Cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon, preparation method and application thereof

Abstract

This application belongs to the technical field of electrocatalytic hydrogen evolution, and specifically relates to a cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon, its preparation method and application. The cobalt-molybdenum-based heterostructure composite material includes nitrogen-doped porous carbon and Co crystals, Mo2C crystals, and Co3Mo3N crystals supported on the nitrogen-doped porous carbon. The content of Co in the cobalt-molybdenum-based heterostructure composite material is 2 wt% - 8 wt%; the content of Mo in the cobalt-molybdenum-based heterostructure composite material is 10 wt% - 30 wt%. The present invention uses the nitrogen-doped porous carbon structure as a carrier to provide enhanced attachment sites for the mass transfer of the electrolyte and products. At the same time, the combination of the cobalt-molybdenum-based heterostructure supported on the nitrogen-doped porous carbon structure can significantly expose a large number of active sites in the electrolytic water hydrogen evolution reaction. Through experimental results, it shows that this composite material exhibits high efficiency activity and can still maintain stable activity and structure after continuously catalyzing hydrogen production for 20 hours.

Description

Technical Field

[0001] This application belongs to the technical field of electrocatalytic hydrogen evolution, and specifically relates to a cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon, its preparation method and application. Background Art

[0002] The emergence of severe challenges such as global warming and fossil fuel depletion is mainly due to the widespread use of non-renewable energy sources. Hydrogen (H2), due to its extremely high energy density, is considered the most promising clean energy carrier in the future. However, although noble metal-based catalysts (such as Pt / C) have ideal performance, their high cost limits their feasibility in large-scale practical applications. Therefore, there is an urgent need to develop electrocatalytic hydrogen evolution catalysts that are both efficient, stable, and low-cost to promote the widespread application of hydrogen energy. Summary of the Invention

[0003] The purpose of the present invention is to solve the deficiencies of the prior art and provide a cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon, its preparation method and application. Specifically, the following technical solutions are adopted:

[0004] In the first aspect, the present invention provides a cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon. The cobalt-molybdenum-based heterostructure composite material includes nitrogen-doped porous carbon and Co crystals, Mo2C crystals, and Co3Mo3N crystals supported on the nitrogen-doped porous carbon. The content of Co in the cobalt-molybdenum-based heterostructure composite material is 2 wt%-8 wt%; the content of Mo in the cobalt-molybdenum-based heterostructure composite material is 10 wt%-30 wt%.

[0005] By loading and constructing a heterostructure of multiple transition metals (Co, Mo) and their compounds on porous carbon, the present invention utilizes the synergistic effect generated by the interaction between the heterogenous components to enhance the ability of the material to dissociate water molecules and adsorb / desorb hydrogen species, and can effectively improve its hydrogen evolution reaction (HER) performance. Thus, it solves the problems of easy aggregation, insufficient exposure of active sites, and poor conductivity in the catalytic process of single transition metal compounds in the prior art.

[0006] By loading and constructing a heterostructure of multiple transition metals (Co, Mo) and their compounds on porous carbon, in which the crystalline phase interfaces with ternary hydrogen evolution activity combine to form nanoparticle heterojunctions, which can more efficiently promote charge transfer, thereby enhancing the dissociation of water molecules by metal sites and the ability to adsorb / desorb hydrogen species; while nitrogen-doped porous carbon provides a good and stable conductive support for the dispersed ternary heterojunctions, and also facilitates the exposure of more active sites. Therefore, this composite material can effectively improve its hydrogen evolution reaction (HER) performance.

[0007] As a further preferred embodiment, in the cobalt-molybdenum-based heterostructure composite material, heterostructure nanoparticles are formed by Co crystallization, Mo2C crystallization, and Co3Mo3N crystallization, and the particle size of the heterostructure nanoparticles is 5 nm to 20 nm.

[0008] As a further preferred embodiment, the lattice distances in the cobalt-molybdenum-based heterostructure composite material correspond to Co(111), Mo2C(101), and Co3Mo3N(331). In the present invention, the crystal planes of these three crystalline phases, Co(111), Mo2C(101), and Co3Mo3N(331), are all active crystal planes for the hydrogen evolution reaction. Combining them to form nanoparticles at the nanoscale can significantly enhance charge transfer, reduce the overpotential of the hydrogen evolution reaction, and thus improve the catalytic efficiency.

[0009] In a second aspect, the present invention provides a method for preparing the above-mentioned cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon, comprising the following steps:

[0010] Drop a mixed solution of urea and anhydrous citric acid into an ethanol solution containing cobalt salt and molybdenum salt, and stir to form a uniform sol; then heat and stir the sol to form a gel, and dry it to obtain a composite precursor powder;

[0011] Place the composite precursor powder in a tubular furnace, maintain a flowing hydrogen / argon mixed atmosphere, and perform high-temperature annealing treatment to obtain the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon.

[0012] In the present invention, a uniform Co, Mo complex sol is prepared by a liquid phase method as a precursor, and then the Co, Mo complex sol is heated and stirred at a constant temperature, causing the sol to evaporate to form a Co, Mo compound gel, which is dried as a composite precursor. Finally, a cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon is obtained through a high-temperature annealing reaction.

[0013] As a further preferred embodiment, the volume ratio of the mixed solution of urea and anhydrous citric acid to the ethanol solution containing cobalt salt and molybdenum salt is 2:15. In the present invention, if this ratio is too high, the metal content of the cobalt-molybdenum-based heterostructure in the composite material will decrease; if the ratio is too low, the cobalt-molybdenum-based heterostructure nanoparticles in the composite material will aggregate, both of which will lead to a decrease in the hydrogen evolution catalytic activity.

[0014] As a further preferred embodiment, the mass concentration of urea is 350 mg·mL -1 ~ 400 mg·mL -1 and the mass concentration of anhydrous citric acid is 40 mg·mL -1 ~ 50 mg·mL -1。During the preparation process of the present invention, the concentrations of urea and anhydrous citric acid are crucial for the formation of nitrogen-doped porous carbon in the composite material; if the concentration is too low, it cannot provide the conductive support for the formation of cobalt-molybdenum-based heterostructure nanoparticles; if the concentration is too high, excessive formation of nitrogen-doped porous carbon will cause a reduction in the proportion of cobalt-molybdenum-based heterostructure nanoparticles, reducing the active sites for the hydrogen evolution reaction.

[0015] As a further preferred embodiment, the cobalt salt is a CoCl2·6H2O solution; the molybdenum salt is MoCl5; the mass concentration of the CoCl2·6H2O solution is 0.90 mg·mL -1 ~ 1.00 mg·mL -1 and the mass concentration of the MoCl5 is 4.00 mg·mL -1 ~ 5.00 mg·mL -1 。During the preparation process of the present invention, the concentration of the metal salt within the above range is conducive to the controllable formation of ternary metal crystalline phase nanoparticle heterojunctions inside the composite material.

[0016] As a further preferred embodiment, the temperature of the heating and stirring is 65 °C to 85 °C, and the time of the heating and stirring is 1 h to 3 h; the temperature of the drying is 90 °C to 120 °C, and it is maintained for 8 h to 12 h. During the preparation process of the present invention, heating for too long a time and at too high a temperature will cause the structure of the composite material precursor to be damaged, and it will be difficult to form the cobalt-molybdenum-based heterostructure composite material on the nitrogen-doped porous carbon subsequently; while heating and stirring for too short a time and at too low a temperature will excessively prolong the formation time of the precursor.

[0017] As a further preferred embodiment, the specific parameters of the high-temperature annealing treatment are as follows:

[0018] Raise the temperature from room temperature to 400 °C at a heating rate of 5 °C·min -1 , then maintain it for 1 h to 3 h, and then raise the temperature to 900 °C to 1100 °C at a heating rate of 5 °C·min -1 , then maintain it for 1 h to 3 h, and finally cool naturally to room temperature.

[0019] Thirdly, the present invention provides the application of the above cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon as an electrocatalyst in the hydrogen evolution reaction of electrolytic water.

[0020] The beneficial effects of the present invention are as follows:

[0021] (1) The present invention uses a nitrogen-doped porous carbon structure as a carrier, which provides enhanced attachment sites for the mass transfer of the electrolyte and products. At the same time, the cobalt-molybdenum-based heterostructure loaded on the nitrogen-doped porous carbon structure can significantly expose a large number of active sites in the electrocatalytic hydrogen evolution reaction. This composite material shows high efficiency, especially when generating 10 mA cm -2 current density, only requiring a very low overpotential (83 mV) and a very small Tafel slope (56.5 mV dec -1 ). In addition, after continuous catalytic hydrogen production for 20 hours, the composite material can still maintain stable activity and structure.

[0022] (2) The preparation method provided by the present invention is simple, highly reproducible, and uses inexpensive and easily available raw materials. The cobalt-molybdenum heterostructure hydrogen evolution electrocatalytic composite material obtained by this method shows good conductivity due to the loading of active substances on nitrogen-doped porous carbon. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0024] Figure 1 The figure shows the XRD pattern of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon.

[0025] Figure 2 The figure shows the EDX-Mapping pattern of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon.

[0026] Figure 3 The figure shows the SEM image of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon.

[0027] Figure 4 The figure shows the TEM image of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon.

[0028] Figure 5 The figure shows the linear voltammetry polarization curve of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon as an electrocatalyst in 1M KOH.

[0029] Figure 6 The figure shows the Tafel slope curve of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon as an electrocatalyst.

[0030] Figure 7Shown is the chronopotentiometry curve of the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon as an electrocatalyst tested at a constant current density of 10 mA·cm –2 for 20 h.

[0031] Figure 8 Shown is the SEM image of the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon as an electrocatalyst after 20 h of chronopotentiometric stability testing. Detailed implementation manners

[0032] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0033] The crystal structure of the product prepared by the present invention is determined by an X-ray diffractometer (XRD), the morphology of the material is determined by a scanning electron microscope (SEM) and a transmission electron microscope (TEM), the elemental composition is determined by energy-dispersive X-ray spectroscopy imaging (EDX-mapping), and the catalytic activity of electrolytic water hydrogen evolution is measured on a Shanghai Chenhua electrochemical workstation.

[0034] Example 1

[0035] A preparation method of a cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon specifically includes the following steps:

[0036] (1) Prepare a 10 mL deionized water solution containing 3.8 g of urea and 0.4572 g of anhydrous citric acid, add it to a 75 mL ethanol solution containing 0.328 g of CoCl2·6H2O and 0.071 g of MoCl5 mixed metal salts, stir for 10 min to form a homogeneous sol; then place the sol on a heating magnetic stirrer and continue stirring, and volatilize it at 75 °C with an open mouth for 1.5 h until a gel is formed; place the gel in an oven and dry it at 100 °C for 10 h to form a composite material precursor;

[0037] (2) Weigh 1.5 g of the composite material precursor and place it in the center of a tubular furnace, maintain a flowing hydrogen / argon mixed atmosphere, and set a temperature program for high-temperature annealing treatment. The program for high-temperature annealing treatment is as follows: increase the temperature from room temperature to 400 °C at a heating rate of 5 °C·min -1 , then hold for 2 h, and then increase the temperature to 5 °C·min -1The heating rate is increased from 400 °C to 1000 °C, then maintained for 2 h, and finally cooled naturally, thus obtaining the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon.

[0038] The cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon prepared above was subjected to characterization performance tests (XRD, EDX-mapping, SEM, TEM), and the specific results are as Figures 1-4 shown:

[0039] As revealed by Figure 1 X-ray powder diffraction (XRD) analysis in, the obtained cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon contains Co, Mo2C, and Co3Mo3N crystals; the energy-dispersive X-ray spectrometer imaging map (EDX-mapping, see Figure 2 ) analysis proves that elements such as Co, Mo, N, and C are evenly distributed in the composite material, and Co and Mo elements show granular aggregation; the scanning electron microscope photograph (see Figure 3 ) shows that the composite material is assembled from a large number of crystal particles and a nitrogen-doped porous carbon substrate, and its surface is rich in rough structures; the transmission electron microscope photograph (see Figure 4 ) shows that a large number of Co / Mo2C / Co3Mo3N crystal particles are supported on nitrogen-doped porous carbon, constituting a heterocomposite material.

[0040] As can be seen from the above, in the present invention, a heterostructure of multiple transition metals (Co, Mo) and their compounds is constructed by loading on porous carbon, and the crystalline phase interface of the ternary hydrogen evolution activity forms a nanoparticle heterojunction, which can more efficiently promote charge transfer, and then enhance the dissociation of metal sites on water molecules and the ability to adsorb / desorb hydrogen species; while the nitrogen-doped porous carbon provides a good and stable conductive support for the dispersed ternary heterojunction, and at the same time is also conducive to the exposure of more active sites. Therefore, this composite material can effectively improve its hydrogen evolution reaction (HER) performance.

[0041] Example 2

[0042] Performance test of the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon as an electrolytic water hydrogen evolution reaction catalyst, and the specific process is as follows:

[0043] (1) Preparation of the electrocatalyst working electrode:

[0044] 8 mg of cobalt-molybdenum-based heterostructure composite powder loaded on nitrogen-doped porous carbon, 2 mg of Super P, and 17 μL of 10 wt% PTFE solution were mixed. During this process, a small amount of ethanol was added and continuously ground for 30 min until a uniform catalyst film was formed; the catalyst film was evenly pasted on the surface of nickel foam, and the pasting area was 3 cm −2 , vacuum dried at 60 °C for 12 h, and a pressure of 7 Mpa was applied using a tablet press for 3 min. The catalyst pasting area was cut to 0.5 cm −2 of nickel foam as the working electrode.

[0045] (2) Electrochemical performance study:

[0046] The electrochemical characteristics of the prepared samples were tested on a CHI 760E electrochemical workstation (Chenhua Instruments, Shanghai, China). A traditional three-electrode system was used, that is, a graphite electrode as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and nickel foam modified with the catalyst as the working electrode. 1 mol·L -1 KOH aqueous solution was used as the supporting electrolyte. Unless otherwise specified, all potentials in the test were converted to the reversible hydrogen electrode potential according to the Nernst equation ( E RHE ). The overpotential η can be further obtained by the following formula: η = -E RHE .

[0047] The above test results are as Figures 5-8 shown:

[0048] Figure 5 is the linear voltammetric scanning polarization curve of the cobalt-molybdenum-based heterostructure composite loaded on nitrogen-doped porous carbon as an electrocatalyst in 1 mol·L -1 KOH. The figure shows that the composite material has good electrocatalytic activity, and its initial potential for electrocatalytic water splitting hydrogen evolution reaction (defined as the overpotential to obtain a current density of 1 mA·cm -2 ) is only 8 mV, while only 83 mV overpotential is required to generate a reference current density of 10 mA·cm -2 (equivalent to the current density generated by a solar water splitting device with an efficiency of 12.3%).

[0049] Figure 6 is the Tafel slope curve of the cobalt-molybdenum-based heterostructure composite loaded on nitrogen-doped porous carbon as an electrocatalyst. The figure shows that the composite material has a low Tafel slope (only 56.5 mV·dec -1 ), indicating that as the overpotential increases, the hydrogen evolution reaction rate will increase sharply.

[0050] Figure 7 The chronopotentiometry curve of the cobalt-molybdenum-based heterostructure composite material loaded on nitrogen-doped porous carbon as an electrocatalyst at a current density of 10 mA·cm -2 During the continuous test for 20 hours at a constant current density of (10 mA·cm -2 ), only an overpotential not exceeding 118 mV needs to be applied;

[0051] Figure 8 This is a scanning electron microscope photograph of the composite material as an electrocatalyst after 20 h of chronopotentiometry stability test, showing that it still maintains a morphology similar to the initial state. These reflect that the material has high electrocatalytic activity and structural stability in alkaline electrolyte.

[0052] The embodiments of the present application have been described above in conjunction with the accompanying drawings. Specific examples are used herein to elaborate the principles and implementation manners of the present application. The description of the above embodiments is only used to help understand the core idea of the present application. However, the present application is not limited to the above specific implementation manners. The above specific implementation manners are only illustrative and not restrictive. Under the inspiration of the present application, those of ordinary skill in the art can also make many forms without departing from the purpose of the present application and the scope protected by the claims, and all belong to the protection scope of the present application.

Claims

1. A cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon, characterized in that: The cobalt-molybdenum-based heterostructure composite material comprises nitrogen-doped porous carbon and Co crystals, Mo2C crystals, and Co3Mo3N crystals supported on the nitrogen-doped porous carbon, wherein the content of Co in the cobalt-molybdenum-based heterostructure composite material is 2 wt%-8 wt%; the content of Mo in the cobalt-molybdenum-based heterostructure composite material is 10 wt%-30 wt%; In the cobalt-molybdenum-based heterostructure composite material, heterostructure nanoparticles are formed by Co crystals, Mo2C crystals and Co3Mo3N crystals.

2. The cobalt-molybdenum-based heterostructure composite material according to claim 1, characterized in that: The particle size of heterostructured nanoparticles is 5 nm ~ 20 nm.

3. The cobalt-molybdenum-based heterostructure composite material according to claim 1, characterized in that: The lattice distances in the cobalt-molybdenum-based heterostructure composite material correspond to Co (111), Mo2C (101) and Co3Mo3N (331).

4. The method for preparing the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon according to any one of claims 1 to 3, characterized in that: The following steps are involved: The mixed solution of urea and anhydrous citric acid is dripped into the ethanol solution containing cobalt salt and molybdenum salt, and stirred to form a uniform sol; then the sol is heated and stirred to form a gel, and dried to obtain a composite material precursor powder; The composite material precursor powder is placed in a tube furnace, maintained in a flowing hydrogen / argon mixed atmosphere, and subjected to high-temperature annealing treatment to obtain the cobalt-molybdenum-based heterogeneous structure composite material supported on nitrogen-doped porous carbon.

5. The preparation method according to claim 4, characterized in that: The volume ratio of the mixed solution of urea and anhydrous citric acid to the ethanol solution containing cobalt salt and molybdenum salt is 2:

15.

6. The preparation method according to claim 5, characterized in that: The urea mass concentration is 350 mg·mL -1 ~400 mg mL -1 The mass concentration of anhydrous citric acid is 40 mg·mL -1 ~ 50 mg mL -1 .

7. The preparation method according to claim 5, characterized in that: The cobalt salt is CoCl2·6H2O; the molybdenum salt is MoCl5; the mass concentration of the CoCl2·6H2O is 0.90 mg·mL -1 ~ 1.00 mg mL -1 The mass concentration of MoCl5 is 4.00 mg·mL -1 ~ 5.00 mg mL -1 .

8. The preparation method according to claim 4, wherein the heating and stirring temperature is 65°C to 85°C, and the heating and stirring time is 1 h to 3 h; the drying temperature is 90°C to 120°C, and is maintained for 8 h to 12 h.

9. The preparation method according to claim 4, characterized in that: The specific parameters of the high temperature annealing treatment are as follows: At 5 ℃·min -1 The heating rate was increased from room temperature to 400 °C, then maintained for 1 h to 3 h, and then increased at 5 °C min -1 The heating rate was increased to 900 ℃~1100 ℃, then maintained for 1 h~3 h, and finally cooled naturally to room temperature.

10. An application of the cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon according to any one of claims 1 to 3, characterized in that: The cobalt-molybdenum-based heterostructure composite material supported on nitrogen-doped porous carbon is used as an electrocatalyst for electrolyzing water to generate hydrogen.

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