Nb4n5-mgh2 composite hydrogen storage material and preparation method thereof

Abstract

The application relates to a 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.% in the composite hydrogen storage material according to the mass percentage. The preparation method comprises the following steps: (1) preparing a Nb4N5 catalyst, dissolving NbCl5 in anhydrous ethanol, adding ammonia water to generate a precipitate, washing and drying, and then calcining in an ammonia atmosphere to obtain a Nb4N5 catalyst powder; (2) mixing the Nb4N5 catalyst with MgH2, and preparing a composite hydrogen storage material through mechanical ball milling. 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 positive and negative intermittent ball milling is adopted. The composite material prepared by the application has excellent hydrogen storage kinetic performance. Through the uniform distribution of the Nb4N5 catalyst and the stable phase structure thereof, the hydrogen absorption and release performance and the cycle stability of the hydrogen storage material are remarkably improved. The application is suitable for the hydrogen energy field.

Description

Technical Field

[0001] This invention belongs to the field of hydrogen energy technology, specifically relating to an Nb4N5-MgH2 composite hydrogen storage material and its preparation method. Background Technology

[0002] Magnesium-based hydrogen storage materials possess high hydrogen storage density, with a theoretical mass hydrogen storage density of 7.6 wt.% and a volumetric hydrogen storage density of 110 g / L. They are also inexpensive, abundant, and exhibit stable hydrogen absorption and desorption cycle characteristics, making them one of the most promising hydrogen storage materials currently available. However, their high thermodynamic stability and poor kinetic performance, resulting in high hydrogen desorption temperatures and slow rates, severely restrict their development.

[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. The presence of the transition metal Nb can promote hydrogen transport in MgH2 through a hydrogen pump mechanism. It has also been reported that transition metal nitrides have good catalytic effects on the hydrogen absorption / desorption performance of MgH2. Simultaneously, transition metal nitrides exhibit 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, thus helping to improve the cycling stability of the material. However, current research on transition metal nitrides is relatively limited; therefore, it is necessary to study their impact on the hydrogen storage performance of magnesium hydride to provide guidance for the subsequent preparation of magnesium hydride catalysts. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a Nb4N5-MgH2 composite hydrogen storage material and its preparation method.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

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

[0007] A method for preparing the Nb4N5-MgH2 composite hydrogen storage material as described above includes the following steps:

[0008] (1) Preparation of Nb4N5 catalyst: First, NbCl5 was dissolved in anhydrous ethanol and stirred until the solution was clear. Then, an appropriate amount of ammonia was added to the solution and stirred to obtain a colloidal precipitate. The precipitate was washed several times by centrifugation with deionized water and anhydrous ethanol. Finally, the precipitate was dried in a vacuum drying oven to obtain the precursor. Then, the precursor sample was nitrided by temperature program. The precursor sample was placed in a ceramic crucible and placed in a tube furnace. After the ammonia atmosphere was turned on, it was calcined to obtain Nb4N5 catalyst powder.

[0009] (2) Preparation of Nb4N5-MgH2 composite material: Nb4N5 and MgH2 powders obtained in step (1) are mixed and Nb4N5-MgH2 composite material is prepared by mechanical ball milling.

[0010] Further, in step (1), the mass-to-volume ratio of NbCl5, anhydrous ethanol and ammonia 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.

[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 as follows: centrifuge and wash 3 to 5 times with deionized water and anhydrous ethanol at a speed of 3000 to 8000 rpm; the vacuum drying process is as follows: vacuum dry at 60 to 100℃ for 8 to 16 hours.

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

[0014] Furthermore, in step (2), stainless steel balls are used in the mechanical ball mill, argon is used as the ball milling atmosphere, the ball-to-material ratio is (10-40):1, the ball milling time is 5-12 hours, the ball milling speed is 350-500 rpm, and the ball milling is carried out in both forward and reverse intermittent directions: after each ball milling for 5-30 minutes, the direction is reversed once after a 5-30 minute pause.

[0015] The beneficial effects of this invention are as follows:

[0016] This invention provides an Nb4N5 hydrogen storage catalyst, a hydrogen storage material containing the catalyst, and a preparation method thereof. The catalyst, when combined with magnesium hydride, exhibits excellent catalytic performance.

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

[0018] (2) The hydrogen absorption capacity was increased. After adding 6 wt.% Nb4N5, MgH2 was able to rapidly absorb 3.84 wt.% hydrogen within 1 min at 150℃. When the time was extended to 20 min, the hydrogen adsorption capacity could reach 4.51 wt.%. As the temperature decreased, the composite material could still maintain a relatively fast hydrogen absorption rate. At 100℃, it could absorb 3.40 wt.% hydrogen within 30 min, while pure MgH2 needed to be at 225℃ for 45 min to adsorb the same amount of hydrogen.

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

[0020] In this invention, Nb4N5 is used as the catalyst for hydrogen storage. Its excellent catalytic performance stems from two main factors. First, by ball milling Nb4N5 with MgH2, it can be uniformly dispersed on the surface of the MgH2 matrix, providing uniformly dispersed active sites for the composite hydrogen storage material. Second, during subsequent hydrogen absorption and desorption cycles, Nb4N5 maintains a stable phase structure and does not transform into other new phases. As a stable catalytically active substance, Nb4N5 not only plays a role in charge transfer between Mg / MgH2 but also provides a channel for hydrogen diffusion, thereby significantly improving the hydrogen storage performance of MgH2.

[0021] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0023] Figure 1 The image shows a SEM image of the Nb4N5 catalyst prepared in Example 1.

[0024] Figure 2 The image shows the XRD pattern of the Nb4N5 catalyst prepared in Example 1.

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

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

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

[0028] Figure 6 The graph shows the cyclic hydrogen absorption and desorption performance of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1. Detailed Implementation

[0029] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed 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 representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0030] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0031] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship 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 orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0032] Example 1

[0033] A Nb4N5-MgH2 composite hydrogen storage material and its preparation method are described below:

[0034] (1) Preparation of Nb4N5 catalyst: First, 1g of NbCl5 was dissolved in 1ml of anhydrous ethanol and magnetically stirred for 20min. Then, 10ml of 0.5mol / L ammonia solution was added to the above solution and magnetically stirred for 30min. The resulting precipitate was washed several times with deionized water and anhydrous ethanol. Finally, the precipitate was placed in a vacuum drying oven and dried at 60℃ for 10h to obtain the precursor. Then, temperature-programmed nitriding was performed: The above precursor sample was placed in a ceramic crucible and placed in a tube furnace. Ammonia atmosphere was turned on, and the temperature was raised to 900℃ at a rate of 10℃ / min. When the temperature reached about 300℃, the flow meter was adjusted to increase the gas flow rate appropriately. After calcination for 2h, the sample was cooled to room temperature, removed, and ground to obtain Nb4N5 catalyst powder.

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

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

[0037] Figure 2 The image shows the XRD pattern of the Nb4N5 catalyst prepared in Example 1. Figure 2 It can be seen that the prepared Nb4N5 catalyst corresponds exactly 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 the hydrogen desorption curves of the samples with temperature were tested using a PCTpro high-pressure gas adsorption instrument; the isothermal hydrogen desorption curves at 225℃, 250℃, 275℃, 300℃ and 325℃ with an initial hydrogen pressure of 0.01 MPa; the isothermal hydrogen adsorption curves at 50℃, 100℃ and 150℃ with an initial hydrogen pressure of 3 MPa; and the hydrogen adsorption and desorption curves after 50 cycles at 300℃.

[0040] Figure 3 The temperature-dependent hydrogen desorption (TPD) curve of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 is shown. 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] To compare the performance of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1, ball-milled MgH2 powder (prepared by ball milling 1g of commercial MgH2 for 10h under a high-purity argon atmosphere, wherein the ball-to-powder ratio was 20:1, the rotation speed was 400rpm, the ball milling jar was a 100mL stainless steel jar, and the grinding balls were stainless steel grinding balls with diameters of 8mm and 6mm) was used as a control group.

[0042] Figure 4 Isothermal hydrogen desorption curves for ball-milled MgH2 powder and the magnesium-based hydrogen storage material (MgH2-6wt.% Nb4N5 mixed powder) prepared in Example 1. Figure 4 As can be seen, the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1, when subjected to hydrogen release tests at a constant temperature of 325℃, showed 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 reduced to 300℃, the composite material released 6.00wt.% hydrogen within 5 minutes, while pure MgH2 released almost no hydrogen. At a lower temperature (275℃), the composite material still released 5.85wt.% hydrogen within 10 minutes, and even at a low temperature of 225℃, the composite material released 5.44wt.% hydrogen within 60 minutes. Therefore, it can be concluded that the Nb4N5-MgH2 composite hydrogen storage material not only has a lower initial hydrogen release temperature, but also exhibits significantly improved hydrogen release kinetics.

[0043] Figure 5 Isothermal hydrogen absorption curves for 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.% hydrogen within 1 minute at 150°C, and the hydrogen adsorption capacity can reach 4.51 wt.% after the time is extended to 20 minutes. As the temperature decreases, the composite material still maintains a relatively fast hydrogen absorption rate; at 100°C, it can absorb 3.40 wt.% hydrogen within 30 minutes, while pure MgH2 requires 45 minutes at 225°C to adsorb the same amount of hydrogen. Therefore, it can be concluded that the hydrogen absorption kinetics of the Nb4N5-MgH2 composite hydrogen storage material prepared in Example 1 are significantly improved.

[0044] Figure 6 The graph shows 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 hydrogen release cycle, the hydrogen release of the composite hydrogen storage material was still 5.46 wt.%, and the capacity retention rate was 90.10%, demonstrating excellent cycle reversibility.

[0045] Example 2

[0046] A Nb4N5-MgH2 composite hydrogen storage material and its preparation method are described below:

[0047] (1) Preparation of Nb4N5 catalyst: First, 1g of NbCl5 was dissolved in 1ml of anhydrous ethanol and magnetically stirred for 20min. Then, 10ml of 0.5mol / L ammonia solution was added to the above solution and magnetically stirred for 30min. The resulting precipitate was washed several times with deionized water and anhydrous ethanol. Finally, the precipitate was placed in a vacuum drying oven and dried at 60℃ for 10h to obtain the precursor. Then, temperature-programmed nitriding was performed: The above precursor sample was placed in a ceramic crucible and placed in a tube furnace. Ammonia atmosphere was turned on, and the temperature was raised to 900℃ at a rate of 10℃ / min. When the temperature reached about 300℃, the flow meter was adjusted to increase the gas flow rate appropriately. After calcination for 2h, the sample was cooled to room temperature, removed, and ground to obtain Nb4N5 catalyst powder.

[0048] (2) Preparation of Nb4N5-MgH2 composite material: It was prepared in two proportions. 0.09g of the prepared Nb4N5 powder was used as a catalyst and mixed with 0.91g of MgH2 powder in a high-purity argon atmosphere. The mass ratio of Nb4N5 powder to MgH2 powder was 9:91.

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

[0050] Nb4N5 powder and MgH2 powder in the above two ratios were mechanically ball-milled for 10 hours under an argon atmosphere to prepare Nb4N5-MgH2 mixed powder. The ball-to-powder 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 compares the hydrogen release rates of the three Nb4N5-MgH2 composite hydrogen storage materials and ball-milled MgH2 powder obtained in Examples 1 and 2 at the initial hydrogen release temperature and 5 min at 300°C. After adding 3 wt.% Nb4N5 catalyst, the initial hydrogen release temperature of the system was 247°C. With increasing Nb4N5 doping concentration, the hydrogen release temperature decreased, but the hydrogen release rate also decreased continuously. This is because Nb4N5 does not participate in hydrogen storage, reducing the effective hydrogen storage capacity of the system. The initial hydrogen release temperature of the MgH2 + 6 wt.% Nb4N5 sample had already dropped to 228°C. When the addition concentration was further increased to 9 wt.%, the initial hydrogen release temperature did not decrease significantly, but the hydrogen release rate decreased by 0.2 wt.%. Considering both hydrogen release temperature and hydrogen storage rate, the composite material with a catalyst doping concentration of 6 wt.% exhibited 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 its preparation method are described below:

[0056] (1) Preparation of Nb4N5 catalyst: First, 1g of NbCl5 was dissolved in 1ml of anhydrous ethanol and magnetically stirred for 20min. Then, 10ml of 0.5mol / L ammonia solution was added to the above solution and magnetically stirred for 30min. The resulting precipitate was washed several times with deionized water and anhydrous ethanol. Finally, the precipitate was placed in a vacuum drying oven and dried at 60℃ for 10h to obtain niobium oxide precursor. Then, temperature-programmed nitriding was performed: The above precursor sample was placed in a ceramic crucible and placed in a tube furnace. Ammonia atmosphere was turned on, and the temperature was raised to 600℃ at a rate of 10℃ / min. When the temperature reached about 300℃, the flow meter was adjusted to increase the gas flow rate appropriately. After calcination for 2h, the sample was cooled to room temperature, removed, and ground to obtain Nb4N5 catalyst powder.

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

[0058] Example 4

[0059] A Nb4N5-MgH2 composite hydrogen storage material and its preparation method are described below:

[0060] (1) Preparation of Nb4N5 catalyst: First, 1g of NbCl5 was dissolved in 1ml of anhydrous ethanol and magnetically stirred for 20min. Then, 10ml of 0.5mol / L ammonia solution was added to the above solution and magnetically stirred for 30min. The resulting precipitate was washed several times with deionized water and anhydrous ethanol. Finally, the precipitate was placed in a vacuum drying oven and dried at 60℃ for 10h to obtain niobium oxide precursor. Then, temperature-programmed nitriding was performed: The above precursor sample was placed in a ceramic crucible and placed in a tube furnace. Ammonia atmosphere was turned on, and the temperature was raised to 900℃ at a rate of 10℃ / min. When the temperature reached about 300℃, the flow meter was adjusted to increase the gas flow rate appropriately. After calcination for 3h / 4h, the sample was cooled to room temperature, removed, and ground to obtain Nb4N5 catalyst powder.

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

[0062] Example 5

[0063] A Nb4N5-MgH2 composite hydrogen storage material and its preparation method are described below:

[0064] (1) Preparation of Nb4N5 catalyst: First, 1g of NbCl5 was dissolved in 2ml of anhydrous ethanol and magnetically stirred for 10min. Then, 40ml of 0.3mol / L ammonia solution was added to the above solution and magnetically stirred for 30min. The resulting precipitate was washed several times with deionized water and anhydrous ethanol. Finally, the precipitate was placed in a vacuum drying oven and dried at 60℃ for 12h to obtain niobium oxide precursor. Then, temperature-programmed nitriding was performed: The above precursor sample was placed in a ceramic crucible and placed in a tube furnace. Ammonia atmosphere was turned on, and the temperature was raised to 900℃ at a rate of 10℃ / min. When the temperature reached about 300℃, the flow meter was adjusted to increase the gas flow rate appropriately. After calcination for 2h, the sample was cooled to room temperature, removed, and ground to obtain Nb4N5 catalyst powder.

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

[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 intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within 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, wherein Nb4N5 accounts for 3 wt.% to 9 wt.% by mass percentage. The preparation method of the composite hydrogen storage material includes the following steps: (1) Preparation of Nb4N5 catalyst: First, NbCl5 was dissolved in anhydrous ethanol and stirred until the solution was clear. Then, an appropriate amount of ammonia was added to the solution and stirred to obtain a colloidal precipitate. The precipitate was washed several times by centrifugation with deionized water and anhydrous ethanol. The mass-volume ratio of NbCl5, anhydrous ethanol and ammonia was (0.5~1):(0.5~5):(10~30), g:mL:mL. The reaction was stirred at a rate of 200~800 rpm for 10~60 min. Finally, the precipitate was dried in a vacuum drying oven to obtain the precursor. Then, temperature-programmed nitriding was performed. The precursor sample was placed in a ceramic crucible and placed in a tube furnace. After the ammonia atmosphere was turned on, it was calcined to obtain Nb4N5 catalyst powder. The temperature-programmed nitriding parameters were: heating at a rate of 2~10 ℃ / min to 600℃~900℃ and calcining for 2~8 h. (2) Preparation of Nb4N5-MgH2 composite material: Nb4N5 and MgH2 powders obtained in step (1) are mixed and Nb4N5-MgH2 composite material is prepared by mechanical ball milling.

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

3. The Nb4N5-MgH2 composite hydrogen storage material according to claim 1, characterized in that: In step (1), the centrifugal washing process is as follows: centrifuge and wash 3 to 5 times with deionized water and anhydrous ethanol at a speed of 3000 to 8000 rpm; the vacuum drying process is as follows: vacuum dry at 60 to 100℃ for 8 to 16 h.

4. The Nb4N5-MgH2 composite hydrogen storage material according to claim 1, characterized in that: In step (2), stainless steel balls are used in the mechanical ball mill, 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 carried out in both forward and reverse intermittent directions: after each ball milling for 5~30 min, the direction is reversed after a 5~30 min pause.

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