Nitrogen-doped carbon quantum dot / Co2V2O7 composite material as well as preparation method and application thereof

Abstract

The invention provides a nitrogen-doped carbon quantum dot / Co2V2O7 composite material as well as a preparation method and application thereof. The preparation method of the composite material comprises the following steps: fully mixing and uniformly dispersing a carbon source, a nitrogen source and deionized water, performing hydrothermal reaction, filtering, and drying filtrate to obtain nitrogen-doped carbon quantum dots; the nitrogen-doped carbon quantum dots, deionized water, ammonium metavanadate, cobalt salt and hexamethylenetetramine are fully mixed and uniformly dispersed, and the nitrogen-doped carbon quantum dots are obtained through reaction, centrifugation, washing, drying and calcination. The preparation method is simple, and the obtained composite material is formed by uniformly loading nitrogen-doped carbon quantum dots on the surface of a hexagonal porous Co2V2O7 micron sheet and has relatively high specific surface area and porosity; the composite material disclosed by the invention is applied to the field of magnesium-based solid hydrogen storage, the nitrogen-doped carbon quantum dots and Co2V2O7 can generate a synergistic effect, and by combining a high specific surface area and porosity, the composite material shows excellent magnesium-based solid hydrogen storage catalytic modification performance.

Description

Technical Field

[0001] The present invention belongs to the field of composite material preparation, and specifically relates to a nitrogen-doped carbon quantum dot / Co2V2O7 composite material and a preparation method and application thereof. Background Art

[0002] Magnesium hydride (MgH2) is a solid-state hydrogen storage material that has attracted widespread attention due to its high hydrogen storage capacity, abundant reserves, and low cost. Lowering the operating temperature of MgH2 and improving the kinetics of hydrogen absorption and desorption is a key challenge, and adding catalysts through ball milling is a simple and efficient modification strategy. Current research shows that adding metal oxides can significantly improve the kinetic performance while lowering the operating temperature of MgH2.

[0003] Co2V2O7 is a transition metal oxide based on cobalt-based vanadate. Compared with single-component metal oxides, the multivalent states and synergistic effects of cobalt ions and vanadium ions bring it more excellent catalytic ability. The morphology of the catalyst has a great influence on the catalytic ability. The three-dimensional structure has the advantages of high specific surface area, good dispersibility, and not easy to agglomerate, while the porous structure can further reduce the material density and increase the specific surface area. Therefore, constructing a catalyst with a three-dimensional porous structure can bring more reactive active sites and improve the catalytic ability.

[0004] Carbon quantum dots (CQDs) are small in size, environmentally friendly, and highly catalytic, and are highly valued in the fields of energy storage and electrocatalysis. In addition, carbon quantum dots can be prepared on a large scale through a variety of simple synthesis methods such as hydrothermal, microwave-assisted, and organic pyrolysis, overcoming the high cost of traditional carbon nanomaterials. However, carbon quantum dots are small in size and are prone to agglomeration when used directly as catalysts, which affects their catalytic ability.

[0005] Therefore, it is of great significance to develop a composite material of carbon quantum dots (N-CQDs) and three-dimensional porous Co2V2O7 to increase the loading rate of carbon quantum dots, avoid agglomeration, and significantly improve its magnesium-based solid-state hydrogen storage catalytic modification performance. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a nitrogen-doped carbon quantum dot / Co2V2O7 composite material and a preparation method and application thereof to solve the current problems of high temperature and slow kinetics of MgH2 dehydrogenation. The preparation method of the present invention is simple, and the obtained nitrogen-doped carbon quantum dot / Co2V2O7 composite material is a hexagonal porous Co2V2O7 micron sheet with uniformly loaded nitrogen-doped carbon quantum dots on its surface, which has a high specific surface area and porosity; the nitrogen-doped carbon quantum dot / Co2V2O7 composite material of the present invention is applied to the field of magnesium-based solid-state hydrogen storage, and the nitrogen-doped carbon quantum dots and Co2V2O7 will produce a synergistic effect, combined with high specific surface area and porosity, showing excellent magnesium-based solid-state hydrogen storage catalytic modification performance.

[0007] The technical solution of the present invention is as follows:

[0008] A nitrogen-doped carbon quantum dot / Co2V2O7 composite material, wherein the nitrogen-doped carbon quantum dot / Co2V2O7 composite material is a hexagonal porous Co2V2O7 micron sheet with nitrogen-doped carbon quantum dots uniformly loaded on its surface.

[0009] According to the preferred embodiment of the present invention, the size of the Co2V2O7 micron sheet is 1.5-2.5 μm, the thickness is 0.1-0.25 μm, and the pore size is 5-50 nm. The size of the Co2V2O7 micron sheet refers to the length of the diagonal line of the hexagon.

[0010] The preparation method of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material comprises the following steps:

[0011] (1) fully mixing and dispersing a carbon source, a nitrogen source, and deionized water, performing a hydrothermal reaction, filtering, and drying the filtrate to obtain nitrogen-doped carbon quantum dots;

[0012] (2) The nitrogen-doped carbon quantum dots, deionized water, ammonium metavanadate, cobalt salt and hexamethylenetetramine are fully mixed and dispersed uniformly, and then reacted, centrifuged, washed, dried and calcined to obtain the nitrogen-doped carbon quantum dots / Co2V2O7 composite material.

[0013] Preferably according to the present invention, in step (1), the carbon source is one of glucose, ascorbic acid or citric acid.

[0014] Preferably according to the present invention, in step (1), the nitrogen source is one of urea, ethylenediamine or melamine.

[0015] According to the preferred embodiment of the present invention, in step (1), the molar ratio of the carbon source to the nitrogen source is 1: (1 to 5), preferably 1: (3 to 4), and more preferably 1: 3. The type and ratio of the carbon source and the nitrogen source will change the type of functional groups on the surface of the nitrogen-doped carbon quantum dots, thereby affecting their properties, such as water solubility, and further affecting the performance of the final composite material.

[0016] Preferably according to the present invention, in step (1), the mass ratio of the carbon source to deionized water is 1:(10-60).

[0017] According to the preferred embodiment of the present invention, in step (1), the temperature of the hydrothermal reaction is 150-200°C; the time of the hydrothermal reaction is 2-10 hours. Appropriate raw materials and reaction conditions are conducive to the generation of carbon quantum dots of uniform size. If the reaction temperature is low and the reaction time is short, the reaction will be insufficient, and the synthesized carbon quantum dots will be small and the impurities will be large. On the contrary, the carbon quantum dots will aggregate and precipitate, affecting the yield.

[0018] Preferably according to the present invention, in step (2), the cobalt salt is one of cobalt nitrate hexahydrate or cobalt chloride hexahydrate.

[0019] According to the preferred embodiment of the present invention, in step (2), the molar ratio of ammonium metavanadate to cobalt salt is (2.5-5):1, preferably (3.5-5):1, and more preferably 5:1; the mass ratio of nitrogen-doped carbon quantum dots, ammonium metavanadate, hexamethylenetetramine and deionized water is (0.01-0.1):(0.23-0.58):(1.4-2.24):(50-100), preferably 0.05:0.58:1.82:100. Hexamethylenetetramine is very important for the formation of hexagonal sheet structure Co2V2O7. It plays a role in regulating pH and controlling size and morphology in the reaction. If the amount of hexamethylenetetramine is too low, a uniform hexagonal sheet structure cannot be formed. If the amount is too high, the sheet structure size will be too large, affecting the catalytic performance.

[0020] According to the preferred embodiment of the present invention, in step (2), the reaction temperature is 50-90°C; the reaction time is 4-12 hours; and the reaction is carried out under stirring. A higher reaction temperature is conducive to the decomposition of ammonium metavanadate, while sufficient reaction time is conducive to the assembly of hexagonal sheet structures.

[0021] According to the preferred embodiment of the present invention, in step (2), the calcination temperature is 400-500°C; the calcination atmosphere is an inert gas; preferably, the inert gas is nitrogen or argon; and the calcination time is 1-3 hours. Calcination can remove crystal water, generate a porous structure and improve crystallinity, and calcination in an inert gas can prevent the decomposition of carbon quantum dots.

[0022] Application of the above nitrogen-doped carbon quantum dots / Co2V2O7 composite material in magnesium hydride hydrogen storage.

[0023] Preferably, according to the present invention, the application method comprises the steps of: ball-milling and mixing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material with magnesium hydride to obtain a composite material for magnesium hydride hydrogen storage.

[0024] Preferably, the mass ratio of nitrogen-doped carbon quantum dots / Co2V2O7 composite material to magnesium hydride is 1:8-20; the ball milling speed is 400-600 r / min, and the ball milling time is 2-4 h.

[0025] The technical features and beneficial effects of the present invention are as follows:

[0026] (1) The present invention first disperses the carbon source and the nitrogen source in deionized water, and the carbon source and the nitrogen source are decomposed and carbonized under high temperature and high pressure environment, and finally obtains nitrogen-doped carbon quantum dots. The surface of the nitrogen-doped carbon quantum dots contains abundant hydrophilic groups such as hydroxyl, carboxyl and carbonyl, so that they have good solubility and dispersibility in aqueous solution, which is conducive to the synthesis of nitrogen-doped carbon quantum dots / Co2V2O7 composite materials with high nitrogen-doped carbon quantum dots content and uniform dispersion. Ammonium metavanadate, cobalt salt and hexamethylenetetramine are dispersed in the nitrogen-doped carbon quantum dots aqueous solution, and the ammonium metavanadate and cobalt salt are hydrolyzed at a certain temperature and react to generate Co2V2O7, and the hexamethylenetetramine controls the formation of hexagonal sheet structures, and the nitrogen-doped carbon quantum dots are doped into Co2V2O7 by electrostatic adsorption. Subsequently, the crystal water is removed by calcination to obtain a hexagonal porous nitrogen-doped carbon quantum dots / Co2V2O7 composite material. The synthesis method of the hexagonal porous nitrogen-doped carbon quantum dots / Co2V2O7 composite material of the present invention is simple, the reaction conditions are mild, and the cost is low. The nitrogen-doped carbon quantum dots / Co2V2O7 composite material with a hexagonal porous structure can be prepared by simple hydrothermal reaction and calcination.

[0027] (2) The nitrogen-doped carbon quantum dots / Co2V2O7 composite material obtained by the present invention is a hexagonal porous Co2V2O7 micron sheet with nitrogen-doped carbon quantum dots uniformly loaded on its surface. The porous Co2V2O7 micron sheet (size is about 1.5-2.5 μm, thickness is about 0.1-0.25 μm) is uniformly loaded with nitrogen-doped carbon quantum dots on its surface, and has the characteristics of porous structure (pore size is 5-50 nm), high specific surface area, uniform distribution of nitrogen-doped carbon quantum dots and high loading rate, which is conducive to the improvement of catalytic performance.

[0028] (3) In the nitrogen-doped carbon quantum dots / Co2V2O7 composite material obtained by the present invention, the multivalent states and synergistic effects of cobalt ions and vanadium ions in Co2V2O7 improve its catalytic ability, and the hexagonal porous structure can increase the specific surface area of ​​the material, and can provide more active sites for the absorption / desorption of hydrogen by MgH2. Nitrogen doping can effectively improve the electronegativity of carbon quantum dots, and can accelerate the decomposition and transfer of hydrogen molecules on the surface of the material. Therefore, nitrogen-doped carbon quantum dots / Co2V2O7 enhance the catalytic effect on the hydrogen storage performance of MgH2 through synergistic effects. The preparation method of the present invention is taken as a whole, and each step and each condition work together to obtain the nitrogen-doped carbon quantum dots / Co2V2O7 composite material with the structure and performance of the present invention. The nitrogen-doped carbon quantum dots / Co2V2O7 composite material of the present invention is applied to the field of magnesium-based solid-state hydrogen storage. Due to the synergistic effect of nitrogen-doped carbon quantum dots and Co2V2O7, combined with the hexagonal porous sheet structure, the effect of catalyzing the absorption / desorption of hydrogen by MgH2 is enhanced, showing excellent catalytic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 XRD pattern of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material prepared in Example 1;

[0030] Figure 2 This is a SEM image of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material prepared in Example 1;

[0031] Figure 3 Isothermal hydrogen release curves of nitrogen-doped carbon quantum dots / Co2V2O7 modified magnesium hydride and unmodified magnesium hydride prepared in Example 1;

[0032] Figure 4 This is an infrared spectrum of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material prepared in Example 2;

[0033] Figure 5 The isothermal hydrogen absorption curves of the nitrogen-doped carbon quantum dots / Co2V2O7 modified magnesium hydride and the unmodified magnesium hydride prepared in Example 2;

[0034] Figure 6 The temperature-dependent hydrogen release curves of the nitrogen-doped carbon quantum dots / Co2V2O7 modified magnesium hydride and the unmodified magnesium hydride prepared in Example 3;

[0035] Figure 7 TEM image of nitrogen-doped carbon quantum dots / Co2V2O7 modified magnesium hydride prepared in Example 3;

[0036] Figure 8 The temperature-dependent hydrogen release curves of the nitrogen-doped carbon quantum dots / Co2V2O7 modified magnesium hydride and the unmodified magnesium hydride prepared in Example 4;

[0037] Fig. 9 XRD pattern of hexagonal porous Co2V2O7 prepared in Comparative Example 1;

[0038] Fig.10 The temperature-dependent hydrogen release curves of the Co2V2O7-modified magnesium hydride and the unmodified magnesium hydride prepared in Comparative Example 2;

[0039] Fig.11 This is the XRD pattern of nitrogen-doped carbon quantum dots prepared in Comparative Example 2;

[0040] Fig.12 This is an infrared spectrum of nitrogen-doped carbon quantum dots prepared in Comparative Example 2;

[0041] Fig.13 This is a temperature-dependent hydrogen release curve of the nitrogen-doped carbon quantum dot-modified magnesium hydride and the unmodified magnesium hydride prepared in Comparative Example 2. DETAILED DESCRIPTION

[0042] The present invention is further described below with reference to specific embodiments, but is not limited thereto.

[0043] Meanwhile, the experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0044] Example 1

[0045] A method for preparing a nitrogen-doped carbon quantum dot / Co2V2O7 composite material comprises the following steps:

[0046] (1) 1.59 g (0.009 mol) of ascorbic acid and 1.62 g (0.027 mol) of urea were dissolved in 50 mL of deionized water and subjected to ultrasonic treatment for 30 min. The mixture was hydrothermally reacted at 190 °C for 6 h. After the reaction, the supernatant was collected by centrifugation, filtered through a 0.22 μm microporous filter membrane, and the filtrate was dried to obtain nitrogen-doped carbon quantum dots.

[0047] (2) 0.04 g of nitrogen-doped carbon quantum dots, 0.23 g (0.002 mol) of ammonium metavanadate and 0.12 g (0.0005 mol) of cobalt chloride hexahydrate were added to 50 mL of deionized water and ultrasonicated for 20 min. The mixture was then heated in a water bath at 75 °C and 1.4 g of hexamethylenetetramine was added under stirring. The mixture was stirred in a constant temperature water bath for 5 h. After the reaction was completed, the precipitate was collected by centrifugation and washed and dried to obtain a hexagonal flaky nitrogen-doped carbon quantum dots / Co2V2O7·nH2O composite material.

[0048] (3) The composite material obtained in the previous step was calcined at 450°C in a nitrogen atmosphere for 2 h to obtain a nitrogen-doped carbon quantum dot / Co2V2O7 composite material.

[0049] like Figure 1 Shown is the XRD pattern of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material prepared in this example. It can be seen from the figure that the diffraction peak of the composite material is consistent with the standard card of Co2V2O7 (PDF#70-1189), and there are no diffraction peaks of other phases, which proves that the Co2V2O7 synthesized by this method has a high purity. Due to the low content of nitrogen-doped carbon quantum dots or the unclear diffraction peaks, no diffraction peaks belonging to nitrogen-doped carbon quantum dots were observed.

[0050] like Figure 2 The SEM image of the nitrogen-doped carbon quantum dot / Co2V2O7 composite material prepared in this embodiment is shown. It can be seen from the figure that the composite material has a hexagonal porous sheet structure with a size (hexagonal diagonal length) of about 1.5-2.5μm, a thickness of about 0.1-0.25μm, and a pore size of 5-50nm. Since the size of carbon quantum dots is generally below 10nm, the presence of carbon quantum dots was not observed.

[0051] The application of the above nitrogen-doped carbon quantum dots / Co2V2O7 composite material in magnesium hydride hydrogen storage is as follows:

[0052] 0.07g of nitrogen-doped carbon quantum dots / Co2V2O7 composite material was ball-milled with 0.93g of commercial magnesium hydride at room temperature, with a rotation speed of 450r / min and a ball-milling time of 4h. The composite sample was subjected to isothermal dehydrogenation and variable temperature dehydrogenation performance tests. The isothermal dehydrogenation performance test was as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was increased from room temperature to 300℃ at a heating rate of 5℃ / min. The sample was kept at 300℃ for 30min, and the amount of hydrogen released during the 300℃ insulation process was measured and drawn into an isothermal dehydrogenation curve. The obtained performance was compared with that of unmodified magnesium hydride (also ball-milled under the above conditions). The variable temperature dehydrogenation test conditions were as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was increased from room temperature to 450℃ at a heating rate of 5℃ / min. The amount of hydrogen released during this process was measured.

[0053] According to the isothermal hydrogen desorption test, the magnesium hydride modified by nitrogen-doped carbon quantum dots / Co2V2O7 catalysis can significantly increase the hydrogen desorption rate and improve the hydrogen desorption capacity. Figure 3As shown, the unmodified magnesium hydride (ball-milled MgH2) releases almost no hydrogen at 300°C, and can only release 0.64wt% of hydrogen within 30 minutes, with a hydrogen release rate of 8.42%. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), 6.76wt% of hydrogen can be released within 5 minutes at 300°C, and the hydrogen release rate is 95.6%, which is a significant improvement. After the variable temperature hydrogen release test, the magnesium hydride catalytically modified by nitrogen-doped carbon quantum dots / Co2V2O7 prepared in this embodiment can significantly reduce the initial hydrogen release temperature and increase the hydrogen release capacity, but the modification effect is worse than that of Example 3. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), the initial hydrogen release temperature is reduced to 192°C, and the hydrogen release rate is 96.2%.

[0054] Example 2

[0055] A method for preparing a nitrogen-doped carbon quantum dot / Co2V2O7 composite material comprises the following steps:

[0056] (1) 0.96 g (0.005 mol) of citric acid and 1.20 g (0.02 mol) of ethylenediamine were dissolved in 50 mL of deionized water, and subjected to ultrasonic treatment for 30 min. The reaction was hydrothermally reacted at 170 °C for 7 h. After the reaction was completed, the supernatant was collected by centrifugation, filtered through a 0.22 μm microporous filter membrane, and the filtrate was dried to obtain nitrogen-doped carbon quantum dots;

[0057] (2) 0.06 g of nitrogen-doped carbon quantum dots, 0.33 g (0.0028 mol) of ammonium metavanadate and 0.23 g (0.0008 mol) of cobalt nitrate hexahydrate were added to 80 mL of deionized water and ultrasonicated for 20 min, then heated in a water bath at 70 ° C, 2.24 g of hexamethylenetetramine was added under stirring conditions, and the reaction was continued in a constant temperature water bath with stirring for 6 h. After the reaction was completed, the precipitate was collected by centrifugation, washed and dried to obtain a hexagonal flaky nitrogen-doped carbon quantum dots / Co2V2O7·nH2O composite material;

[0058] (3) The composite material obtained in the previous step was calcined at 400°C in a nitrogen atmosphere for 2.5 h to obtain a nitrogen-doped carbon quantum dot / Co2V2O7 composite material.

[0059] like Figure 4 The infrared spectrum of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material prepared in this example is shown. It can be seen from the figure that at 1405 cm -1 The absorption peak is the bending vibration of the CN bond, corresponding to the presence of nitrogen-doped carbon quantum dots, located at 1000-500cm -1The strong peaks at are the VO bonds and Co-O bonds in Co2V2O7, corresponding to the existence of Co2V2O7. Therefore, the above test results prove the successful preparation of nitrogen-doped carbon quantum dots / Co2V2O7 composite materials.

[0060] The application of the above nitrogen-doped carbon quantum dots / Co2V2O7 composite material in magnesium hydride hydrogen storage is as follows:

[0061] 0.07g of nitrogen-doped carbon quantum dots / Co2V2O7 composite material was ball-milled with 0.93g of commercial magnesium hydride at room temperature, with a rotation speed of 450r / min and a ball-milling time of 4h. The isothermal hydrogen absorption and variable temperature hydrogen release performance tests were performed on the obtained composite sample. The isothermal hydrogen absorption performance test was as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was increased from room temperature to 200℃ at a heating rate of 5℃ / min. A hydrogen pressure of 4MPa was provided at 200℃ and kept warm for 1h. The amount of hydrogen absorbed during the 200℃ insulation process was measured and an isothermal hydrogen absorption curve was drawn. The obtained performance was compared with that of unmodified magnesium hydride (also ball-milled under the above conditions). The variable temperature hydrogen release test conditions were as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was increased from room temperature to 450℃ at a heating rate of 5℃ / min, and the amount of hydrogen released during this process was measured.

[0062] According to the isothermal hydrogen absorption test, the magnesium hydride modified by nitrogen-doped carbon quantum dots / Co2V2O7 catalysis can significantly increase the hydrogen absorption speed and capacity. Figure 5 As shown, unmodified magnesium hydride (ball-milled MgH2) can only absorb 4.91wt% of hydrogen within 60min at 200°C, and the hydrogen absorption rate is 64.6%. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), 6.60wt% of hydrogen can be absorbed within 1min at 200°C, and the hydrogen absorption rate is 93.4%, which is a significant improvement. According to the variable temperature hydrogen release test, the magnesium hydride catalytically modified by nitrogen-doped carbon quantum dots / Co2V2O7 prepared in this embodiment can significantly reduce the initial hydrogen release temperature and increase the hydrogen release capacity, but the modification effect is worse than that of Example 3. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), the initial hydrogen release temperature is reduced to 208°C, and the hydrogen release rate is 95.9%.

[0063] Example 3

[0064] A method for preparing a nitrogen-doped carbon quantum dot / Co2V2O7 composite material comprises the following steps:

[0065] (1) 1.73 g (0.009 mol) of citric acid and 1.62 g (0.027 mol) of urea were dissolved in 30 mL of deionized water, and subjected to ultrasonic treatment for 30 min. The mixture was subjected to hydrothermal reaction at 180 °C for 8 h. After the reaction, the supernatant was collected by centrifugation, filtered through a 0.22 μm microporous membrane, and the filtrate was dried to obtain nitrogen-doped carbon quantum dots.

[0066] (2) 0.05 g of nitrogen-doped carbon quantum dots, 0.58 g (0.005 mol) of ammonium metavanadate and 0.24 g (0.001 mol) of cobalt chloride hexahydrate were added to 100 mL of deionized water and ultrasonicated for 20 min, then heated in a water bath at 80 °C, 1.82 g of hexamethylenetetramine was added under stirring conditions, and the reaction was continued in a constant temperature water bath with stirring for 4 h. After the reaction was completed, the precipitate was collected by centrifugation, washed and dried to obtain a hexagonal flaky nitrogen-doped carbon quantum dots / Co2V2O7·nH2O composite material;

[0067] (3) The composite material obtained in the previous step was calcined at 500°C in a nitrogen atmosphere for 2 h to obtain a nitrogen-doped carbon quantum dot / Co2V2O7 composite material.

[0068] The application of the above nitrogen-doped carbon quantum dots / Co2V2O7 composite material in magnesium hydride hydrogen storage is as follows:

[0069] 0.07g of nitrogen-doped carbon quantum dots / Co2V2O7 composite material was ball-milled with 0.93g of commercial magnesium hydride at room temperature, with a rotation speed of 450r / min and a ball-milling time of 4h. The obtained composite sample was subjected to variable temperature hydrogen release and isothermal hydrogen absorption performance tests. The variable temperature hydrogen release test conditions are as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was increased from room temperature to 450℃ at a heating rate of 5℃ / min. The amount of hydrogen released during this process was measured and plotted as a variable temperature hydrogen release curve, and the obtained performance was compared with that of unmodified magnesium hydride (also ball-milled under the above conditions). The isothermal hydrogen absorption test conditions are as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was increased from room temperature to 200℃ at a heating rate of 5℃ / min. A hydrogen pressure of 4MPa was provided at 200℃ and kept warm for 1h, and the amount of hydrogen absorbed during the 200℃ insulation process was measured.

[0070] The variable temperature hydrogen desorption test showed that the magnesium hydride modified by nitrogen-doped carbon quantum dots / Co2V2O7 catalysis can significantly reduce the initial hydrogen desorption temperature and increase the hydrogen desorption capacity. Figure 6As shown, the initial dehydrogenation temperature of unmodified magnesium hydride (ball-milled MgH2) is 310°C, and the dehydrogenation rate is 95.2%. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), the initial dehydrogenation temperature is reduced to 184°C, and the dehydrogenation rate is 96.3%, which is significantly improved. According to the isothermal hydrogen absorption test, magnesium hydride catalytically modified with nitrogen-doped carbon quantum dots / Co2V2O7 can significantly increase the hydrogen absorption rate and hydrogen absorption capacity. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), 6.65wt% of hydrogen can be absorbed within 1 minute at 200°C, and the hydrogen absorption rate is 94.1%, which is a significant improvement.

[0071] like Figure 7 Shown is a TEM image of nitrogen-doped carbon quantum dots / Co2V2O7 modified magnesium hydride prepared in this example. It can be seen from the image that the size of the composite material is about 400-500nm.

[0072] Example 4

[0073] A method for preparing a nitrogen-doped carbon quantum dot / Co2V2O7 composite material comprises the following steps:

[0074] (1) 1.73 g (0.009 mol) of citric acid and 1.62 g (0.027 mol) of urea were dissolved in 30 mL of deionized water, and subjected to ultrasonic treatment for 30 min. The mixture was subjected to hydrothermal reaction at 180 °C for 8 h. After the reaction, the supernatant was collected by centrifugation, filtered through a 0.22 μm microporous membrane, and the filtrate was dried to obtain nitrogen-doped carbon quantum dots.

[0075] (2) 0.01 g of nitrogen-doped carbon quantum dots, 0.58 g (0.005 mol) of ammonium metavanadate and 0.24 g (0.001 mol) of cobalt chloride hexahydrate were added to 100 mL of deionized water and subjected to ultrasonic treatment for 20 min. The mixture was then heated in a water bath at 80 °C and 1.82 g of hexamethylenetetramine was added under stirring conditions. The mixture was stirred in a constant temperature water bath for 4 h. After the reaction was completed, the precipitate was collected by centrifugation, washed and dried to obtain a hexagonal flaky nitrogen-doped carbon quantum dots / Co2V2O7·nH2O composite material.

[0076] (3) The composite material obtained in the previous step was calcined at 500°C in a nitrogen atmosphere for 2 h to obtain a nitrogen-doped carbon quantum dot / Co2V2O7 composite material.

[0077] The application of the above nitrogen-doped carbon quantum dots / Co2V2O7 composite material in magnesium hydride hydrogen storage is as follows:

[0078] 0.07g of nitrogen-doped carbon quantum dots / Co2V2O7 composite material was ball-milled at room temperature with 0.93g of commercial magnesium hydride at a speed of 450r / min for 4h, and the resulting composite sample was tested for variable temperature hydrogen release performance. The test conditions are as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was raised from room temperature to 450℃ at a heating rate of 5℃ / min. The amount of hydrogen released during this process was measured and plotted as a variable temperature hydrogen release curve, and the obtained performance was compared with that of unmodified magnesium hydride (also ball-milled under the above conditions).

[0079] After testing, magnesium hydride modified by nitrogen-doped carbon quantum dots / Co2V2O7 catalysis can significantly reduce the initial hydrogen desorption temperature and increase the hydrogen desorption capacity. However, due to the low doping amount of nitrogen-doped carbon quantum dots, the modification effect is worse than that of Example 3. Figure 8 As shown, the initial dehydrogenation temperature of unmodified magnesium hydride (ball-milled MgH2) is 310°C, and the dehydrogenation rate is 95.2%. After modification with nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), the initial dehydrogenation temperature is reduced to 228°C, and the dehydrogenation rate is 96.1%, which is a significant improvement.

[0080] Example 5

[0081] A method for preparing a nitrogen-doped carbon quantum dot / Co2V2O7 composite material comprises the following steps:

[0082] (1) 1.73 g (0.009 mol) of citric acid and 1.62 g (0.027 mol) of urea were dissolved in 30 mL of deionized water, and subjected to ultrasonic treatment for 30 min. The mixture was subjected to hydrothermal reaction at 180 °C for 8 h. After the reaction, the supernatant was collected by centrifugation, filtered through a 0.22 μm microporous membrane, and the filtrate was dried to obtain nitrogen-doped carbon quantum dots.

[0083] (2) 0.1 g of nitrogen-doped carbon quantum dots, 0.58 g (0.005 mol) of ammonium metavanadate and 0.24 g (0.001 mol) of cobalt chloride hexahydrate were added to 100 mL of deionized water and subjected to ultrasonic treatment for 20 min. The mixture was then heated in a water bath at 80 °C and 1.82 g of hexamethylenetetramine was added under stirring conditions. The mixture was stirred in a constant temperature water bath for 4 h. After the reaction was completed, the precipitate was collected by centrifugation, washed and dried to obtain a hexagonal flaky nitrogen-doped carbon quantum dots / Co2V2O7·nH2O composite material.

[0084] (3) The composite material obtained in the previous step was calcined at 500°C in a nitrogen atmosphere for 2 h to obtain a nitrogen-doped carbon quantum dot / Co2V2O7 composite material.

[0085] The application of the above nitrogen-doped carbon quantum dots / Co2V2O7 composite material in magnesium hydride hydrogen storage is as follows:

[0086] 0.07g of nitrogen-doped carbon quantum dots / Co2V2O7 composite material was ball-milled with 0.93g of commercial magnesium hydride at room temperature, with a rotation speed of 450r / min and a ball-milling time of 4h, and the resulting composite sample was subjected to a variable temperature hydrogen release performance test. The test conditions are as follows: 150mg of the composite sample was transferred to a hydrogen storage tester, and the temperature was raised from room temperature to 450℃ at a heating rate of 5℃ / min, and the amount of hydrogen released during this process was measured.

[0087] After the variable temperature hydrogen desorption test, the magnesium hydride modified by nitrogen-doped carbon quantum dots / Co2V2O7 catalysis can significantly reduce the initial hydrogen desorption temperature and increase the hydrogen desorption capacity, but the modification effect is worse than that of Example 3. After modification by nitrogen-doped carbon quantum dots / Co2V2O7 (N-CQD / Co2V2O7 modified MgH2), the initial hydrogen desorption temperature is reduced to 214°C, and the hydrogen desorption rate is 95.9%, which is significantly improved.

[0088] Comparative Example 1

[0089] A method for preparing hexagonal porous Co2V2O7 comprises the following steps:

[0090] (1) 0.58 g (0.005 mol) of ammonium metavanadate and 0.24 g (0.001 mol) of cobalt chloride hexahydrate were added to 100 mL of deionized water, and ultrasonicated for 20 min. Then, the mixture was heated in a water bath at 80° C., 1.82 g of hexamethylenetetramine was added under stirring, and the mixture was stirred in a constant temperature water bath for 4 h. After the reaction, the precipitate was collected by centrifugation, washed, and dried to obtain hexagonal flake Co2V2O7·nH2O;

[0091] (2) The Co2V2O7·nH2O obtained in the previous step was calcined at 500°C for 2 h in a nitrogen atmosphere to obtain hexagonal porous Co2V2O7.

[0092] like Fig. 9 Shown is the XRD pattern of Co2V2O7 prepared in this comparative example. It can be seen from the figure that the diffraction peaks of the material are consistent with the standard card of Co2V2O7 (PDF#70-1189), and there are no diffraction peaks of other phases, indicating that the Co2V2O7 synthesized by this method has a higher purity.

[0093] The application of the above Co2V2O7 material in magnesium hydride hydrogen storage is as follows:

[0094] 0.07g Co2V2O7 and 0.93g commercial magnesium hydride were ball-milled at room temperature, with a rotation speed of 450r / min and a ball-milling time of 4h, and the obtained sample was tested for variable temperature hydrogen release performance. The test conditions are as follows: 150mg of the sample was transferred to a hydrogen storage tester, and the temperature was raised from room temperature to 450℃ at a heating rate of 5℃ / min. The amount of hydrogen released during this process was measured and plotted into a variable temperature hydrogen release curve, and the obtained performance was compared with that of unmodified magnesium hydride (also ball-milled under the above conditions).

[0095] After testing, magnesium hydride modified by Co2V2O7 catalysis can reduce the hydrogen release temperature and increase the hydrogen release rate, but the modification effect is worse than that of Example 3. Fig.10 As shown, the initial dehydrogenation temperature of unmodified magnesium hydride (ball-milled MgH2) is 310°C, and the dehydrogenation rate is 95.2%. After modification with Co2V2O7 (Co2V2O7-modified MgH2), the initial dehydrogenation temperature is reduced to 233°C, and the dehydrogenation rate is 96.2%.

[0096] Comparative Example 2

[0097] A method for preparing nitrogen-doped carbon quantum dots comprises the following steps:

[0098] 1.73 g (0.009 mol) of citric acid and 1.62 g (0.027 mol) of urea were dissolved in 30 mL of deionized water, and subjected to ultrasonic treatment for 30 min. The mixture was hydrothermally reacted at 180 °C for 8 h. After the reaction, the supernatant was collected by centrifugation, filtered through a 0.22 μm microporous membrane, and the filtrate was dried to obtain nitrogen-doped carbon quantum dots.

[0099] like Fig.11 The figure shows the XRD pattern of the nitrogen-doped carbon quantum dots prepared in this comparative example. It can be seen from the figure that there is a diffraction peak at about 27°, corresponding to the (002) crystal plane of graphite, indicating that there is a graphite structure in the prepared carbon quantum dots.

[0100] like Fig.12 The infrared spectrum of nitrogen-doped carbon quantum dots prepared in this comparative example is shown in the figure. It can be seen from the figure that at 3201cm -1 and 3041cm -1 There are two broad and strong absorption peaks nearby, corresponding to the stretching vibration of OH and NH, 1665cm -1 、1567cm -1 The absorption peaks near this wavelength can be attributed to the stretching vibrations of C=O and C=C, which are located at 1400 cm -1 The absorption peak near 1199cm -1The small peak at is related to the stretching vibration of CO. Therefore, the presence of nitrogen-containing groups proves that nitrogen-doped carbon quantum dots are successfully synthesized, while the presence of hydrophilic groups such as carboxyl and hydroxyl groups indicates that the prepared nitrogen-doped carbon quantum dots have good water solubility.

[0101] The application of the above nitrogen-doped carbon quantum dots in magnesium hydride hydrogen storage is as follows:

[0102] 0.07g of nitrogen-doped carbon quantum dots and 0.93g of commercial magnesium hydride were ball-milled at room temperature, with a rotation speed of 450r / min and a ball-milling time of 4h, and the obtained sample was tested for variable temperature hydrogen release performance. The test conditions are as follows: 150mg of the sample was transferred to a hydrogen storage tester, and the temperature was raised from room temperature to 450℃ at a heating rate of 5℃ / min. The amount of hydrogen released during this process was measured and plotted into a variable temperature hydrogen release curve, and the obtained performance was compared with that of unmodified magnesium hydride (also ball-milled under the above conditions).

[0103] After testing, magnesium hydride catalytically modified by nitrogen-doped carbon quantum dots can reduce the hydrogen release temperature and increase the hydrogen release rate, but the modification effect is worse than that of Example 3 and Comparative Example 1. Fig.13 The unmodified magnesium hydride (ball-milled MgH2) has an initial dehydrogenation temperature of 310°C and a dehydrogenation rate of 95.2%. After modification with nitrogen-doped carbon quantum dots (NCQDs-modified MgH2), the initial dehydrogenation temperature is reduced to 290°C and the dehydrogenation rate is 95.9%.

Claims

1. A nitrogen-doped carbon quantum dot / Co2V2O7 composite material, characterized in that: The nitrogen-doped carbon quantum dots / Co2V2O7 composite material is a hexagonal porous Co2V2O7 micron sheet with nitrogen-doped carbon quantum dots uniformly loaded on its surface.

2. The nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 1, characterized in that: The size of Co2V2O7 micron flakes is 1.5-2.5μm, the thickness is 0.1-0.25μm, and the pore size is 5-50nm.

3. The method for preparing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 1 or 2, comprising the steps of: (1) fully mixing and dispersing a carbon source, a nitrogen source, and deionized water, performing a hydrothermal reaction, filtering, and drying the filtrate to obtain nitrogen-doped carbon quantum dots; (2) The nitrogen-doped carbon quantum dots, deionized water, ammonium metavanadate, cobalt salt and hexamethylenetetramine are fully mixed and dispersed uniformly, and then reacted, centrifuged, washed, dried and calcined to obtain the nitrogen-doped carbon quantum dots / Co2V2O7 composite material.

4. The method for preparing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 3, characterized in that: In step (1), one or more of the following conditions are included: i. The carbon source is one of glucose, ascorbic acid or citric acid; ii. The nitrogen source is one of urea, ethylenediamine or melamine.

5. The method for preparing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 3, characterized in that: In step (1), one or more of the following conditions are included: i. The molar ratio of the carbon source to the nitrogen source is 1:(1-5), preferably 1:(3-4), and more preferably 1:3; ii. The mass ratio of carbon source to deionized water is 1:(10-60).

6. The method for preparing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 3, characterized in that: In step (1), the temperature of the hydrothermal reaction is 150 to 200° C.; the time of the hydrothermal reaction is 2 to 10 hours.

7. The method for preparing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 3, characterized in that: In step (2), one or more of the following conditions are included: i. The cobalt salt is one of cobalt nitrate hexahydrate or cobalt chloride hexahydrate; ii. The molar ratio of ammonium metavanadate to cobalt salt is (2.5-5):1, preferably (3.5-5):1, and further preferably 5:1; the mass ratio of nitrogen-doped carbon quantum dots, ammonium metavanadate, hexamethylenetetramine and deionized water is (0.01-0.1):(0.23-0.58):(1.4-2.24):(50-100), preferably 0.05:0.58:1.82:

100.

8. The method for preparing the nitrogen-doped carbon quantum dots / Co2V2O7 composite material according to claim 3, characterized in that: In step (2), one or more of the following conditions are included: i. The reaction temperature is 50-90°C; the reaction time is 4-12h; the reaction is carried out under stirring; ii. The calcination temperature is 400-500° C.; the calcination atmosphere is an inert gas; preferably, the inert gas is nitrogen or argon; and the calcination time is 1-3 hours.

9. Use of the nitrogen-doped carbon quantum dots / Co2V2O7 composite material as claimed in claim 1 or 2 in magnesium hydride hydrogen storage.

10. The use according to claim 9, characterized in that: The application method comprises the steps of: mixing the nitrogen-doped carbon quantum dot / Co2V2O7 composite material with magnesium hydride by ball milling to obtain a composite material for magnesium hydride hydrogen storage; Preferably, the mass ratio of nitrogen-doped carbon quantum dots / Co2V2O7 composite material to magnesium hydride is 1:8-20; the ball milling speed is 400-600 r / min, and the ball milling time is 2-4 h.

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