A molybdenum carbide / zirconium oxide composite electrocatalyst, a preparation method and application thereof

Abstract

The application belongs to the technical field of electrocatalytic material preparation, and relates to a molybdenum carbide / zirconium oxide composite electrocatalyst and a preparation method and application thereof. The application adopts a hydrothermal method to prepare UiO-66 (a molybdenum carbide carbon source); and then a molybdenum carbide composite catalyst is prepared through pyrolysis reduction. Compared with the prior art, the material synthesis reaction of the application is controllable, the structure is stable, the material has a framework structure, and good service life is exhibited; the preparation method of the application is strong in systematicness, mass transfer is effectively promoted by providing rich pore structures by UiO-66, and therefore the catalytic efficiency is significantly improved; the obtained molybdenum carbide composite catalyst has good electrocatalytic performance, and can significantly reduce the cost when applied to PEM water electrolysis hydrogen production.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic material preparation technology, specifically relating to a molybdenum carbide / zirconia composite photocatalyst, its preparation method, and its application. Background Technology

[0002] Against the backdrop of ever-increasing energy demand and environmental pollution challenges, exploring new energy sources to replace traditional fossil fuels has become an urgent task. As a clean energy source, hydrogen (H2) is hailed as one of the most promising energy alternatives to fossil fuels. Electrocatalytic water splitting for hydrogen production is considered a promising method. The selection of electrocatalytic materials largely depends on their overpotential, intrinsic activity and density of active sites, material cost, and stability. However, most existing electrocatalysts suffer from high overpotentials, insufficient active site density, and dependence on precious metals. Therefore, constructing a carbon material framework and introducing a polarized electric field to reduce the hydrogen evolution overpotential and increase the density of active sites is crucial for improving electrocatalytic performance.

[0003] In the field of electrocatalytic material preparation technology, molybdenum carbide (Mo2C) is considered one of the most promising non-noble metal catalysts for the hydrogen evolution reaction (HER). Carbon atom insertion makes the d-band electronic state density of molybdenum highly similar to that of the noble metal platinum (Pt), thus endowing molybdenum carbide with both high intrinsic catalytic activity and metallic-grade conductivity. Simultaneously, its raw material cost is far lower than that of noble metals, making it an ideal candidate material to replace platinum-based catalysts. Furthermore, Mo2C exhibits certain electrochemical stability over a wide pH range, these advantages providing feasibility for its large-scale application. Zirconia (ZrO2), as a transition metal oxide with stable chemical properties, high thermal stability, and dual acid-base function, is receiving increasing attention. ZrO2's excellent surface stability over a wide range of media and temperatures, especially in acidic HER environments, can significantly inhibit the aggregation and dissolution of active components, thereby extending the catalyst's lifetime. In addition, the porous structure of ZrO2 facilitates the construction of efficient charge transport channels and mass diffusion pathways. In summary, introducing ZrO2 as a support or functional component into electrocatalysts (such as the Mo2C / ZrO2 composite system) not only enables the confined growth of active nanoparticles but also enhances intrinsic catalytic activity and stability through interfacial synergistic effects. Therefore, it exhibits unique advantages in the rational design of alternative precious metal electrocatalytic materials, resulting in better electrocatalytic performance and possessing significant scientific and practical value.

[0004] Currently, commonly used methods for constructing carbon supports mainly include hard template methods, soft template methods, chemical activation methods, and MOF derivatization methods. Among them, the hard template method uses SiO2 or SBA-15 as templates, which can achieve precise control of pore size and high structural order. However, the preparation process involves multiple steps, such as template filling, high-temperature carbonization, and etching removal of the template with strong corrosive reagents (such as HF or NaOH). This is not only cumbersome and energy-intensive, but the use of strong acids and alkalis also causes serious environmental pollution, making it difficult to meet the requirements of green chemistry. The soft template method uses surfactant micelles as structure directing agents. Although it avoids the etching step, the template is prone to decomposition during high-temperature carbonization, leading to pore structure collapse. It requires precise control of pyrolysis temperature and heating rate, and the pore wall thickness and long-range order are limited. The chemical activation method uses activators such as KOH to etch and create pores. Although it can significantly increase the specific surface area, the pore structure is disordered. Excessive micropore development may hinder electrolyte mass transfer and diffusion, and activator residues are difficult to completely remove, affecting the intrinsic performance of electrocatalytic active sites. These methods each have their limitations, so there is an urgent need to adopt a simple strategy that allows for the design of precursor structures, controllability of processes, and the construction of hierarchical porous carbon supports to achieve the construction of high-performance electrocatalyst supports. Summary of the Invention

[0005] The primary objective of this invention is to overcome the shortcomings and deficiencies of existing traditional Mo2C electrocatalysts and provide a method for preparing a highly efficient molybdenum carbide / zirconium oxide (Mo2C / ZrO2) composite electrocatalyst with good catalytic performance, thereby solving the problems of complex preparation process, difficulty in controlling phase purity, and severe insufficient exposure of active sites in existing synthesis methods.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a molybdenum carbide / zirconia composite electrocatalyst, comprising the following steps: S1. The derivative porous carbon support precursor UiO-66 was prepared by hydrothermal method; S2. The derived porous carbon support precursor UiO-66 and water-soluble molybdate are calcined at a mass ratio of (3-5):1 under vacuum or inert atmosphere at a temperature of 2-5℃ / min to 500-900℃, and the temperature is maintained for 4-6 hours to obtain the composite electrocatalyst.

[0007] Preferably, the method for preparing the derived porous carbon support precursor UiO-66 includes: S11. Dissolve ZrCl4 and terephthalic acid in N,N-dimethylformamide (DMF) solution, disperse by ultrasonication, add glacial acetic acid, and stir for 1 h; the molar ratio of ZrCl4 to terephthalic acid is 1:(1-2). S12. Take out the above solution and add it to a hydrothermal reactor. React at 120°C for 16 h. S13. After the reaction is complete, cool to room temperature, wash three times with DMF and ethanol respectively by centrifugation, dry at 60°C, collect the white powder and grind it to obtain the precursor.

[0008] Preferably, the preparation steps of the composite electrocatalyst are as follows: S21. The derived porous carbon support precursor UiO-66 and water-soluble molybdate are added to a pure water system. S22. The mixed solution is ultrasonically stirred for 1 hour, then placed in an oven to dry and the mixed powder is collected. S23. The mixed powder is placed in a porcelain boat, which is placed in the middle of the vacuum tube furnace chamber, and the furnace opening is sealed. S24. Use argon gas to flush the furnace tubes and maintain a vacuum inside the furnace tubes; S25. Heating tube furnace, first heat from room temperature to 500 ℃ at 2℃ / min, and hold at that temperature for 2h; S26. After the heat preservation is completed, continue to increase the temperature to 800 ℃ at a rate of 2℃ / min and keep it warm for 3 hours; S27. After the heat preservation is completed, cool to room temperature, open the air inlet and purge with argon to atmospheric pressure, open the two closed ends, take out the ceramic boat containing the Mo2C / ZrO2 composite electrocatalyst, and obtain a black powder product. Grind to obtain the composite electrocatalyst.

[0009] Preferably, during argon purging, the argon gas flow rate is 40-60 sccm, and the purging time is 8-12 min.

[0010] Furthermore, the present invention also provides a molybdenum carbide / zirconia composite electrocatalyst prepared by the method described above.

[0011] Furthermore, the present invention also provides the application of the molybdenum carbide / zirconia composite electrocatalyst in the total water splitting for hydrogen production.

[0012] This application describes a one-step pyrolysis reduction process on a derived porous carbon support precursor UiO-66 to achieve the conversion of ammonium molybdate to molybdenum trioxide to molybdenum dioxide to molybdenum carbide. Furthermore, a molybdenum carbide / zirconia composite electrocatalyst is prepared using the UiO-66-derived porous carbon framework as a support. Compared with existing technologies, the advantages of this invention are as follows: (1) In this invention, ammonium molybdate is used as the molybdenum source for molybdenum carbide. Compared with other conventional solid molybdenum sources, it has fewer impurities and higher purity of molybdenum carbide. (2) This invention has good catalytic performance, and electrocatalytic reactions can be carried out well using this catalyst; (3) The synthesis process of the present invention is relatively systematic, and the cost is lower than that of traditional PEM electrolysis for hydrogen production, and the stability of the catalytic reaction can be guaranteed. Attached Figure Description

[0013] Figure 1 This is a scanning electron microscope image of the Mo2C / ZrO2 composite electrocatalyst obtained in step S3 of Example 1; Figure 2 The X-ray diffraction analysis (XRD) of the Mo2C / ZrO2 composite electrocatalyst obtained at different heating temperatures in step S3 of Example 1 is shown. Figure 3 X-ray diffraction analysis (XRD) of the Mo2C / ZrO2 composite electrocatalysts obtained in step S2 of Example 1 with different UiO-66 and ammonium molybdate in different mass ratios. Figure 4 The catalytic performance of Mo2C / ZrO2 obtained in step S3 of Example 1 is tested by LSV, Tafel slope and stability in 0.5M H2SO4 electrolyte. Figure 5 This is a schematic diagram of the PEM system in step S3 of Example 1 and the catalytic performance under the PEM system; wherein, a. schematic diagram of the PEM system; b. catalytic performance under the PEM system; c. stability under the PEM system. Detailed Implementation

[0014] To facilitate understanding of the present invention, it will be described in more detail below with reference to the accompanying drawings and specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described in this specification. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0015] Example 1: A method for preparing a molybdenum carbide / zirconia composite electrocatalyst, comprising the following steps: S1. Preparation of the derived porous carbon support precursor UiO-66 (molybdenum carbide carbon source): 3 mmol ZrCl4 and 3 mmol terephthalic acid were dissolved in 40 mL N,N-dimethylformamide (DMF) solution, ultrasonically dispersed for 15 min, and then 4 mL glacial acetic acid was added. The mixture was stirred at 1000 r / min for 1 h. After stirring, the mixture was added to a 50 mL hydrothermal reactor and reacted at 120 °C for 16 h. After the reaction, the mixture was washed three times by centrifugation with DMF and ethanol respectively, dried at 60 °C, and the white powder was collected and ground.

[0016] The derived porous carbon support precursor UiO-66 is a metal-organic framework material with a regular and uniformly sized pore structure. When a molybdenum source is introduced, these nanoscale pores act like tiny "reactors," confining the growth of molybdenum carbide within the nanoscale. This "confinement effect" effectively prevents the aggregation of molybdenum carbide particles during high-temperature carbonization, resulting in highly dispersed ultrasmall nanoparticles. The ultrasmall particle size means more exposed active sites, which is crucial for enhancing catalytic activity.

[0017] The carbon materials derived from the porous carbon support precursor UiO-66 perfectly inherit the high porosity of its parent material. This porous structure not only provides ample space when loading molybdenum species, but more importantly, it acts as a "highway" in catalytic reactions, ensuring that reactant molecules can quickly contact the molybdenum carbide active sites distributed within the pores, while allowing product molecules to leave rapidly, thereby significantly improving catalytic efficiency.

[0018] The porous carbon framework derived from the UiO-66 porous carbon support precursor is not only a support but also a stable "anchor." It can effectively "lock" molybdenum carbide particles within the carbon framework, preventing them from migrating, agglomerating, and becoming deactivated during the reaction, thereby improving the overall structural stability and lifespan of the catalyst.

[0019] S2. Using ammonium molybdate as the molybdenum source, high-temperature calcination pre-decomposes the UiO-66 framework of the derived porous carbon support precursor to produce a porous carbon support: The derived porous carbon support precursor UiO-66 and ammonium molybdate are added to 20 mL of high-purity water at a mass ratio of 4:1, ultrasonically stirred for 1 h, and then dried in a 60℃ oven. The mixed powder is collected. The mixed powder is placed in a ceramic boat and placed in the center of a vacuum tube furnace, and the furnace opening is sealed. The furnace tube is flushed with argon gas at a flow rate of 50 sccm for 10 min. After the air is exhausted, the tube furnace is heated. The calcination program is to first raise the temperature from room temperature to 500 ℃ at a rate of 2℃ / min, and hold at that temperature for 2 h.

[0020] The reasons for using ammonium molybdate as the carbon source for molybdenum carbide are as follows: Certain types of ammonium molybdate (such as ammonium dodecylmolybdate) have very low solubility in water, making them easy to crystallize and purify, and they have low ammonia content, ensuring the purity and high quality of the final molybdenum carbide powder from the source. Furthermore, less ammonia is released during the heat treatment process, making it more environmentally friendly. Compared to other conventional solid molybdenum sources such as sodium molybdate and phosphomolybdic acid, there is no issue of incomplete removal of sodium and phosphorus, which could affect the purity of the molybdenum carbide powder.

[0021] This step mainly involves the pre-decomposition of the UiO-66 framework, a precursor for the derived porous carbon support, and the pyrolysis of ammonium molybdate into MoO3.

[0022] S3. Mo2C / ZrO2 composite electrocatalyst was prepared by pyrolysis reduction technology. After the initial heat treatment, the temperature was increased to 800 °C at a rate of 2 °C / min and maintained for 3 hours. Finally, the mixture was allowed to cool naturally to room temperature, and the black powder was collected and ground to obtain the Mo2C / ZrO2 composite electrocatalyst.

[0023] This step completely pyrolyzes UiO-66 into C / ZrO2, providing a carbon source and electronic structure control carrier for molybdenum carbide, and MoO3 undergoes a process of reduction to MoO2 and then to Mo2C.

[0024] In this step, the calcination temperature affects the chemical reaction from ammonium molybdate to molybdenum trioxide to molybdenum dioxide to molybdenum carbide, or further reduction to molybdenum nanoparticles. Molybdenum nanoparticles exhibit significantly lower HER catalytic activity than molybdenum carbide and are prone to agglomeration, reducing conductivity. Therefore, it is crucial to avoid inappropriate calcination temperatures that result in insufficient reduction to molybdenum carbide or excessive reduction to molybdenum nanoparticles. Controlling the maximum calcination time is also important, as it involves the pyrolysis of the derived porous carbon support precursor UiO-66. Therefore, extending the holding time is essential for providing sufficient carbon source; a holding time of 3 hours is more suitable compared to the 1 hour or 2 hours set in other studies.

[0025] The product obtained in step S3 was scanned by electron microscopy, such as... Figure 1 As shown. Figure 1 This indicates that the Mo2C / ZrO2 morphology obtained in step S3 retains the high specific surface area octahedral morphology of the derived porous carbon support precursor UiO-66. Because the derived porous carbon support precursor UiO-66 has the advantage of high porosity, Mo2C is distributed both inside its pores and on its surface.

[0026] X-ray diffraction analysis (700-Mo2C / ZrO2, 800-Mo2C / ZrO2, 900-Mo2C / ZrO2) was performed by adjusting the heating temperature in step S3 (700 ℃, 800 ℃, 900 ℃). Figure 2 As shown, 800 ℃ better ensures the acquisition of molybdenum carbide with higher purity. Comparison reveals that 700 ℃ is not the optimal temperature for molybdenum carbide crystalline phase synthesis, as the molybdenum carbide diffraction peaks are not obvious. At 800 ℃, typical diffraction peaks of β-Mo2C appear without other molybdenum oxides or molybdenum nanoparticle impurities, indicating stable molybdenum carbide crystal form at this temperature. At 900 ℃, both molybdenum carbide and molybdenum nanoparticles are present, indicating that excessively high temperatures lead to over-reduction of the molybdenum source. Furthermore, the sharper ZrO2 diffraction peaks indicate that high temperatures increase the size of ZrO2 particles, which is detrimental to conductivity and uniform dispersion of molybdenum carbide particles.

[0027] The mass ratio of UiO-66 to ammonium molybdate in step S2 was adjusted (8:1, 6:1, 4:1, and 2:1) to (8-Mo2C / ZrO2, 6-Mo2C / ZrO2, 4-Mo2C / ZrO2, and 2-Mo2C / ZrO2). X-ray diffraction analysis of the materials is as follows: Figure 3As shown, by comparison, it can be found that the mass ratio of carbon source to molybdenum source of 8:1 and 6:1 is not the optimal ratio for the synthesis of molybdenum carbide crystal phase. The diffraction peak of molybdenum carbide is not obvious, indicating that the molybdenum source is too small. The ratio of 4:1 shows the typical diffraction peak of β-Mo2C and has no other molybdenum oxide or molybdenum nanoparticle impurities. Molybdenum carbide with a high loading is obtained under this ratio. The ratio of 2:1 shows both molybdenum carbide and molybdenum nanoparticles, indicating that there is too much molybdenum source and not enough carbon source to participate in the reaction, resulting in some of the molybdenum source being reduced to molybdenum nanoparticles.

[0028] Figure 4 The advantages of the molybdenum carbide / zirconia composite electrocatalytic material are demonstrated, mainly including: 1. Under acidic conditions (0.5 M H2SO4), the Mo2C / ZrO2 with a heating temperature of 800℃ and an ammonium molybdate mass ratio of 4:1 exhibits the best catalytic performance, with the lowest overpotential and Tafel slope, reaching 10 mA·cm⁻¹. -2 The overpotential required for the current density is only 24 mV, and the Tafel slope is as low as 16 mV·dec. -1 The performance is close to that of commercial Pt / C, indicating that the molybdenum carbide / zirconia composite electrocatalyst exhibits excellent electrocatalytic activity and the most favorable reaction kinetics in the hydrogen evolution reaction (HER). 2. Catalytic stability: At a constant potential of -0.2 V (relative to the reversible hydrogen electrode, RHE), the Mo2C / ZrO2 composite catalyst can operate stably for over 350 hours continuously, during which the hydrogen evolution current density does not show significant decay, fully demonstrating the corrosion resistance and structural durability of this composite material in acidic environments.

[0029] Figure 5 This demonstrates the practical application value of molybdenum carbide / zirconia composite electrocatalysts. Specifically, a PEM system assembled with 4-Mo₂C / ZrO₂ (800-Mo₂C / ZrO₂) as the PEM cathode catalyst and commercial IrO₂ as the anode catalyst achieved 1 A cm⁻¹ at 1.725 V and 1.96 V, respectively. -2 and 2 A cm -2 The voltages are close to those of commercial Pt / C||IrO2 (1.671 V and 1.883 V), but the absence of precious metals in the cathode significantly reduces the cost of PEM electrolysis for hydrogen production. 4-Mo2C / ZrO2 ||IrO2 at 1 A cm⁻¹ -2 After 500 hours of stable operation with virtually no degradation, it confirms its potential to replace commercial Pt / C.

[0030] A comparative experiment was designed: changing the catalyst type, the performance of single materials Mo2C, C / ZrO2 and the composite material Mo2C / ZrO2 were compared. The comparison results showed that the formation of the Mo2C / ZrO2 composite material significantly improved the material performance.

[0031] The relevant test specifications involved in this application: The instrument used for characterizing the surface morphology of the samples using scanning electron microscopy was a Hitachi S-4800 cold field emission scanning electron microscope. This instrument was also equipped with a Genesis XM2 energy dispersive spectroscopy (EDS) instrument, manufactured by EDAX, for analyzing the composition and elemental distribution of the samples. This scanning electron microscope has a magnification range of 30-2Kx in low-magnification mode and 100-800Kx in high-magnification mode; the accelerating voltage range is 0.5-30kV, with an acceleration of ≥0.1kV, and adjustable resolutions of 1.0nm and 1.4nm. The parameters set for characterizing the surface morphology of CoO samples were: accelerating voltage of 5.0kV, normal current, and an electron gun distance of approximately 8mm from the sample.

[0032] Phase analysis of the samples was performed using an X-ray diffractometer, model D8advance, manufactured by Bruker Corporation. The parameters set for X-ray diffraction analysis in this invention were: scanning range of 20°–80°, and scanning speed of 2° / min. -1 The positioning rate is 1500°min. -1 The step size is 0.0001°.

[0033] The electrocatalytic activity was evaluated by coating the prepared Mo2C / ZrO2 composite electrocatalyst with the best performance onto a glassy carbon electrode (or carbon paper) as the working electrode, with a platinum sheet as the counter electrode and a saturated calomel electrode as the reference electrode, forming a three-electrode system in 0.5 MH2SO4 electrolyte, and electrolysis was performed at a constant potential (e.g. -0.2 V vs. RHE) applied at an electrochemical workstation.

[0034] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for preparing a molybdenum carbide / zirconia composite electrocatalyst, characterized in that, Includes the following steps: S1. The derivative porous carbon support precursor UiO-66 was prepared by hydrothermal method; S2. The derived porous carbon support precursor UiO-66 and water-soluble molybdate are calcined at a mass ratio of (3-5):1 under vacuum or inert atmosphere at a temperature of 2-5℃ / min to 500-900℃, and the temperature is maintained for 4-6 hours to obtain the composite electrocatalyst.

2. The preparation method of the molybdenum carbide / zirconia composite electrocatalyst according to claim 1, characterized in that, Step S1 includes: S11. Dissolve ZrCl4 and terephthalic acid in N,N-dimethylformamide solution, disperse by ultrasonication, add glacial acetic acid, and stir for 1 h; the molar ratio of ZrCl4 to terephthalic acid is 1:(1-2). S12. Take out the above solution and add it to a hydrothermal reactor. React at 120°C for 16 h. S13. After the reaction is complete, cool to room temperature, wash three times each with N,N-dimethylformamide and ethanol by centrifugation, dry, collect the white powder and grind it to obtain the precursor.

3. The preparation method of the molybdenum carbide / zirconia composite electrocatalyst according to claim 1, characterized in that, Step S2 includes: S21. The derived porous carbon support precursor UiO-66 and water-soluble molybdate are added to a pure water system. S22. The mixed solution is ultrasonically stirred for 1 hour, then placed in an oven to dry and the mixed powder is collected. S23. The mixed powder is placed in a porcelain boat, which is placed in the middle of the vacuum tube furnace chamber, and the furnace opening is sealed. S24. Use argon gas to flush the furnace tubes and maintain a vacuum inside the furnace tubes; S25. Heating tube furnace, first heat from room temperature to 500 ℃ at 2℃ / min, and hold at that temperature for 2h; S26. After the heat preservation is completed, continue to increase the temperature to 800 ℃ at a rate of 2℃ / min and keep it warm for 3 hours; S27. After the heat preservation is completed, cool to room temperature, open the air inlet and purge with argon to atmospheric pressure, open the two closed ends, take out the ceramic boat containing the Mo2C / ZrO2 composite electrocatalyst, and obtain a black powder product. Grind to obtain the composite electrocatalyst.

4. The preparation method of the molybdenum carbide / zirconia composite electrocatalyst according to claim 3, characterized in that, During argon purging, the argon gas flow rate is 40-60 sccm, and the purging time is 8-12 min.

5. The method for preparing the molybdenum carbide / zirconia composite electrocatalyst according to claim 1, characterized in that, The water-soluble molybdate is ammonium molybdate.

6. A molybdenum carbide / zirconia composite electrocatalyst prepared by the method according to any one of claims 1-5.

7. The application of the molybdenum carbide / zirconia composite electrocatalyst according to claim 6 in the total water splitting for hydrogen production.

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