel particles. By leveraging the interfacial regulation and structural protection of the carbon coating structure, a UOR catalyst was successfully prepared. Through the core-shell structure of the carbon coating, this invention combines the high specific surface area and porous properties of lignin-derived carbon with the catalytic activity of nickel particles, achieving a synergistic adaptation effect between the carbon coating structure of the support and the active component.

[0130] Based on the above performance and kinetics of HER and UOR, the catalyst of this invention achieves reaction performance comparable to commercial materials, while significantly reducing costs through lignin-derived carbon support, and has the potential for application in industrial-grade urea electrolysis systems.

[0131] It should be noted that although the above embodiments have been described herein, this does not limit the scope of patent protection of the present invention. Therefore, any changes and modifications made to the embodiments described herein based on the innovative concept of the present invention, or equivalent structural or procedural transformations made using the content of the present invention's specification and drawings, directly or indirectly applying the above technical solutions to other related technical fields, are all included within the scope of patent protection of the present invention.

Claims

1. A method for preparing a lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst, comprising the following steps: 1) Dissolve lignin and nickel salt in deionized water sequentially to obtain a mixed solution; the mass ratio of nickel salt to lignin is 2-18:

1. 2) The pretreated nickel-based support and the mixed solution obtained in step 1) are sequentially added to a reaction vessel. A hydrothermal reaction is carried out at 120-240℃ for 3-18 hours. After the reaction, the mixture is cooled to room temperature, and the nickel-based support is removed from the reaction vessel for washing and drying to obtain a preliminary sample. The ratio of the mass of the nickel salt added to the surface area of ​​the pretreated nickel-based support is 0.08-0.56:1 g / cm³. 2 ; 3) Place the preliminary sample obtained in step 2) in a calcination furnace, introduce a mixed gas of hydrogen and argon, and heat it to 400-1200℃ at a heating rate of 3-7℃ / min. Calcinate at high temperature for 2-9 hours to obtain the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst.

2. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 1, characterized in that: The lignin in step 1) is one of the following: sulfate lignin, sulfite lignin, hydrolyzed lignin, basic lignin, organic solvent lignin, enzymatic hydrolyzed lignin, dealkalized lignin, dilute acid lignin, concentrated acid lignin, carboxymethylated lignin, sulfonated lignin, aminated lignin, phosphorylated lignin, carboxylate-modified lignin, acetylated lignin, oxidized lignin, hydroxypropylated lignin, and hydroxyethylated lignin. The nickel salt in step 1) is one of the following: nickel nitrate, nickel acetate, nickel formate, nickel malonate, nickel phthalate, nickel chloride, nickel citrate, nickel sulfate, nickel carbonate, nickel aminosulfonate, nickel tartrate, nickel maleate, nickel gluconate, nickel lactate, nickel oxalate, nickel bromide, nickel iodide, and nickel hypophosphite.

3. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 1, characterized in that, The nickel-based carrier in step 2) is one of the following: nickel wire, nickel foil, nickel mesh, nickel fiber felt, nickel foam, or nickel sheet.

4. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 3, characterized in that, The nickel mesh is one of the following: angled nickel mesh, honeycomb nickel mesh, rectangular nickel mesh, spherical nickel mesh, square nickel mesh, pentagonal ring nickel mesh, triangular ring nickel mesh, folded interlaced nickel mesh, curved corrugated nickel mesh, tetrahedral nickel mesh, star-shaped ring nickel mesh, straight cylindrical nickel mesh, and octagonal ring nickel mesh. The nickel wire is one of the following: ultrafine nickel wire, coarse nickel wire, fine nickel wire, straight nickel wire, or coiled nickel wire; The nickel foil is one of the following: straight nickel foil, thin nickel foil, thick nickel foil, porous nickel foil, embossed nickel foil, or laminated nickel foil; The nickel fiber felt is one of the following: porous nickel fiber felt, ultra-thin nickel fiber felt, thickened nickel fiber felt, composite layer nickel fiber felt, or oriented nickel fiber felt; The nickel foam is one of the following: standard porous nickel foam, ultrathin high-density nickel foam, gradient porous nickel foam, directional through-cell nickel foam, and composite layer nickel foam; The nickel sheet is one of the following: flat nickel sheet, ultra-thin nickel sheet, porous nickel sheet, or laminated composite nickel sheet.

5. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 1, characterized in that: The pretreatment method for the nickel-based support is as follows: the untreated nickel-based support is rinsed sequentially with anhydrous ethanol, 0.2-3.5 mol / L hydrochloric acid, and deionized water, with each rinse lasting 1-4 times for 10-30 minutes.

6. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 1, characterized in that: In step 1), lignin and nickel salt are dissolved in deionized water in sequence, and then dispersed by ultrasonic stirring or shaking until all components are completely dissolved to obtain a mixed solution.

7. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 1, characterized in that, In step 2), the pretreated nickel-based support and the mixed solution obtained in step 1) are added to the reaction vessel for hydrothermal reaction. The resulting preliminary sample forms nickel hydroxide and oxide and is loaded onto the nickel-based support. The nickel hydroxide and oxide are one or a mixture of the following: NiOOH, Ni(OH)2, NiO, Ni2O3, Ni3O4.

8. The preparation method of the lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst according to claim 1, characterized in that, In step 3), the volume fraction of hydrogen in the mixed gas is 2-15%.

9. A lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst, characterized in that: The lignin-derived carbon-coated nickel particle electrolytic urea hydrogen production catalyst is prepared by the preparation method described in any one of claims 1-8.

Images