Nb4N5-MgH2 composite hydrogen storage material and preparation method thereof

Abstract

The invention relates to an Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof, the composite hydrogen storage material comprises Nb4N5 and MgH2, and the Nb4N5 accounts for 3wt.%-9wt.% of the composite hydrogen storage material according to the mass percentage. The preparation method comprises the following steps: (1) preparing an Nb4N5 catalyst: dissolving NbCl5 in absolute ethyl alcohol, adding ammonia water to generate a precipitate, washing, drying, and calcining in an ammonia atmosphere to obtain Nb4N5 catalyst powder; and (2) mixing the Nb4N5 catalyst with MgH2, and performing mechanical ball milling to obtain the composite hydrogen storage material. In the ball milling process, stainless steel balls are adopted, argon is used as a protective atmosphere, the ball-to-material ratio is (10-40): 1, the ball milling time is 5-12 hours, the rotating speed is 350-500 rpm, and forward and reverse intermittent ball milling is adopted. The prepared composite material shows excellent hydrogen storage dynamic performance. Through uniform distribution of the Nb4N5 catalyst and a stable phase structure of the Nb4N5 catalyst, the hydrogen absorption and desorption performance and the cycle stability of the hydrogen storage material are remarkably improved. The invention is suitable for the hydrogen energy field.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydrogen energy, and in particular relates to a Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof. Background Art

[0002] Magnesium-based hydrogen storage materials have high hydrogen storage densities, with theoretical mass and volumetric hydrogen storage densities of 7.6 wt.% and 110 g / L, respectively. They are also inexpensive, abundant, and exhibit stable hydrogen absorption and desorption cycles, making them one of the most promising hydrogen storage materials. However, their development is severely hampered by their high thermodynamic stability and poor kinetic properties, resulting in high desorption temperatures and slow desorption rates.

[0003] Among all modification strategies, catalyst doping is one of the simplest and most effective methods to accelerate the reaction kinetics of the MgH2 system, and it has important theoretical significance for its practical application. In previous studies, transition metals and their compounds have been shown to be highly active catalysts for MgH2. Among them, the presence of transition metal Nb can promote the transport of hydrogen in MgH2 through a hydrogen pump mechanism. It is reported that transition metal nitrides also have a good catalytic effect on the hydrogen absorption / desorption performance of MgH2. At the same time, transition metal nitrides have good stability in the MgH2 / Mg system, which can avoid the loss of hydrogen storage capacity caused by the reaction of Mg or MgH2 with the catalyst, and help to improve the cyclic stability of the material. However, there is relatively little research on transition metal nitrides, so it is necessary to study their effect on the hydrogen storage performance of magnesium hydride to provide guidance for the subsequent preparation of magnesium hydride catalysts. Summary of the Invention

[0004] In order to solve the above problems, the present invention proposes a Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A Nb4N5-MgH2 composite hydrogen storage material comprises Nb4N5 and MgH2. Calculated by mass percentage, Nb4N5 in the composite hydrogen storage material accounts for 3 wt.% to 9 wt.%.

[0007] A method for preparing the above-mentioned Nb4N5-MgH2 composite hydrogen storage material comprises the following steps:

[0008] (1) Preparation of Nb4N5 catalyst: First, NbCl5 is dissolved in anhydrous ethanol and stirred until the solution is clear. Then, an appropriate amount of ammonia water is added to the solution and stirred to react to obtain a colloidal precipitate. The obtained precipitate is centrifuged and washed multiple times with deionized water and anhydrous ethanol. Finally, the precipitate is dried in a vacuum drying oven to obtain a precursor. Then, a temperature-programmed nitridation is performed. The precursor sample is placed in a ceramic crucible, placed in a tube furnace, connected to an ammonia atmosphere, and calcined to obtain Nb4N5 catalyst powder.

[0009] (2) Preparation of Nb4N5-MgH2 composite material: The Nb4N5 and MgH2 powders prepared in step (1) were mixed and mechanically ball milled to obtain the Nb4N5-MgH2 composite material.

[0010] Furthermore, in step (1), the mass volume ratio of NbCl5, anhydrous ethanol and ammonia water is: (0.5-1): (0.5-5): (10-30), g: mL: mL; and the reaction is stirred at a rate of 200-800 rpm for 10-60 min.

[0011] Furthermore, in step (1), the concentration of ammonia water is 0.3 to 0.5 mol / L.

[0012] Furthermore, in step (1), the centrifugal washing process is: centrifugal washing 3 to 5 times with deionized water and anhydrous ethanol at a speed of 3000 to 8000 rpm; the vacuum drying process is: vacuum drying at 60 to 100° C. for 8 to 16 hours.

[0013] Furthermore, in step (1), the parameters of the temperature-programmed nitriding are: heating to 600°C to 900°C at a heating rate of 2 to 10°C / min, and calcining for 2 to 8 hours. When the temperature reaches 300°C, the flow meter is adjusted to increase the flow rate of ammonia.

[0014] Furthermore, in step (2), stainless steel balls are used in the mechanical ball milling, argon is used as the ball milling atmosphere, the ball-to-material ratio is (10-40):1, the ball milling time is 5-12 h, the ball milling speed is 350-500 rpm, and the ball milling is performed intermittently in forward and reverse rotations: after each ball milling for 5-30 min, the direction is reversed after a pause of 5-30 min.

[0015] The beneficial effects of the present invention are:

[0016] The present invention provides a Nb4N5 hydrogen storage material catalyst, a hydrogen storage material containing the catalyst, and a preparation method. The catalyst exhibits excellent catalytic effects when combined with magnesium hydride:

[0017] (1) Improved hydrogen release rate: At 325°C, the Nb4N5-MgH2 composite hydrogen storage material releases 6.15 wt.% of hydrogen in just 4 minutes, while pure MgH2 releases only 0.23 wt.% of hydrogen under the same conditions. When the temperature is lowered to 300°C, the composite material can release 6.00 wt.% of hydrogen in 5 minutes, while pure MgH2 releases almost no hydrogen.

[0018] (2) Increased hydrogen absorption capacity: By adding 6 wt.% Nb4N5, MgH2 can rapidly absorb 3.84 wt.% of hydrogen within 1 minute at 150°C. When the time is extended to 20 minutes, the hydrogen adsorption amount reaches 4.51 wt.%. As the temperature decreases, the composite material maintains a relatively fast hydrogen absorption rate. At 100°C, it can absorb 3.40 wt.% of hydrogen within 30 minutes, while pure MgH2 requires 45 minutes at 225°C to absorb the same amount of hydrogen.

[0019] (3) It has stable cycle performance. The hydrogen release amount in the 50th cycle can still be maintained at 5.46wt.%, and the capacity retention rate is 90.10%.

[0020] The present invention employs Nb4N5 as a hydrogen storage material catalyst, resulting in excellent catalytic performance. This is due to the fact that, after ball-milling with MgH2, Nb4N5 can be evenly dispersed on the surface of the MgH2 matrix, providing uniformly dispersed active sites for the composite hydrogen storage material. Furthermore, during subsequent hydrogen absorption and desorption cycles, Nb4N5 maintains a stable physical structure and does not transform into new phases. As a stable catalytically active species, Nb4N5 not only facilitates charge transfer between Mg and MgH2 but also provides a pathway for hydrogen diffusion, significantly enhancing the hydrogen storage performance of MgH2.

[0021] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0023] Figure 1 This is the SEM image of the Nb4N5 catalyst prepared in Example 1.

[0024] Figure 2 This is the XRD pattern of the Nb4N5 catalyst prepared in Example 1.

[0025] Figure 3 This is the temperature-dependent hydrogen desorption curve (TPD) of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1.

[0026] Figure 4 These are the isothermal hydrogen desorption curves of ball-milled MgH2 powder and the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1.

[0027] Figure 5 These are the isothermal hydrogen absorption curves of ball-milled MgH2 powder and the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1.

[0028] Figure 6 This is a diagram of the cyclic hydrogen absorption and desorption performance of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1. DETAILED DESCRIPTION

[0029] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0030] Among them, the accompanying drawings are only for illustrative purposes and represent only schematic diagrams rather than actual pictures, and should not be understood as limiting the present invention. In order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the dimensions of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions may be omitted in the accompanying drawings.

[0031] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "back", etc. indicating directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operate in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0032] Example 1

[0033] A Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof, the specific preparation method is as follows:

[0034] (1) Preparation of Nb4N5 catalyst: First, dissolve 1g of NbCl5 in 1ml of anhydrous ethanol and stir magnetically for 20min. Then, add 10ml of 0.5mol / L ammonia water to the above solution and stir magnetically for 30min. Wash the resulting precipitate several times with deionized water and anhydrous ethanol. Finally, place the precipitate in a vacuum drying oven and dry it at 60℃ for 10h to obtain the precursor. Then, perform programmed temperature nitridation: place the above precursor sample in a ceramic crucible, place it in a tube furnace, connect it to an ammonia atmosphere, and heat it to 900℃ at a rate of 10℃ / min. When the temperature reaches about 300℃, adjust the flowmeter to increase the gas flow appropriately. After calcination for 2h, cool it to room temperature, remove the sample, and grind it to obtain Nb4N5 catalyst powder.

[0035] (2) Preparation of Nb4N5-MgH2 composite material: 0.06 g of the prepared Nb4N5 powder as a catalyst was mixed with 0.94 g of MgH2 powder in a high-purity argon atmosphere, wherein the mass ratio of Nb4N5 powder to MgH2 powder was 6:94; the Nb4N5-MgH2 mixed powder was prepared by mechanical ball milling in an argon atmosphere for 10 h, wherein the ball-to-material ratio was 20:1, the ball milling speed was 400 rpm, the ball milling jar was a 100 mL stainless steel jar, and the grinding balls were stainless steel grinding balls with diameters of 8 mm and 6 mm.

[0036] Figure 1 This is the SEM image of the Nb4N5 catalyst prepared in Example 1. Figure 1 It can be seen that the Nb4N5 catalyst prepared in Example 1 is in the form of powder particles with a size of 0.05 to 0.2 μm.

[0037] Figure 2 The XRD pattern of the Nb4N5 catalyst prepared in Example 1 is shown in FIG. Figure 2 It can be seen that the prepared Nb4N5 catalyst corresponds accurately to the standard card 74-0606.

[0038] The hydrogen storage performance of the Nb4N5-MgH2 composite hydrogen storage material (MgH2-6wt.% Nb4N5 mixed powder) prepared in Example 1 is as follows:

[0039] 50 mg of the Nb4N5-MgH2 composite material prepared in Example 1 was weighed in a glove box, and a PCTpro high-pressure gas adsorption instrument was used to test the temperature-dependent hydrogen desorption curve of the sample; the isothermal hydrogen desorption curve at an initial hydrogen pressure of 0.01 MPa at 225°C, 250°C, 275°C, 300°C and 325°C; the isothermal hydrogen absorption curve at an initial hydrogen pressure of 3 MPa at 50°C, 100°C and 150°C; and the 50-cycle hydrogen absorption and desorption curve at 300°C.

[0040] Figure 3 The temperature-dependent hydrogen release curve (TPD) of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 is shown in FIG. Figure 3 It can be seen that the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 begins to release hydrogen at 228°C.

[0041] In order to compare the performance of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1, ball-milled MgH2 powder (1 g of commercial MgH2 was ball-milled for 10 h in a high-purity argon atmosphere to prepare ball-milled MgH2 powder, wherein the ball-to-material ratio was 20:1, the rotation speed was 400 rpm, the ball mill used was a stainless steel mill with a volume of 100 mL, and the grinding balls were stainless steel grinding balls with diameters of 8 mm and 6 mm) was used as the control group.

[0042] Figure 4 The isothermal hydrogen release curves of the ball-milled MgH2 powder and the magnesium-based hydrogen storage material (MgH2-6wt.% Nb4N5 mixed powder) prepared in Example 1. Figure 4 It can be seen that the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 was subjected to a hydrogen desorption test at a constant temperature of 325°C. It was found that the MgH2+6wt.% Nb4N5 composite material released 6.15wt.% hydrogen within 4 minutes, while pure MgH2 released only 0.23wt.% hydrogen under the same conditions. When the temperature was lowered to 300°C, the composite material released 6.00wt.% hydrogen within 5 minutes, while pure MgH2 released almost no hydrogen. At a lower temperature (275°C), the composite material still released 5.85wt.% hydrogen within 10 minutes. Even at a low temperature of 225°C, the composite material released 5.44wt.% hydrogen within 60 minutes. This shows that the Nb4N5-MgH2 composite hydrogen storage material not only has a lower initial hydrogen desorption temperature, but also has significantly improved hydrogen desorption kinetics.

[0043] Figure 5 The isothermal hydrogen absorption curves of the ball-milled MgH2 powder and the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1. Figure 5It can be seen that the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 can rapidly absorb 3.84 wt.% of hydrogen within 1 minute at 150°C. When the time is extended to 20 minutes, the hydrogen adsorption amount reaches 4.51 wt.%. As the temperature decreases, the composite material can still maintain a relatively fast hydrogen absorption rate. At 100°C, it can absorb 3.40 wt.% of hydrogen within 30 minutes, while pure MgH2 requires 45 minutes at 225°C to absorb the same amount of hydrogen. This shows that the hydrogen absorption kinetics of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 are significantly improved.

[0044] Figure 6 This is a graph showing the cyclic hydrogen absorption and desorption performance of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1. Figure 6 It can be seen that during the 50th cycle of hydrogen release, the hydrogen release amount of the composite hydrogen storage material is still 5.46wt.%, and the capacity retention rate is 90.10%, showing excellent cycle reversibility.

[0045] Example 2

[0046] A Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof, the specific preparation method is as follows:

[0047] (1) Preparation of Nb4N5 catalyst: First, dissolve 1g of NbCl5 in 1ml of anhydrous ethanol and stir magnetically for 20min. Then, add 10ml of 0.5mol / L ammonia water to the above solution and stir magnetically for 30min. Wash the resulting precipitate several times with deionized water and anhydrous ethanol. Finally, place the precipitate in a vacuum drying oven and dry it at 60℃ for 10h to obtain the precursor. Then, perform programmed temperature nitridation: place the above precursor sample in a ceramic crucible, place it in a tube furnace, connect it to an ammonia atmosphere, and heat it to 900℃ at a rate of 10℃ / min. When the temperature reaches about 300℃, adjust the flowmeter to increase the gas flow appropriately. After calcination for 2h, cool it to room temperature, remove the sample, and grind it to obtain Nb4N5 catalyst powder.

[0048] (2) Preparation of Nb4N5-MgH2 composite materials: 0.09 g of the prepared Nb4N5 powder as a catalyst was mixed with 0.91 g of MgH2 powder under a high-purity argon atmosphere, wherein the mass ratio of Nb4N5 powder to MgH2 powder was 9:91;

[0049] 0.03 g of the prepared Nb4N5 powder was used as a catalyst and mixed with 0.97 g of MgH2 powder under a high-purity argon atmosphere, wherein the mass ratio of Nb4N5 powder to MgH2 powder was 3:97.

[0050] The two ratios of Nb4N5 powder and MgH2 powder were mechanically ball milled for 10 h under an argon atmosphere to prepare Nb4N5-MgH2 mixed powder, wherein the ball-to-material ratio was 20:1, the ball milling speed was 400 rpm, the ball milling jar was a 100 mL stainless steel jar, and the grinding balls were stainless steel grinding balls with diameters of 8 mm and 6 mm.

[0051] Table 1 shows a performance comparison of the initial dehydrogenation temperature and the amount of hydrogen released in 5 minutes at 300°C for the three Nb4N5-MgH2 composite hydrogen storage materials and ball-milled MgH2 powder obtained in Examples 1 and 2. After adding 3wt.% Nb4N5 catalyst, the system's initial dehydrogenation temperature was 247°C. As the Nb4N5 doping level increased, the sample's dehydrogenation temperature also decreased, but the amount of hydrogen released also continued to decrease. This is because Nb4N5 does not participate in hydrogen storage, and the system's effective hydrogen storage capacity decreases. The initial dehydrogenation temperature of the MgH2+6wt.% Nb4N5 sample has dropped to 228°C. When the addition level continued to increase to 9wt.%, the initial dehydrogenation temperature did not decrease significantly, but its dehydrogenation amount decreased by 0.2wt.%. Taking into account the dehydrogenation temperature and hydrogen storage capacity, the composite material with a catalyst doping level of 6wt.% has the best hydrogen storage performance.

[0052] Table 1 TPD data of composite materials with different catalyst doping amounts

[0053]

[0054] Example 3

[0055] A Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof, the specific preparation method is as follows:

[0056] (1) Preparation of Nb4N5 catalyst: First, dissolve 1g of NbCl5 in 1ml of anhydrous ethanol and stir magnetically for 20min. Then, add 10ml of 0.5mol / L ammonia water to the above solution and stir magnetically for 30min. Wash the resulting precipitate several times with deionized water and anhydrous ethanol. Finally, place the precipitate in a vacuum drying oven and dry it at 60℃ for 10h to obtain a niobium oxide precursor. Then, perform programmed temperature nitriding: place the above precursor sample in a ceramic crucible, place it in a tube furnace, connect it to an ammonia atmosphere, and heat it to 600℃ at a rate of 10℃ / min. When the temperature reaches about 300℃, adjust the flowmeter to increase the gas flow appropriately. After calcination for 2h, cool it to room temperature, remove the sample, and grind it to obtain Nb4N5 catalyst powder.

[0057] (2) Preparation of Nb4N5-MgH2 composite material: 0.06 g of the prepared Nb4N5 powder as a catalyst was mixed with 0.94 g of MgH2 powder in a high-purity argon atmosphere, wherein the mass ratio of Nb4N5 powder to MgH2 powder was 6:94; mechanical ball milling was carried out in an argon atmosphere for 10 h to prepare Nb4N5-MgH2 mixed powder, wherein the ball-to-material ratio was 20:1, the ball milling speed was 400 rpm, the ball milling jar was a 100 mL stainless steel jar, and the grinding balls used were stainless steel grinding balls with diameters of 8 mm and 6 mm.

[0058] Example 4

[0059] A Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof, the specific preparation method is as follows:

[0060] (1) Preparation of Nb4N5 catalyst: First, dissolve 1g of NbCl5 in 1ml of anhydrous ethanol and stir magnetically for 20min. Then, add 10ml of 0.5mol / L ammonia water to the above solution and stir magnetically for 30min. Wash the resulting precipitate several times with deionized water and anhydrous ethanol. Finally, place the precipitate in a vacuum drying oven and dry it at 60℃ for 10h to obtain a niobium oxide precursor. Then, perform programmed temperature nitriding: place the above precursor sample in a ceramic crucible, place it in a tube furnace, connect it to an ammonia atmosphere, and heat it to 900℃ at a rate of 10℃ / min. When the temperature reaches about 300℃, adjust the flowmeter to increase the gas flow appropriately. After calcination for 3h / 4h, cool it to room temperature, remove the sample, and grind it to obtain Nb4N5 catalyst powder.

[0061] (2) Preparation of Nb4N5-MgH2 composite material: 0.06 g of the prepared Nb4N5 powder as a catalyst was mixed with 0.94 g of MgH2 powder in a high-purity argon atmosphere, wherein the mass ratio of Nb4N5 powder to MgH2 powder was 6:94; mechanical ball milling was carried out in an argon atmosphere for 10 h to prepare Nb4N5-MgH2 mixed powder, wherein the ball-to-material ratio was 20:1, the ball milling speed was 400 rpm, the ball milling jar was a 100 mL stainless steel jar, and the grinding balls were stainless steel grinding balls with diameters of 8 mm and 6 mm.

[0062] Example 5

[0063] A Nb4N5-MgH2 composite hydrogen storage material and a preparation method thereof, the specific preparation method is as follows:

[0064] (1) Preparation of Nb4N5 catalyst: First, dissolve 1g of NbCl5 in 2ml of anhydrous ethanol and stir magnetically for 10min. Then, add 40ml of 0.3mol / L ammonia water to the above solution and stir magnetically for 30min. Wash the resulting precipitate several times with deionized water and anhydrous ethanol. Finally, place the precipitate in a vacuum drying oven and dry it at 60℃ for 12h to obtain a niobium oxide precursor. Then, perform programmed temperature nitriding: place the above precursor sample in a ceramic crucible, place it in a tube furnace, connect it to an ammonia atmosphere, and heat it to 900℃ at a rate of 10℃ / min. When the temperature reaches about 300℃, adjust the flowmeter to increase the gas flow appropriately. After calcination for 2h, cool it to room temperature, remove the sample, and grind it to obtain Nb4N5 catalyst powder.

[0065] (2) Preparation of Nb4N5-MgH2 composite material: 0.06 g of the prepared Nb4N5 powder as a catalyst was mixed with 0.94 g of MgH2 powder in a high-purity argon atmosphere, wherein the mass ratio of Nb4N5 powder to MgH2 powder was 6:94. The Nb4N5-MgH2 mixed powder was prepared by mechanical ball milling for 10 h in an argon atmosphere, wherein the ball-to-material ratio was 20:1, the ball milling speed was 400 rpm, the ball milling jar was a 100 mL stainless steel jar, and the grinding balls were stainless steel grinding balls with diameters of 8 mm and 6 mm.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A Nb4N5-MgH2 composite hydrogen storage material, characterized in that: The composite hydrogen storage material comprises Nb4N5 and MgH2. In terms of mass percentage, the Nb4N5 content in the composite hydrogen storage material is 3 wt.% to 9 wt.%.

2. A method for preparing the Nb4N5-MgH2 composite hydrogen storage material according to claim 1, characterized in that: The steps include: (1) Preparation of Nb4N5 catalyst: First, NbCl5 is dissolved in anhydrous ethanol and stirred until the solution is clear. Then, an appropriate amount of ammonia water is added to the solution and stirred to react to obtain a colloidal precipitate. The obtained precipitate is centrifuged and washed multiple times with deionized water and anhydrous ethanol. Finally, the precipitate is dried in a vacuum drying oven to obtain a precursor. Then, a temperature-programmed nitridation is performed. The precursor sample is placed in a ceramic crucible, placed in a tube furnace, connected to an ammonia atmosphere, and calcined to obtain Nb4N5 catalyst powder. (2) Preparation of Nb4N5-MgH2 composite material: The Nb4N5 and MgH2 powders prepared in step (1) were mixed and mechanically ball milled to obtain the Nb4N5-MgH2 composite material.

3. The method for preparing the Nb4N5-MgH2 composite hydrogen storage material according to claim 2, characterized in that: In step (1), the mass volume ratio of NbCl5, anhydrous ethanol and ammonia water is: (0.5-1):(0.5-5):(10-30), g:mL:mL; the reaction is stirred at a rate of 200-800 rpm for 10-60 min.

4. The method for preparing the Nb4N5-MgH2 composite hydrogen storage material according to claim 3, characterized in that: In step (1), the concentration of aqueous ammonia is 0.3 to 0.5 mol / L.

5. The method for preparing the Nb4N5-MgH2 composite hydrogen storage material according to claim 2, characterized in that: In step (1), the centrifugal washing process is: centrifugal washing 3 to 5 times with deionized water and anhydrous ethanol at a speed of 3000 to 8000 rpm; the vacuum drying process is: vacuum drying at 60 to 100° C. for 8 to 16 hours.

6. The method for preparing the Nb4N5-MgH2 composite hydrogen storage material according to claim 2, characterized in that: In step (1), the parameters of the programmed temperature nitriding are: heating to 600°C to 900°C at a heating rate of 2 to 10°C / min, and calcining for 2 to 8 hours. When the temperature reaches 300°C, adjust the flow meter to increase the flow of ammonia.

7. The method for preparing the Nb4N5-MgH2 composite hydrogen storage material according to claim 2, characterized in that: In step (2), stainless steel balls are used in the mechanical ball milling, argon is used as the ball milling atmosphere, the ball-to-material ratio is (10-40):1, the ball milling time is 5-12 h, the ball milling speed is 350-500 rpm, and the ball milling is performed in forward and reverse intermittent rotations: after each ball milling for 5-30 min, the direction is reversed after pausing for 5-30 min.

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