Co-B / CDNS composite material and preparation method and application thereof

Abstract

The invention discloses a Co-B / CDNS composite material which is prepared by the following steps: firstly, preparing carbon dot CDs by adopting a hydrothermal method, then freeze-drying to obtain solid CDs, then carrying out high-temperature sintering to form a carbon dot nanosheet CNS, and finally, loading Co-B particles by taking the CNS as a carrier and adopting a chemical reduction method to obtain Co-B / CNS. CDs is used as a precursor, and CNS is grown through splicing under high-temperature sintering; the CDNS is used as a carrier, and the microstructure of the carrier is of an amorphous lamellar structure; co-B serves as an active substance, the microstructure of the Co-B is in a nano particle shape, and the Co-B is loaded on the surface of the CDNS. The preparation method comprises the following steps: 1, preparing the CDNS; and 2, preparing the Co-B / CDNS. When the catalyst is used as a sodium borohydride hydrolysis hydrogen production catalyst, the maximum hydrogen production rate is 5000-5500 mL * min <-1 > * g <-1 > under the condition of 303 K, and the hydrogen desorption amount reaches 100% of a theoretical value; 72.4%-75.1% of the initial catalytic activity is reserved after the catalyst is recycled / repeatedly used for five times; and the activation energy Ea is equal to 39.5 to 40.6 kJ.mol <-1 >.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogen production by catalytic hydrolysis of sodium borohydride, and particularly relates to Co-B / CDNS composite materials, a preparation method thereof, and an application thereof. Background Art

[0002] Hydrogen production by hydrolysis of NaBH4 has the advantages of a theoretical hydrogen production density of up to 10.8 wt%, a low hydrogen release temperature, a controllable reaction process, high hydrogen purity, environmental friendliness, etc., and is considered to be one of the most promising hydrogen production methods. However, the hydrogen release rate of NaBH4 by self-hydrolysis at room temperature is very low and cannot meet the actual application requirements.

[0003] To solve the above problems, a common method is to add a catalyst to reduce the reaction activation energy and improve the hydrogen production efficiency. Common types of catalysts are noble metal catalysts. For example, simple substances or compounds containing elements such as ruthenium, platinum, rhodium, palladium, etc. Noble metal catalysts have high catalytic activity, but the high price of noble metals greatly limits their large-scale application in commerce.

[0004] Therefore, the current research direction tends to use non-noble metal catalysts, such as simple substances or compounds of cobalt, nickel, iron, manganese, etc. to improve the catalytic activity. For example, in the existing literature 1 (Jeong S U, Kim R K, Cho E A, et al. A study on hydrogen generation from NaBH4 solution using the high-performance Co-B catalyst[J]. Journal of Power Sources, 2005, 144(1): 129-134.), a Co-B catalyst was synthesized by a chemical reduction method, and its maximum hydrogen production rate was 1100 mL·min -1 ·g -1 . The fundamental reason for the poor catalytic activity of this technical solution is that the Co-B particles are in the form of aggregates, and the contact area with the NaBH4 solution during the reaction is small, resulting in the technical problem of a relatively small maximum hydrogen production rate.

[0005] Currently, to solve the problem of severe agglomeration that occurs when the above-mentioned catalyst is used, a method of introducing a suitable carrier material is usually adopted to enhance the hydrogen production performance and cyclic stability of the catalyst. Common carrier materials include carbon materials, metal-organic frameworks, graphene, etc. For example, the existing literature 2 (Xu JN, Du XX, Wei QL, et al. Efficient Hydrolysis of Sodium Borohydride by Co-B Supported on Nitrogen-doped Carbon[J]. ChemistrySelect, 2020, 5(22): 6683-6690.) synthesized Co-B / N-C catalyst by chemical purge method and impregnation-chemical reduction method, with a maximum hydrogen production rate of 2649 mL·min -1 ·g -1 , and the activation energy was 37.57 kJ·mol -1 . This experimental scheme reduces the agglomeration phenomenon of Co-B particles by introducing carbon as a carrier, increases the specific surface area of Co-B particles, and thus improves the catalytic efficiency of the catalyst. However, the problem with this technology is that Co-B particles are easily detached from the carrier, directly resulting in poor catalytic activity and cyclicity. The main reason for the easy detachment of Co-B particles on the carbon material is that the method adopted in this technical scheme is the chemical reduction method - the interaction between the Co-B particles prepared by this scheme and the carbon material is mainly physical adsorption or electrostatic interaction, that is, no strong chemical bond is formed, which directly leads to a weak interfacial interaction between Co-B particles and the carbon material. Therefore, when Co-B particles are subjected to external forces, they are easily detached from the surface of the carbon material.

[0006] Therefore, the current existing technologies have the following technical problems:

[0007] During the cyclic use of the catalyst for hydrogen production by hydrolysis of sodium borohydride, there are problems of agglomeration and detachment, which directly lead to a decrease in the catalytic activity of the catalyst and poor cyclicity. Summary of the Invention

[0008] The purpose of the present invention is to provide a Co-B / CDNS composite material, its preparation method and application.

[0009] To address the technical problems existing in the current technical scheme, the following principles and methods can be used for improvement:

[0010] Using CDs as a precursor, sintering to obtain Co-B particles supported on a CDNS carrier, improving the microscopic morphology of the catalyst, increasing the specific surface area of Co-B particles, increasing active sites, inhibiting the agglomeration of Co-B particles, thereby increasing the contact area between Co-B and NaBH4 and improving the catalytic performance.

[0011] To achieve the above-mentioned invention object, the technical solution adopted by the present invention is as follows:

[0012] A Co-B / CDNS composite material. First, carbon dots CDs are prepared by a hydrothermal method, and then solid CDs are obtained by freeze-drying. Then, carbon dot nanosheets CDNS are formed by high-temperature sintering. Finally, Co-B particles are loaded by a chemical reduction method using CDNS as a carrier to obtain a Co-B composite material supported by carbon dot nanosheet CDNS carrier, simply referred to as Co-B / CDNS;

[0013] The raw materials for preparing the CDs are citric acid monohydrate and ethylenediamine;

[0014] The raw materials for preparing the Co-B are cobalt sulfate heptahydrate and sodium borohydride;

[0015] The Co-B / CDNS is obtained by compounding Co-B and CDNS;

[0016] As a carrier, the microscopic morphology of the CDNS is a lamellar amorphous structure;

[0017] As an active substance, the microscopic morphology of the Co-B is nanoparticle-shaped and is loaded on the surface of the CDNS.

[0018] A preparation method of a Co-B / CDNS composite material includes the following steps:

[0019] Step 1, preparation of CDNS. First, citric acid monohydrate and ethylenediamine are placed in deionized water to obtain a mixture A. Secondly, the mixture A is stirred under certain conditions. After stirring, the mixture A is placed in a polytetrafluoroethylene inner liner reaction kettle and subjected to hydrothermal synthesis under certain conditions. Then, the obtained product is freeze-dried under certain conditions to obtain CDs. Finally, the CDs are subjected to high-temperature sintering under certain conditions to obtain CDNS;

[0020] In the step 1, the mass of citric acid monohydrate is 21.014 g, and the volume of ethylenediamine is 6.7 mL;

[0021] In the step 1, the stirring condition is that the stirring time of the mixture A is 30 min;

[0022] In the step 1, the hydrothermal synthesis condition is that the temperature is 200 °C and the time is 5 h;

[0023] In the step 1, the freeze-drying condition is that the temperature is -45 °C and the time is 144 h;

[0024] In the step 1, the high-temperature sintering condition is that the temperature is 600 °C and the heating rate is 5 °C·min -1, The heat preservation time is 2 h, and the gas atmosphere is argon;

[0025] Step 2, Preparation of Co-B / CDNS: First, place CDNS in deionized water to obtain mixture B, place cobalt sulfate heptahydrate in deionized water to obtain mixture C, stir mixture B and mixture C under certain conditions to obtain mixture D. At the same time, place sodium borohydride in deionized water to obtain solution E. Then, under stirring conditions, slowly add solution E to mixture D at certain conditions. After the addition is complete, stir and ultrasonicate under certain conditions to obtain mixture F. Finally, filter and wash the obtained product with deionized water, and perform vacuum drying under certain conditions to obtain the Co-B composite supported on the carbon dot nanosheet CDNS carrier, abbreviated as Co-B / CDNS;

[0026] In the said step 2, the mass of CDNS is 200 mg, and the molar ratio of cobalt sulfate heptahydrate to sodium borohydride is 1:10;

[0027] In the said step 2, the conditions for preparing mixture D are: the dropping rate is 0.05 mL / s, and mixture C is added to mixture B; the stirring time is 12 h;

[0028] In the said step 2, the conditions for preparing mixture F are: the dropping conditions are that the dropping rate is 0.02 mL / s; the stirring conditions after the addition is complete are that the stirring time is 3 h; the ultrasonication conditions are that the ultrasonication time is 30 min;

[0029] In the said step 2, the drying conditions are: the drying temperature is 60 °C, and the drying time is 12 h.

[0030] When a Co-B / CDNS composite material is used as a catalyst for sodium borohydride hydrolysis to produce hydrogen, the maximum hydrogen production rate is 5000 - 5500 mL·min -1 ·g -1 at 303 K, and the hydrogen release amount reaches 100% of the theoretical value; after 5 times of recycling / reusing, 72.4 - 75.1% of the initial catalytic activity is retained;

[0031] The activation energy for catalytic hydrogen release is E a = 39.5 - 40.6 kJ·mol -1 .

[0032] The technical effects of the present invention can be known through the following detections:

[0033] It can be known through SEM detection that the average particle size of the CDs is 7 nm.

[0034] It can be known through UV-Vis detection that a strong absorption peak is observed at 341 nm, which can be attributed to the n→π of C=O *Electronic transition.

[0035] It can be known from fluorescence spectrum detection that: CDs have the best excitation and emission wavelengths at 377 nm and 455 nm, and show bright blue under a handheld ultraviolet lamp. Combining the results of SEM, UV-Vis, fluorescence spectrum test and ultraviolet lamp test shows that: CDs are successfully synthesized.

[0036] It can be known from XRD detection that: CDs and CDNS each have a diffraction peak near 22° and 24°, which is related to the characteristic (002) crystal plane of typical carbon materials. In addition, the diffraction peak intensities of CDs and CDNS are relatively low, indicating the generation of a disordered layered structure, which means that the skeletons of CDs and CDNS are mainly composed of randomly oriented microcrystals distributed throughout the carbon matrix. No obvious Co-B diffraction peak was observed for Co-B / CDNS, indicating that Co-B is in an amorphous state.

[0037] It can be known from SEM detection that: The basic microscopic morphology of Co-B / CDNS remains the same sheet-like structure as CDNS. The difference is that the surface of Co-B / CDNS becomes rough and is loaded with nanoparticle-like substances.

[0038] It can be known from EDS detection that: Co-B / CDNS contains elements C, O, N, Co and B. Combining the XRD and EDS test results can prove the successful loading of Co-B / CDNS, and Co-B is amorphous;

[0039] It can be known from hydrogen production by hydrolysis detection that the maximum hydrogen production rate of sodium borohydride is 5000 - 5500 mL·min -1 ·g -1 -1 at 303 K, and the hydrogen release amount reaches 100% of the theoretical value;

[0040] It can be known from the detection of reaction kinetic performance that: the apparent activation energy E a of the reaction is 39.5 - 40.6 kJ·mol -1 -1;

[0041] It can be known from the detection of cyclic performance that: after cycling 5 times at 303 K, it still retains 72.4 - 75.1% of its initial catalytic activity for the hydrolysis of NaBH4.

[0042] Therefore, the present invention has the following advantages:

[0043] 1. The present invention uses CDs as a precursor, and Co-B particles are loaded on the CDNS carrier by sintering, which improves the microscopic morphology of the material, enhances the dispersibility of Co-B, inhibits its agglomeration, and thus increases the contact area between Co-B and NaBH4, improving the catalytic performance;

[0044] 2. The present invention uses a CDNS carrier to load Co-B particles, increasing the specific surface area of Co-B and exposing more active sites, thereby enhancing the hydrogen production rate of the catalyst.

[0045] 3. The present invention uses a non-noble metal Co instead of a noble metal as the catalyst. Loaded by the CDNS carrier, it not only increases the hydrogen production rate but also reduces the catalytic cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 SEM image of CDs in Example 1, and the inset is the particle size distribution histogram of CDs;

[0047] Figure 2 UV-Vis and fluorescence spectra of CDs in Example 1, and the inset shows the photos of CDs in aqueous solution under ultraviolet (left) and visible light (right);

[0048] Figure 3 XRD patterns of Co-B, CDs, CDNS, and Co-B / CDNS composites in Example 1;

[0049] Figure 4 SEM image of CDNS in Example 1;

[0050] Figure 5 SEM image of Co-B / CDNS in Example 1;

[0051] Figure 6 Hydrogen production diagram of the hydrolysis of sodium borohydride catalyzed by CDNS and Co-B / CDNS at 303K in Example 1;

[0052] Figure 7 EDS diagram of Co-B / CDNS in Example 1;

[0053] Figure 8 Hydrogen production diagram of the hydrolysis of sodium borohydride catalyzed by Co-B / CDNS at different temperatures in Example 1;

[0054] Figure 9 Activation energy diagram of Co-B / CDNS in Example 1;

[0055] Figure 10 Cyclic performance diagram of Co-B / CDNS for 5 cycles of hydrogen evolution from the hydrolysis of sodium borohydride at 303K in Example 1;

[0056] Figure 11 Comparison diagram of hydrogen evolution from the hydrolysis of sodium borohydride catalyzed by Example 1 and Comparative Example 1 at 303K;

[0057] Figure 12 SEM image of Co-B in Comparative Example 1. Detailed implementation mode

[0058] The present invention will be further described in detail in combination with the accompanying drawings of the specification through examples, but it is not a limitation of the present invention.

[0059] Example 1

[0060] A preparation method of Co-B / CDNS composite material comprises the following specific steps:

[0061] Step 1, preparation of CDNS. First, 21.014 g of citric acid monohydrate and 6.7 mL of ethylenediamine are placed in 200 mL of deionized water to obtain a mixture A. Secondly, the mixture A is stirred for 30 min. After stirring, the mixture A is placed in a polytetrafluoroethylene inner liner reactor and subjected to hydrothermal synthesis at a temperature of 200 °C and a heat preservation time of 5 h. Then, the obtained product is freeze-dried at a temperature of -45 °C and a freeze-drying time of 144 h to obtain solid CDs. Finally, the CDs are subjected to high-temperature sintering at a temperature of 600 °C, a heating rate of 5 °C·min -1 , a heat preservation time of 2 h, and an argon gas atmosphere to obtain CDNS;

[0062] To prove the successful synthesis of CDs obtained in step 1, SEM, UV-Vis, fluorescence spectrum tests, ultraviolet lamp and sunlight tests are carried out. The test results are as Figure 1-2 shown. The average particle size of CDs is 7 nm; a strong absorption peak is observed at 341 nm for CDs, attributed to the n→π * electronic transition of C=O; CDs have the best excitation and emission wavelengths at 377 nm and 455 nm and show bright blue under a handheld ultraviolet lamp. The test results show that the CDs are successfully synthesized.

[0063] To prove the crystal structures of CDs and CDNS obtained in step 1, XRD tests are carried out. The test results are as Figure 3 shown. CDs and CDNS have a diffraction peak near 22° and 24° respectively, which is related to the characteristic (002) crystal plane of typical carbon materials. In addition, the diffraction peak intensities of CDs and CDNS are low, indicating the generation of a disordered layered structure, which means that the skeletons of CDs and CDNS are mainly composed of randomly oriented microcrystals distributed in the entire carbon matrix.

[0064] To prove the microscopic morphology of CDNS obtained in step 1, SEM tests are carried out. The test results are as Figure 4 shown. CDNS is an amorphous sheet-like structure.

[0065] To prove the hydrogen production performance of the CDNS obtained in Step 1 as a catalyst for sodium borohydride hydrolysis, a hydrogen production test by hydrolysis was carried out. The specific method of the test was as follows: The hydrogen production test was carried out under the temperature conditions set in the experiment. A solution containing 1.5 wt.% NaBH4 and 5 wt.% NaOH was placed in a constant temperature water bath. After reaching equilibrium at the aforementioned set temperature, 10 mL was taken out and added to a wide-mouth bottle containing the catalyst. The volume of hydrogen generated per unit time was collected by the water displacement method and recorded, and thus the hydrogen release rate could be obtained. Among them, when the temperature conditions were not specifically stated, it was 303 K.

[0066] The hydrogen production test results of CDNS are as Figure 6 shown. Under the temperature condition of 303 K, the hydrogen production rate was 0 mL·min -1 ·g -1 , indicating that CDNS has no catalytic activity.

[0067] Step 2, Preparation of Co-B / CDNS. First, 200 mg of CDNS was placed in deionized water to obtain mixture B, and 562.2 mg of cobalt sulfate heptahydrate was placed in deionized water to obtain mixture C. Under stirring conditions, solution C was slowly added dropwise to mixture B at a dropping rate of 0.05 mL / s to obtain mixture D. After the dropping was completed, stirring was carried out for 12 h. At the same time, 760 mg of sodium borohydride was placed in deionized water to obtain solution E. Then, under stirring conditions, solution E was slowly added dropwise to mixture D at a dropping rate of 0.02 mL / s to obtain mixture F. After the dropping was completed, stirring was carried out for 3 h and ultrasonic treatment was carried out for 30 min. Finally, the obtained product was filtered and washed with deionized water, and vacuum dried at a drying temperature of 60 °C and a drying time of 12 h, and thus a composite material of Co-B loaded on a carbon dot nanosheet CDNS carrier, abbreviated as Co-B / CDNS, could be obtained;

[0068] To prove the composition of the Co-B / CDNS obtained in Step 2, an XRD test was carried out. The test results are as Figure 3 shown. No obvious Co-B diffraction peaks were observed in Co-B / CDNS, indicating that Co-B is in an amorphous state.

[0069] Since the characteristic peaks of Co-B were not detected, the composition of Co-B / CDNS was further confirmed by an EDS test. The EDS test results are as Figure 7 shown. Co-B / CDNS has C element, O element, N element, Co element, and B element.

[0070] Combining the XRD and EDS test results can prove that Co-B particles were successfully loaded, and Co-B is amorphous.

[0071] To prove the microscopic morphology of Co-B / CDNS obtained in step 2, SEM tests were carried out. The SEM test results are as Figure 5 shown. The basic microscopic morphology of Co-B / CDNS remains an amorphous structure with a lamellar shape, similar to that of CDNS. The difference is that Co-B particles are attached to the surface of CDNS through step 2.

[0072] To prove the hydrogen production performance of Co-B / CDNS obtained in step 2 as a catalyst for sodium borohydride hydrolysis to produce hydrogen, hydrogen production performance tests were carried out. The hydrolysis hydrogen production test results of Co-B / CDNS are as Figure 6 shown. At a temperature of 303K, the maximum hydrogen production rate is 5262 mL·min -1 ·g -1 , and the hydrogen release amount reaches 100% of the theoretical value. Comparing with the test results of CDNS obtained in step 1, it can be seen that the Co-B loaded in step 2 is the active substance, which directly catalyzes the hydrolysis of sodium borohydride to produce hydrogen. At the same time, it is proved that CDNS only acts as a carrier in the technical solution and has no catalytic performance.

[0073] To prove the reaction kinetic performance of Co-B / CDNS, hydrolysis hydrogen production tests were carried out at temperatures of 303K, 313K, 323K, and 333K respectively. The test results were fitted by the Arrhenius equation as shown in Table 1 and Figure 9 shown. The apparent activation energy of the reaction Ea = 39.9 kJ·mol -1 . At the same time, in order to compare with the existing technology, the data of the reference literature are summarized in Table 1.

[0074] Table 1 Hydrogen production rate and activation energy of different catalysts for catalyzing the hydrolysis of NaBH4

[0075]

[0076] Note: The reference numbers in Table 1 correspond to the following literature:

[0077] [1]Li Q M,Yang W,Li F,et al.Preparation of CoB / ZIF-8supportedcatalyst by single step reduction and its activity in hydrogen production[J].International Journal of Hydrogen Energy,2018,43(1):271-282.

[0078] [2] Peng C L, Li T S, Zou Y J, et al. Bacterial cellulose derived carbon as a support for catalytically active Co-B alloy for hydrolysis of sodium borohydride[J]. International Journal of Hydrogen Energy, 2021, 46(1): 666-675.

[0079] [3] Li M B, Guan S Y, An L L, et al. Protection and confinement effect of carbon on Co / Co x O y nano-catalyst for efficient NaBH4 hydrolysis[J]. International Journal of Hydrogen Energy, 2022, 47(46): 20185-20193.

[0080] [4] Jiang J, Yang S Y, Lei H L, et al. Highly dispersed cobalt nanoparticles onto nitrogen-doped carbon nanosheets for efficient hydrogen generation via catalytic hydrolysis of sodium borohydride[J]. International Journal of Hydrogen Energy, 2021, 46(64): 32403-32412.

[0081] Based on the above test results and comparative analysis with existing references, the following conclusions can be preliminarily drawn:

[0082] 1. For References [1] and [2], with the same loading of Co-B, different carriers can also have a significant impact on the maximum production rate;

[0083] The present invention is also based on the same load Co-B, using CDNS as the carrier. Compared with the cited references [1] and [2], it can not only significantly improve the maximum hydrogen production rate, but also has a significant effect on improving the reaction kinetic performance, that is, the activation energy, which proves that the carrier CDNS of the present invention can significantly improve the reaction thermodynamic performance and reaction kinetic performance.

[0084] 2. It can be seen from the comparison of reference [3] and reference [4] that based on similar carbon materials as carriers, loading different Co and its compounds can have a significant impact on the catalytic performance.

[0085] Based on CDNS as the carrier and Co-B as the load, the present invention can further significantly improve the maximum hydrogen production rate, with the improvement amplitudes reaching 21% and 104.4% respectively compared with reference [3] and reference [4].

[0086] Based on the above analysis, compared with the prior art, the catalytic performance of the present invention is significantly improved. The reasons are as follows:

[0087] 1. The present invention uses CDNS as the carrier, which increases the specific surface area of Co-B particles, obtains more active sites, and thus effectively improves the hydrogen production rate.

[0088] 2. The present invention uses CDNS as the carrier. While improving the loading uniformity of Co-B particles, it also inhibits the agglomeration of Co-B particles, that is, improves the dispersion degree of Co-B particles, making the change of catalyst activity with temperature very small, and thus effectively reducing the reaction activation energy.

[0089] In order to prove the role of Co-B in the technical solution, Comparative Example 1 is provided, where Co-B is prepared alone as the catalyst.

[0090] Comparative Example 1

[0091] A preparation method of Co-B, the steps not specifically described are the same as those in Example 1, except that: step 1 is not carried out, and in step 2, the CDNS obtained in step 1 is not added, and the material obtained in step 2 is Co-B.

[0092] The SEM test results of Co-B are as Figure 12 shown. Co-B is in the form of grain-like aggregates. Combining with the SEM test results of Co-B / CDNS, it can be seen that introducing CDNS as the carrier can change and adjust the microstructure of the composite material, obtaining the technical effect of avoiding agglomeration, which proves the role of CDNS in the technical solution.

[0093] The test results of Co-B are as Figure 11 shown. Under the temperature condition of 303K, the maximum hydrogen production rate is 882 mL·min-1 ·g -1 The hydrogen release amount reaches 100% of the theoretical value. Combining with the test results of Co-B / CDNS, it can be seen that after introducing CDNS, the maximum hydrogen release rate can be significantly increased, and the increase amplitude reaches 496.6%, which proves the role of CDNS in the technical solution.

[0094] It can be seen from the comparison between Comparative Example 1 and Example 1 that although Co-B is the active substance and directly using Co-B as the catalyst can achieve the basic function of catalyzing the hydrolysis of sodium borohydride, however, when used alone as the catalyst, agglomeration will occur, seriously reducing the maximum hydrogen release rate. Furthermore, it is proved that by using CDNS as the carrier, the microscopic morphology of the composite material can be directly adjusted, the agglomeration phenomenon can be inhibited, the specific surface area can be significantly increased, and thus the catalytic hydrogen release rate can be significantly increased, which proves the role of CDNS in the technical solution.

Claims

1. A Co-B / CDNS composite material, characterized in that: First, carbon dots CDs were prepared by a hydrothermal method, and then solid CDs were obtained by freeze-drying. Then, carbon dot nanosheets CDNS were formed by high-temperature sintering. Finally, Co-B particles were loaded by a chemical reduction method using CDNS as a carrier to obtain a carbon dot nanosheet CDNS-supported Co-B composite material, simply referred to as Co-B / CDNS; The raw materials for preparing the CDs were citric acid monohydrate and ethylenediamine; The raw materials for preparing the Co-B were cobalt sulfate heptahydrate and sodium borohydride.

2. The Co-B / CDNS composite material according to claim 1, wherein: The Co-B / CDNS was obtained by compounding Co-B and CDNS, As a carrier, the CDNS had a lamellar amorphous structure in its microscopic morphology; As an active substance, the Co-B had a nanoparticle-like microscopic morphology and was loaded on the surface of the CDNS.

3. A method for preparing a Co-B / CDNS composite material, characterized in that It included the following steps: Step 1, Preparation of CDNS. First, citric acid monohydrate and ethylenediamine were placed in deionized water to obtain mixture A. Second, mixture A was stirred under certain conditions. After stirring, mixture A was placed in a polytetrafluoroethylene inner-liner reaction kettle and subjected to hydrothermal synthesis under certain conditions. Then, the obtained product was freeze-dried under certain conditions to obtain solid CDs. Finally, the CDs were subjected to high-temperature sintering under certain conditions to obtain CDNS; Step 2, Preparation of Co-B / CDNS. First, CDNS was placed in deionized water to obtain mixture B, and cobalt sulfate heptahydrate was placed in deionized water to obtain mixture C. Mixture B and mixture C were stirred under certain conditions to obtain mixture D. At the same time, sodium borohydride was placed in deionized water to obtain solution E. Then, under stirring conditions, solution E was slowly dropped into mixture D at certain conditions. After dropping, stirring and ultrasonic treatment were carried out at certain conditions to obtain mixture F. Finally, the obtained product was filtered and washed with deionized water and vacuum-dried under certain conditions to obtain a carbon dot nanosheet CDNS-supported Co-B composite material, simply referred to as Co-B / CDNS.

4. The preparation method according to claim 3, characterized in that: In step 1, the mass of citric acid monohydrate was 21.014 g, and the volume of ethylenediamine was 6.7 mL; In step 2, the mass of CDNS was 200 mg, and the molar ratio of cobalt sulfate heptahydrate to sodium borohydride was 1:

10.

5. The preparation method according to claim 3, characterized in that: In step 1, the stirring conditions were that the stirring time of mixture A was 30 min; In step 1, the hydrothermal synthesis conditions were a temperature of 200 °C and a time of 5 h; In step 1, the freeze-drying conditions were a temperature of -45 °C and a time of 144 h; In the said step 1, the conditions for high-temperature sintering are as follows: the temperature is 600 °C, the heating rate is 5 °C·min -1 , the holding time is 2 h, and the gas atmosphere is argon.

6. The preparation method according to claim 3, characterized in that: In step 2, the conditions for preparing mixture D were a dropping rate of 0.05 mL / s, and mixture C was added to mixture B; the stirring time was 12 h; In step 2, the conditions for preparing mixture F were that the dropping conditions were a dropping rate of 0.02 mL / s; the stirring conditions after dropping were a stirring time of 3 h; the ultrasonic conditions were an ultrasonic time of 30 min; In step 2, the drying conditions were a drying temperature of 60 °C and a drying time of 12 h.

7. A Co-B / CDNS composite material, characterized in that: When used as a catalyst for hydrogen production by sodium borohydride hydrolysis, the maximum hydrogen production rate is 5000 - 5500 mL·min -1 ·g -1 at 303K, and the hydrogen release amount reaches 100% of the theoretical value; after 5 recycling / reuse, 72.4 - 75.1% of the initial catalytic activity is retained.

8. A Co-B / CDNS composite material, characterized in that: When used as a catalyst for hydrogen production by sodium borohydride hydrolysis, the activation energy for catalytic hydrogen release is E a = 39.5 - 40.6 kJ·mol -1 .

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