Metal borohydride composite material, preparation method thereof and hydrogen storage material

Abstract

The invention belongs to the technical field of hydrogen storage materials, and particularly relates to a metal borohydride composite material, a preparation method thereof and a hydrogen storage material. The preparation method comprises the following steps: activating the carbon-based carrier by adopting an alkaline substance; performing gas-phase fluorination treatment on the activated carbon-based carrier; the carbon fluoride-based carrier is mixed with a metal source, a boron source and a solvent, a solvothermal reaction is driven by hydrogen, metal borohydride is synthesized in situ on the surface of the carbon fluoride-based carrier, metal atoms in the generated metal borohydride react with F atoms in the carbon fluoride-based carrier to form a metal-F bridging phase, and meanwhile, the metal borohydride generates metal vacancies. The metal affinity of fluorine in the carbon fluoride-based carrier is utilized to induce nucleation loading of the metal borohydride in the synthesis process of the metal borohydride, and meanwhile, an interface chemical reaction is carried out to generate vacancies and bridging phases. Vacancies weaken B-H bonds, the hydrogen absorption and desorption heat / dynamics of the material are improved, and the bridging phase improves the structural stability and hydrogen absorption and desorption cycle performance of the material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hydrogen storage materials, and particularly relates to a metal borohydride composite material and a preparation method thereof, and a hydrogen storage material. BACKGROUND

[0002] Metal borohydrides such as LiBH4 are a kind of high-capacity solid-state hydrogen storage materials with great prospects, but there are still problems such as high thermal stability and slow kinetics in practical application. Traditional thermodynamic modification technologies of metal borohydrides such as LiBH4 include anion / cation doping or hydride compounding, which all depend on the introduction of external elements and substances, not only completely changing the chemical composition and hydrogen release path of metal borohydrides, but also reducing the hydrogen capacity and reversibility. The nanocrystallization method can simultaneously regulate the hydrogen storage reaction heat / kinetics of metal borohydrides through size effect. At present, common nanometer construction is to load metal borohydrides on carriers with high surface area, such as graphene, to obtain high hydrogen loading and stable hydrogen storage performance. However, as inert carbon-based carriers, graphene and the like are difficult to change the intrinsic properties of metal borohydrides, resulting in limited performance improvement. In addition, the current loading structure is difficult to completely avoid the detachment and agglomeration of metal borohydride nanoparticles, leading to cycle performance degradation.

[0003] Therefore, it is necessary to synergistically design the functionalization and structuring of the traditional carrier to strengthen the chemical modification effect and structural stability of the carrier, and optimize the heat / kinetics and cycle life of the nanoscale metal borohydride. This has important significance for the development and application of high-performance hydrogen storage materials. SUMMARY

[0004] The application aims to provide a metal borohydride composite material and a preparation method thereof, and a hydrogen storage material, and aims to solve the problems of poor stability of the existing metal borohydride in the carrier, easy agglomeration, and influence on the heat / kinetics and cycle life of the metal borohydride to some extent.

[0005] To achieve the above application purposes, the technical solutions adopted by the application are as follows: In a first aspect, the application provides a preparation method of a metal borohydride composite material, comprising the following steps: Obtaining a carbon-based carrier, activating the carbon-based carrier by using an alkaline substance to obtain an activated carbon-based carrier; Carrying out gas-phase fluorination treatment on the activated carbon-based carrier to obtain a fluorinated carbon-based carrier; Mixing the fluorinated carbon-based carrier with a metal source, a boron source and a solvent, in-situ synthesizing a metal borohydride on the surface of the fluorinated carbon-based carrier by a solvent thermal reaction driven by hydrogen, and forming a metal-F bridging phase by the reaction between metal atoms in the generated metal borohydride and F atoms in the fluorinated carbon-based carrier, while generating metal vacancies in the metal borohydride, to obtain a metal borohydride composite material.

[0006] In some possible implementation manners, the molar content of the metal vacancies in 1 mol of the metal borohydride is 0.1 mol to 0.2 mol.

[0007] In some possible implementation manners, in the metal borohydride composite material, the average particle size of the loaded metal borohydride is 50 nm to 150 nm.

[0008] In some possible implementation manners, in the metal borohydride composite material, the mass percentage content of the loaded metal borohydride is 50% to 80%.

[0009] In some possible implementation manners, in the metal borohydride composite material, the molar ratio of the metal borohydride to the metal-F bridging phase is (0.8-0.9) mol:(0.1-0.2) mol.

[0010] In some possible implementation manners, the carbon-based carrier includes at least one of graphite oxide, activated carbon, carbon nanotubes and biomass-derived carbon.

[0011] In some possible implementation manners, the alkaline substance includes at least one of KOH and NaOH.

[0012] In some possible implementation manners, the step of the activation treatment includes: placing the carbon-based carrier in a solution of the alkaline substance with a concentration of 5 mol / L to 7 mol / L, stirring for 3 h to 5 h, standing for 10 h to 15 h, and then separating, drying the precipitate and treating the precipitate in an inert atmosphere at a temperature of 700℃ to 900℃ for 1 h to 3 h, and then washing and drying to obtain the activated carbon-based carrier.

[0013] In some possible implementation manners, in the activated carbon-based carrier, the average size of the pore size is 1 nm to 5 nm.

[0014] In some possible implementation manners, the step of the gas-phase fluorination treatment includes: performing gas-phase fluorination treatment on the activated carbon-based carrier in a fluorine-containing atmosphere with a temperature of 25℃ to 180℃ and a fluorine volume percentage content of 5% to 8% for 15 min to 60 min to obtain the fluorinated carbon-based carrier.

[0015] In some possible implementation manners, the fluorocarbon-based carrier contains 20% to 30% of fluorine atoms.

[0016] In some possible implementation manners, the metal source includes at least one of a lithium source, a potassium source, a magnesium source, and a calcium source.

[0017] In some possible implementation manners, the boron source includes triethylamine borane.

[0018] In some possible implementation manners, the solvent includes at least one of n-hexane, cyclohexane, and tetrahydrofuran.

[0019] In some possible implementation manners, the mass ratio of the metal source to the boron source is (1 to 3):(0.5 to 0.7).

[0020] In some possible implementation manners, the conditions of the solvent thermal reaction driven by hydrogen include that, under the conditions of a hydrogen pressure of 35 bar to 55 bar and a temperature of 80°C to 100°C, the reaction is performed for 10 h to 20 h.

[0021] In a second aspect, the present application provides a metal borohydride composite material, including a fluorocarbon-based carrier with a porous structure and a metal borohydride loaded in the fluorocarbon-based carrier, the metal borohydride having metal vacancies, and the metal atoms in the metal borohydride forming a metal-F bridging phase with fluorine atoms in the fluorocarbon-based carrier.

[0022] In some possible implementation manners, the fluorocarbon-based carrier includes fluorinated graphene.

[0023] In some possible implementation manners, the metal borohydride includes LiBH4, the molar content of lithium vacancies in 1 mol of LiBH4 is 0.1 mol to 0.2 mol, and the metal-F bridging phase includes a LiF bridging phase.

[0024] In some possible implementation manners, in the metal borohydride composite material, the average particle size of the loaded metal borohydride is 50 nm to 150 nm.

[0025] In some possible implementation manners, in the metal borohydride composite material, the mass percentage content of the loaded metal borohydride is 50% to 80%, and the molar ratio of the metal borohydride to the metal-F bridging phase is (0.8 to 0.9) mol:(0.1 to 0.2) mol.

[0026] In a third aspect, the present application provides a hydrogen storage material, including the metal borohydride composite material prepared by the above method and / or the metal borohydride composite material.

[0027] The preparation method of the metal borohydride composite material provided in the first aspect of the present application is as follows: a carbon fluoride-based carrier is prepared, the fluorine element in the carrier has a gold affinity, and in the process of the solvothermal synthesis of the metal borohydride, the metal borohydride is induced to nucleate and load at the fluorine site while an interface chemical reaction occurs, a metal borohydride composite material with metal vacancies and a metal-F bridging phase is generated. The metal vacancies generated in the metal borohydride can effectively weaken the internal electronic interaction and B-H bonding of the metal borohydride, improve the thermodynamics and kinetics of the hydrogen absorption and release reaction of the metal borohydride, realize low-temperature hydrogen release, and significantly improve the reversible hydrogen absorption and release performance of the metal borohydride. The metal-F bridging phase can improve the loading stability of the metal borohydride in the carbon fluoride-based carrier and improve the structural stability of the metal borohydride, thereby significantly improving the hydrogen absorption and release cycle performance of the metal borohydride composite material.

[0028] The metal borohydride composite material provided in the second aspect of the present application is prepared by loading a metal borohydride with metal vacancies in a porous carbon fluoride-based carrier and constructing a stable interface chemical connection by means of a metal-F bridging phase formed between metal atoms and fluorine atoms of the carrier, while realizing a synergistic improvement in material structural stability and hydrogen storage performance. The bridging phase effectively fixes the active substance, significantly enhancing the cycle stability; and the metal vacancies in the metal borohydride lattice weaken the B-H bond and metal-boron interaction, significantly optimizing the thermodynamics and kinetics of the material's hydrogen absorption and release, and ultimately enabling the composite material to have both high reversible capacity and long cycle life.

[0029] The third aspect of the present application applies the metal borohydride composite material prepared by the aforementioned method and / or the metal borohydride composite material with the aforementioned specific structure to a hydrogen storage material, so that the hydrogen storage material simultaneously has low-temperature hydrogen release characteristics, fast reaction kinetics, and excellent cycle stability, providing an ideal material solution for the development of a new generation of high-performance solid-state hydrogen storage systems. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0031] Figure 1 is a flowchart of the preparation method of the metal borohydride composite material provided in the embodiments of the present application; Figure 2 is an X-ray diffraction pattern of the metal borohydride composite material provided in Embodiment 1 of the present application; Figure 3is a scanning electron microscope image of the metal borohydride composite provided in Example 1 of the present application 11 B nuclear magnetic resonance spectrum; Figure 4 is a scanning electron microscope image (a), a transmission electron microscope image (b), an EDS element distribution map (c), and a high-resolution transmission electron microscope image (d) of the metal borohydride composite provided in Example 1 of the present application; Figure 5 is a scanning electron microscope image of the metal borohydride composite provided in Example 2 of the present application; Figure 6 is a scanning electron microscope image of the metal borohydride composite provided in Example 3 of the present application; Figure 7 is a temperature-dependent hydrogen release curve (a) and an isothermal hydrogen release curve (b) of the metal borohydride composite provided in Example 1 of the present application; Figure 8 is a temperature-dependent hydrogen release curve of the metal borohydride composite provided in Example 2 of the present application; Figure 9 is a temperature-dependent hydrogen release curve of the metal borohydride composite provided in Example 3 of the present application; Figure 10 is a differential scanning calorimetry curve (a) and a Kissinger fitting graph (b) of the metal borohydride composite provided in Example 1 of the present application; Figure 11 is a photograph after mechanical compression and an isothermal hydrogen release curve of the metal borohydride composite provided in Example 1 of the present application; Figure 12 is an isothermal hydrogen absorption curve (a) and a cycle curve (b) of the metal borohydride composite provided in Example 1 of the present application; Figure 13 is an X-ray diffraction pattern (a) and an 11B nuclear magnetic resonance spectrum (b) of the first hydrogen release product of the metal borohydride composite provided in Example 1 of the present application; Figure 14 is a scanning electron microscope image of the first hydrogen release product of the metal borohydride composite provided in Example 1 of the present application; Figure 15 is a Fourier transform infrared spectroscopy graph after 10 and 30 hydrogen absorptions of the metal borohydride composite provided in Example 1 of the present application; Figure 16 is a scanning electron microscope image of the metal borohydride composite provided in Comparative Example 1 of the present application; Figure 17 is a scanning electron microscope image of the metal borohydride composite provided in Comparative Example 2 of the present application; Figure 18Temperature-dependent hydrogen desorption curve (a) and differential scanning calorimetry curve (b) of the metal borohydride composite provided by Comparative Example 2 of the present application; Figure 19 Isothermal hydrogen desorption curves of the metal borohydride composite provided by Comparative Example 2 of the present application before and after mechanical compression; Figure 20 Isothermal hydrogen desorption curves of the metal borohydride composite provided by Comparative Example 3 of the present application before and after mechanical compression; Figure 21 Cyclic hydrogen desorption curve of the metal borohydride composite provided by Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0032] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects clearer and more apparent, the present application will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present application, and not intended to limit the present application.

[0033] In the present application, the term "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B can mean that A exists alone, A and B exist together, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after it.

[0034] In the present application, "at least one" means one or more, and "multiple" means two or more. "At least one of the following" or similar expressions means any combination of these items, including any combination of single item or multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can mean a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, wherein a, b and c can be single or multiple.

[0035] It should be understood that in various embodiments of the present application, the size of the sequence number of the above processes does not mean the order of execution, and part or all of the steps can be executed in parallel or in sequence, and the execution order of the processes should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0036] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0037] The weight of the related components mentioned in the embodiments of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of each component. Therefore, as long as the content of the related components in the embodiments of the present application is proportionally enlarged or reduced, it is within the scope disclosed by the embodiments of the present application. Specifically, the mass mentioned in the embodiments of the present application can be µg, mg, g, kg and other mass units commonly known in the chemical field.

[0038] The terms "first", "second" are only used for descriptive purposes and are used to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be referred to as the second XX, and similarly, the second XX can also be referred to as the first XX. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features.

[0039] The first aspect of the embodiments of the present application provides a preparation method of a metal borohydride composite material, as shown in the accompanying drawings, comprising the following steps: Figure 1 S10. Obtain a carbon-based carrier, activate the carbon-based carrier with an alkaline substance to obtain an activated carbon-based carrier; S20. Perform gas-phase fluorination treatment on the activated carbon-based carrier to obtain a fluorinated carbon-based carrier; S30. Mix the fluorinated carbon-based carrier with a metal source, a boron source and a solvent, drive the solvent to react by hydrogen, in-situ synthesize metal borohydride on the surface of the fluorinated carbon-based carrier, and form a metal-F bridging phase by the reaction between metal atoms in the generated metal borohydride and F atoms in the fluorinated carbon-based carrier, while making the metal borohydride generate metal vacancies, to obtain a metal borohydride composite material.

[0040] The preparation method of the metal borohydride composite material provided by the first aspect of the embodiments of the present application prepares a fluorinated carbon-based carrier, the fluorine element in the carrier has a gold affinity, and in the process of solvothermal synthesis of the metal borohydride, it induces nucleation and loading at the fluorine site while an interface chemical reaction occurs, generating a metal borohydride with metal vacancies and a metal-F bridging phase, and a metal borohydride composite material is prepared. The metal vacancies generated in the metal borohydride can effectively weaken the internal electronic interaction and B-H bond of the metal borohydride, improve the thermodynamics and kinetics of the hydrogen absorption and desorption reaction of the metal borohydride, realize low-temperature hydrogen desorption, and significantly improve the reversible hydrogen absorption and desorption performance of the metal borohydride. The metal-F bridging phase can improve the loading stability of the metal borohydride in the fluorinated carbon-based carrier and improve the structural stability of the metal borohydride, thereby significantly improving the hydrogen absorption and desorption cycle performance of the metal borohydride composite material.

[0041] ​In some possible implementations, the molar content of metal vacancies in 1 mol of metal borohydride is 0.1 mol to 0.2 mol. In this case, the metal borohydride is rich in metal vacancies, and the content of vacancies can effectively disturb the lattice electron structure, significantly weaken the B-H bond and metal-boron interaction, thereby successfully reducing the hydrogen release temperature of the material and greatly improving the hydrogen absorption and release kinetics. At the same time, the vacancy concentration cooperates with the stable metal-F bridge to ensure the structural integrity and load stability of the composite material during the cycle process, and ultimately together creates its excellent comprehensive performance of low-temperature hydrogen release, fast reaction kinetics and excellent cycle life.

[0042] For example, the molar content of metal vacancies in 1 mol of metal borohydride can be 0.1 mol, 0.12 mol, 0.14 mol, 0.15 mol, 0.16 mol, 0.18 mol, 0.2 mol, and the like, typical but non-limiting any point value or interval value between any two point values.

[0043] In some possible implementations, in the metal borohydride composite material, the average particle size of the loaded metal borohydride is 50 nm to 150 nm. In this case, the nanoscale particle size means that the diffusion path of hydrogen atoms or molecules inside the solid phase is significantly shortened. At the same time, the huge specific surface area provides a large number of surface reaction active sites, so that the hydrogen absorption and release chemical reactions can be carried out more quickly and more fully. This complements the improvement of metal vacancies on kinetics, and together realizes the fast hydrogen charging and discharging performance of the material.

[0044] For example, in the metal borohydride composite material, the average particle size of the loaded metal borohydride can be 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, and the like, typical but non-limiting any point value or interval value between any two point values.

[0045] In some possible implementations, in the metal borohydride composite material, the mass percentage content of the loaded metal borohydride is 50% to 80%. In this case, the metal borohydride is the actual hydrogen storage phase, and its loading amount directly determines the overall hydrogen storage capacity of the composite material. This loading amount range cleverly takes into account high hydrogen storage capacity and excellent structural stability and kinetic performance, while avoiding metal borohydride particle agglomeration, ensuring the efficiency and durability of the composite material in practical applications. The high loading amount of 50% to 80% ensures that the active substance dominates, so that the composite material can store enough hydrogen gas per unit mass or unit volume, and has a practical basis.

[0046] Exemplarily, in the metal borohydride composite, the mass percentage of the loaded metal borohydride can be 50%, 60%, 70%, 80%, and the like, or an interval value between any two point values.

[0047] In some possible implementations, in the metal borohydride composite, the molar ratio of the metal borohydride to the metal-F bridging phase is (0.8-0.9) mol:(0.1-0.2) mol. Exemplarily, the molar ratio of the metal borohydride to the metal-F bridging phase can be 0.8 mol:0.1 mol, 0.81 mol:0.11 mol, 0.82 mol:0.12 mol, 0.84 mol:0.14 mol, 0.845 mol:0.155 mol, 0.85 mol:0.15 mol, 0.86 mol:0.16 mol, 0.88 mol:0.18 mol, 0.9 mol:0.2 mol, 0.8 mol:0.2 mol, 0.9 mol:0.1 mol, and the like, or an interval value between any two point values.

[0048] In the above step S10: In some possible implementations, the carbon-based carrier includes at least one of graphene oxide, activated carbon, carbon nanotubes, and biomass-derived carbon. In this case, these carbon-based carriers all have a high specific surface area, providing an ideal substrate for high loading and high dispersion of nanoscale metal borohydride, and further playing an irreplaceable role in enhancing reaction kinetics, buffering volume stress, preventing particle agglomeration, and the like, through their excellent electrical / thermal conductivity and unique mechanical properties. In particular, graphene oxide, with its surface functional groups, creates superior conditions for successful fluorination treatment and stable interface bridging.

[0049] In some possible implementations, the alkaline substance includes at least one of KOH and NaOH. In this case, KOH / NaOH undergoes a violent redox reaction with the carbon-based carrier at high temperatures, like “etching”, which can etch a large number of micropores and mesopores in the carbon skeleton. This “activation” process significantly improves the specific surface area and pore volume of the carbon-based carrier, providing a large space and attachment site for subsequent loading of a large amount of metal borohydride.

[0050] In some possible implementation manners, the step of the activation treatment comprises: placing the carbon-based carrier in a solution of the alkaline substance with a concentration of 5 mol / L to 7 mol / L, stirring for 3 h to 5 h, standing for 10 h to 15 h, then separating the precipitate, drying, and treating the dried precipitate in an inert atmosphere at a temperature of 700 DEG C to 900 DEG C for 1 h to 3 h, and then washing and drying to obtain the activated carbon-based carrier. In this case, the porous carbon-based carrier is obtained by high-temperature heat treatment in the solution of the alkaline substance; a porous structure is created in the carrier, and abundant hydrogen diffusion channels are provided, so that the material can also quickly mass transfer in a mechanically compressed state, and high volumetric hydrogen storage density is achieved.

[0051] For example, the concentration of the solution of the alkaline substance can be 5 mol / L, 6 mol / L, 7 mol / L, or any interval value between any two point values. The inert atmosphere can be nitrogen, argon, helium, etc.

[0052] In some possible implementation manners, in the activated carbon-based carrier, the average size of the pore size is 1 nm to 5 nm. In this case, the abundant pore structure in the activated carbon-based carrier provides fast diffusion channels for the reaction gas, and significantly improves the hydrogen absorption and desorption kinetics of the material. At the same time, the pore structure of this scale gives the carrier a large specific surface area, which not only lays a foundation for introducing a large number of anchoring sites for subsequent fluorination treatment to form a stable "metal-F bridging phase", but also provides sufficient space for high-quality loading of metal borohydride, thereby synergistically improving the comprehensive hydrogen storage performance of the composite material.

[0053] For example, in the activated carbon-based carrier, the average size of the pore size can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, or any interval value between any two point values.

[0054] In the above step S20: In some possible implementation manners, the step of the gas-phase fluorination treatment comprises: performing gas-phase fluorination treatment on the activated carbon-based carrier in a fluorine-containing atmosphere with a temperature of 25 DEG C to 180 DEG C and a fluorine volume percentage of 5% to 8% for 15 min to 60 min to obtain the fluorinated carbon-based carrier. In this case, the activated carbon-based carrier is modified mildly and efficiently: this optimization scheme can uniformly introduce sufficient fluorine sites on the surface of the carrier while maximizing the retention of the 1 nm to 5 nm porous structure of the carrier. These fluorine sites not only provide ideal nucleation sites for the in-situ loading of metal borohydride, but also significantly enhance the interfacial bonding force by forming a stable metal-F bridging phase, and induce 0.1 to 0.2 mol / mol of metal vacancies, thereby synergistically improving the hydrogen storage performance and cycle stability of the composite material.

[0055] For example, the temperature of the fluorination treatment can be any of the typical but non-limiting values of 25°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 100°C, 120°C, 140°C, 160°C, 150°C, 180°C, or an interval value between any two of the above values. The carrier gas in the fluorine-containing atmosphere can be nitrogen, argon, or the like, and the volume percentage of fluorine in the fluorine-containing atmosphere can be any of the typical but non-limiting values of 5%, 6%, 7%, 8%, or an interval value between any two of the above values. The treatment duration can be any of the typical but non-limiting values of 15 min, 20 min, 30 min, 40 min, 50 min, 60 min, or an interval value between any two of the above values.

[0056] In some possible implementations, the content of fluorine atoms in the fluorinated carbon-based carrier is 20% to 30%. In this case, the surface of the carrier is ensured to have a high density and uniform distribution of active fluorine sites, so that the metal borohydride can be efficiently induced to form a stable metal-F bridging phase in situ in the subsequent reaction. This not only significantly enhances the interfacial bonding force between the active nanoparticles and the carrier, effectively prevents the nanoparticles from falling off and agglomerating during use, and guarantees the excellent structural and cyclic stability of the material, but also fundamentally optimizes the electronic structure of the metal borohydride through the vacancy effect, which together contributes to the excellent performance of the composite material, such as low hydrogen release temperature, fast reaction kinetics, and long cycle life.

[0057] For example, the content of fluorine atoms in the fluorinated carbon-based carrier can be any of the typical but non-limiting values of 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or an interval value between any two of the above values.

[0058] In the above step S30, the fluorinated carbon-based carrier is mixed with a metal source, a boron source, and a solvent, and a metal borohydride is synthesized in situ on the surface of the fluorinated carbon-based carrier through a hydrogen-driven solvothermal reaction. In the generated metal borohydride, metal atoms react with F atoms in the fluorinated carbon-based carrier to form a metal-F bridging phase, and at the same time, metal vacancies are generated in the metal borohydride. Through one-step hydrogen-driven solvothermal reaction, the in-situ synthesis of the metal borohydride, the interfacial chemical modification, and the lattice defect engineering are ingeniously integrated. Under the conditions of high-pressure hydrogen and solvothermal reaction, the high-density fluorine atoms (content of 20% to 30%) in the fluorinated carbon-based carrier act as strong metal-philic sites, which not only induce the metal borohydride to achieve uniform loading (particle size of 50 nm to 150 nm, loading amount of 50% to 80%) with the surface as the nucleation center, but also form a stable metal-F bridging phase through interfacial chemical reaction, and at the same time, orderly create metal vacancies with a concentration of 0.1 to 0.2 mol / mol in the borohydride lattice of the “stolen” metal, thereby simultaneously achieving a qualitative leap in the structural stability of the material and a fundamental improvement in the hydrogen absorption and desorption thermodynamics / kinetics.

[0059] In some possible implementations, the metal source includes at least one of a lithium source, a potassium source, a magnesium source, and a calcium source.

[0060] In some possible implementations, the boron source includes triethylamine borane.

[0061] In some possible implementations, the solvent includes at least one of n-hexane, cyclohexane, and tetrahydrofuran. These solvents have good solubility and dispersibility for the fluorocarbon-based carrier and the metal source and the boron source, so that the insoluble carrier, the metal source, and the boron source can be fully contacted to achieve molecular-level mixing. In the "in-situ synthesis", the solvent can provide a site for the reaction, so that the product crystals nucleate and grow on the surface of the carrier.

[0062] In some possible implementations, the mass ratio of the metal source to the boron source is (1-3):(0.5-0.7). In this case, the ratio ensures that the metal ions are relatively excessive in the reaction system, which not only ensures that the boron source can be fully consumed and converted into the target metal borohydride with high efficiency, but also provides sufficient metal atom supply for the competitive reaction at the interface of the fluorocarbon carrier. These excessive metal atoms are used to build a stable metal-fluorine bridging phase on the one hand, and induce the ideal metal vacancy concentration of 0.1-0.2 mol / mol by being selectively captured by the interface on the other hand, so as to achieve high loading while perfectly synchronizing the construction of the crystal structure defects of the composite material and the interface stability.

[0063] For example, the mass ratio of the metal source to the boron source can be 1:0.5, 2:0.6, 3:0.7, and the like, which are typical but non-limiting point values or interval values between any two point values.

[0064] In some possible implementations, the conditions for driving the solvent-thermal reaction by hydrogen include: under the conditions of a hydrogen pressure of 35 bar-55 bar and a temperature of 80°C-100°C, the reaction is performed for 10 h-20 h. In this case, sufficient thermodynamic driving force and suitable reaction rate are provided for the in-situ generation of the metal borohydride, and the sintering of the active components and the damage to the carrier structure are effectively inhibited by the mild temperature; the continuous high-pressure hydrogen environment not only promotes the conversion of the precursor into the target product, but also, through the synergistic effect with the fluorocarbon carrier, guides the ordered formation of the metal-F bridging phase and the controllable construction of the metal vacancy, so that the obtained hydrogen storage material finally realizes the balance of the interface stability and the catalytic activity at the molecular scale, and exhibits ideal low-temperature hydrogen absorption and desorption performance and cycle durability.

[0065] For example, the hydrogen pressure of the solvothermal reaction can be 35 bar, 40 bar, 45 bar, 50 bar, 55 bar, or the like, and the reaction temperature can be 80℃, 85℃, 90℃, 95℃, 100℃, or the like, and the reaction time can be 10 h, 12 h, 14 h, 15 h, 16 h, 18 h, 20 h, or the like.

[0066] In some embodiments, the carbon fluoride-based carrier adopts a porous fluorinated graphene carrier, the metal source adopts butyl lithium, the boron source adopts triethylamine borane, and the solvent adopts n-hexane or cyclohexane, and the solvothermal reaction is carried out under a hydrogen atmosphere to synthesize LiBH4. In this case, the amount of n-butyl lithium is 1-3 ml, the amount of triethylamine borane is 500-700 μl, the amount of solvent is 60-80 ml, the hydrogen pressure is 35 bar-55 bar, the solvothermal temperature is 80℃-100℃, and the solvothermal time is 10-20 hours. Because of the presence of the carrier, the carrier provides heterogeneous nucleation sites for LiBH4, and the LiBH4 particles are loaded on the carrier. In this embodiment, the carrier is activated to form a porous structure and is fluorinated. Fluorine has strong lithium affinity, and in the process of the solvothermal synthesis of LiBH4, the fluorine atoms with lithium affinity on the carrier induce the nucleation and loading of LiBH4 at the fluorine sites, and at the same time, an interface chemical reaction occurs, so that the fluorine in the carrier combines with the lithium in LiBH4 to form LiF, and lithium vacancies are formed in LiBH4, thereby obtaining a stable lithium vacancy-rich LiBH4 material. The preparation method of this embodiment is simple and flexible. By using the porous fluorinated graphene with lithium affinity as the carrier, the nucleation and loading of LiBH4 at the fluorine sites are induced in the process of the solvothermal synthesis of LiBH4, and at the same time, an interface chemical reaction occurs, so that the lithium lattice vacancies and the LiF bridging phase are generated in one step. On the one hand, the lithium vacancies are introduced into LiBH4 through the interface reaction, and under the premise that other properties and performances are basically unaffected, the lithium vacancies effectively weaken the internal charge interaction and B-H bond of LiBH4, improve the hydrogen storage reaction thermodynamics and kinetics, and realize low-temperature hydrogen release. On the other hand, the LiF bridging phase with stable physical and chemical properties is introduced between LiBH4 and the carrier through the interface reaction. As the bridging phase between the carrier and the lithium vacancy-rich LiBH4, the LiF effectively improves the structural stability of LiBH4, greatly improves the load structural stability and long-cycle hydrogen storage capacity of LiBH4, and improves the hydrogen absorption and release cycle life. Therefore, through the synergistic regulation of the lithium vacancies and the LiF bridging phase on the intrinsic thermodynamics / kinetics and structural stability of LiBH4, the material has excellent low-temperature hydrogen release and long-cycle hydrogen storage capacity.

[0067] In a second aspect, the embodiments of the present application provide a metal borohydride composite material, comprising a porous carbon fluoride-based carrier and a metal borohydride loaded in the carbon fluoride-based carrier, the metal borohydride having metal vacancies, and a metal-F bridging phase formed by metal atoms in the metal borohydride and fluorine atoms in the carbon fluoride-based carrier.

[0068] The metal borohydride composite material provided by the embodiments of the present application loads the metal borohydride having metal vacancies in the porous carbon fluoride-based carrier, and forms a stable interface chemical connection by means of the metal-F bridging phase formed between the metal atoms and the fluorine atoms of the carrier, thereby achieving a synergistic improvement in the material structural stability and hydrogen storage performance. The bridging phase effectively fixes the active substance, thereby significantly enhancing the cycle stability; and the metal vacancies in the metal borohydride lattice weaken the B-H bond and the metal-boron interaction, thereby significantly optimizing the hydrogen absorption and desorption thermodynamic and kinetic characteristics of the material, and finally enabling the composite material to have high reversible capacity and long cycle life.

[0069] In some possible implementation manners, the carbon fluoride-based carrier comprises fluorinated graphene. The embodiments of the present application select fluorinated graphene as the carbon fluoride-based carrier, and achieve a triple synergistic enhancement effect by virtue of the unique two-dimensional layered structure and the high-density fluorine atom distribution on the surface thereof: on the one hand, the inherent high specific surface and excellent electrical conductivity of graphene provide an ideal dispersion substrate and an electronic transmission channel for the metal borohydride; on the other hand, the fluorine atoms (content: 20%-30%) covalently bonded on the carbon skeleton not only induce the formation of a stable metal-F bridging phase by virtue of strong gold affinity, effectively anchor the active particles with a size of 50 nm-150 nm, but also promote the metal borohydride lattice to generate metal vacancies with a content of 0.1-0.2 mol / mol by virtue of the interface charge transfer effect. This micro-coupling mechanism between the carrier and the active phase finally enables the composite material to have optimized hydrogen absorption and desorption thermodynamics, accelerated reaction kinetics and significantly improved cycle stability while maintaining a high loading amount of 50%-80%.

[0070] In some possible implementation manners, the metal borohydride comprises LiBH4, the molar content of lithium vacancies in 1 mol of LiBH4 is 0.1 mol-0.2 mol, and the metal-F bridging phase comprises a LiF bridging phase. The embodiments of the present application take LiBH4 as a specific implementation form of the metal borohydride, and form LiF bridging phases by constructing lithium vacancies with a content of 0.1-0.2 mol / mol in the lattice of LiBH4 and the fluorinated graphene carrier, thereby achieving precise regulation of the material performance: these highly controllable lithium vacancies effectively weaken the BH4] -The B-H bond energy of the cluster and the optimized migration path of lithium ion significantly reduce the dehydrogenation temperature of LiBH4 and accelerate the reaction kinetics; meanwhile, the LiF bridge phase formed at the interface not only firmly anchors the LiBH4 nanoparticles on the surface of the carrier through strong ionic bonding, but also constitutes a durable protective layer as a known thermodynamically stable phase.

[0071] In some possible implementations, in the metal borohydride composite material, the average particle size of the loaded metal borohydride is 50 nm to 150 nm. In this case, the nanoscale particle size means that the diffusion path of hydrogen atoms or molecules inside the solid phase is significantly shortened. At the same time, the huge specific surface area provides a large number of surface reaction active sites, so that the hydrogen absorption and hydrogen release chemical reactions can be carried out more quickly and more fully. This complements the improvement of metal vacancies on kinetics, and together realizes the rapid hydrogen charging and discharging performance of the material.

[0072] In some possible implementations, in the metal borohydride composite material, the mass percentage content of the loaded metal borohydride is 50% to 80%; and the molar ratio of the metal borohydride to the metal-F bridge phase is (0.8-0.9) mol:(0.1-0.2) mol. This loading range ingeniously balances high hydrogen storage capacity, excellent structural stability and kinetic performance, while avoiding agglomeration of metal borohydride particles, ensuring high efficiency and durability of the composite material in practical applications.

[0073] In some specific embodiments, in the metal borohydride composite material, the mass percentage content of the loaded metal borohydride is 70%; and the molar ratio of the metal borohydride to the metal-F bridge phase is 0.845 mol:0.155 mol.

[0074] In some embodiments, in the metal borohydride composite material, LiBH4 particles with a size of 50-150 nm are uniformly loaded on a fluorinated graphene carrier, the LiBH4 particles are rich in lithium vacancies, and LiF is used as a bridge phase between the LiBH4 particles and the carrier.

[0075] In a third aspect, the embodiments of the present application provide a hydrogen storage material, which comprises the metal borohydride composite material prepared by the method described above and / or the metal borohydride composite material described above.

[0076] The embodiments of the present application apply the metal borohydride composite material prepared by the method described above and / or the metal borohydride composite material with the specific structure described above to a hydrogen storage material, so that the hydrogen storage material has low-temperature hydrogen release characteristics, fast reaction kinetics and excellent cycle stability at the same time, and provides an ideal material solution for the development of a new generation of high-performance solid-state hydrogen storage systems.

[0077] In order to make the above-mentioned implementation details and operations of the present application clearly understood by those skilled in the art, and the further performance of the metal borohydride composite material and the preparation method thereof of the present application is significantly embodied, the following will be illustrated by multiple embodiments.

[0078] Embodiment 1 A metal borohydride composite material, comprising a porous structure of a fluorinated graphene carrier and loaded structure-stable lithium-rich vacancy LiBH4 particles, and a LiF bridging phase between the LiBH4 particles and the fluorinated graphene carrier, wherein the molar content of lithium vacancies in 1 mol of LiBH4 is 0.155 mol, the mass percentage content of the LiBH4 particles is 70%, the particle size of the LiBH4 particles is 50-150 nm, and the content of fluorine atoms in the fluorinated graphene carrier is 23.3 at%.

[0079] The preparation thereof comprises the following steps: 1. Preparation of activated porous graphene: 1 gram of graphite oxide is dispersed in 50 milliliters of 6 mol / L KOH solution, stirred for 4 hours, and left standing for 12 hours; then the supernatant is poured off, the precipitate is dried at 65°C for 2 hours, and heat-treated at 800°C under argon flow for 1 hour; after cooling, the activated product is washed with deionized water until pH=7, and dried at 65°C for 2 hours to obtain activated porous graphene.

[0080] 2. Preparation of a fluorinated graphene carrier: the dried product, activated porous graphene, is treated with 5% fluorine / nitrogen mixed gas at 100°C for 30 minutes to obtain a fluorinated graphene carrier.

[0081] 3. Preparation of a composite material: 80 milliliters of anhydrous n-hexane, 37 milligrams of a fluorinated graphene carrier, 2 milliliters of n-butyllithium, and 600 microliters of triethylamine borane are sequentially added to a stainless steel reaction kettle, and 40 bar of hydrogen gas is charged; then the reaction kettle is placed in an oil bath, and stirred at 90°C for 12 hours; LiBH4 is in-situ synthesized on the surface of the fluorinated graphene carrier, Li atoms in the generated LiBH4 react with F atoms in the fluorinated graphene carrier to form a LiF bridging phase, and at the same time, metal vacancies are generated in the LiBH4. After cooling, the reaction product is collected by suction filtration, and vacuum dried at room temperature for 1 hour to obtain a structure-stable lithium-rich vacancy LiBH4 composite material.

[0082] Embodiment 2 A metal borohydride composite material, prepared by the same process as in Embodiment 1, except that the mass ratio of the fluorinated graphene carrier to LiBH4 is 4:6.

[0083] Embodiment 3 A metal borohydride composite material, prepared by the same process as in Embodiment 1, except that the mass ratio of the fluorinated graphene carrier to LiBH4 is 2:8.

[0084] Control group The LiBH4 material was used as a control group, and its preparation included the following steps: 80 mL of anhydrous n-hexane was added to a stainless steel reaction kettle, 2 mL of n-butyllithium was added, 2 mL of n-butyllithium was added, 600 μL of triethylamine borane was added, and 40 bar of hydrogen was charged, and the reaction was carried out at 90°C for 12 hours in an oil bath. The reaction product was collected by suction filtration and dried under dynamic vacuum to obtain the LiBH4 material.

[0085] Comparative Example 1 A metal borohydride composite material, which is different from Example 1 in that step 1 in the process of preparing the fluorinated graphene carrier does not add KOH for activation treatment. The preparation process of the fluorinated graphene loaded LiBH4 hydrogen storage material is the same as that of Example 1.

[0086] Comparative Example 2 A metal borohydride composite material, which is different from Example 1 in that after the preparation of the porous graphene carrier, step 2 of fluorination treatment is not performed. The preparation process of the porous graphene loaded LiBH4 hydrogen storage material is the same as that of Example 1.

[0087] Comparative Example 3 A metal borohydride composite material, which is different from Example 1 in that step 1 in the process of preparing the graphene carrier does not add KOH for activation treatment, and step 2 of fluorination treatment is not performed. The preparation process of the graphene loaded LiBH4 hydrogen storage material is the same as that of Example 1.

[0088] In order to verify the progressiveness of the embodiments of the present application, the following performance tests were performed: 1. The metal borohydride composite material prepared in Example 1 was subjected to X-ray diffraction test, Figure 2 The X-ray diffraction pattern of the metal borohydride composite material prepared in Example 1. In addition to the diffraction peak of LiBH4, the diffraction peak of LiF can also be seen, indicating that the interface chemical reaction between LiBH4 and the carrier has occurred. In addition, the LiBH4 diffraction peak position of the material is slightly shifted to a lower angle compared with the single LiBH4 of the control group, indicating that the lattice spacing is increased. This is because the reaction of lithium consumes lithium vacancies in LiBH4, which will cause the deformation of the adjacent [BH4] group, the elongation of the B-H bond, and thus the lattice expansion.

[0089] 2. The metal borohydride composite material prepared in Example 1 was subjected to nuclear magnetic resonance light test, Figure 3 The nuclear magnetic resonance spectrum of the metal borohydride composite material prepared in Example 1. 11B NMR spectrum. Its resonance peak shifts to a more negative chemical shift value compared with single LiBH4, because the shielding effect of carbon-based carrier on B nucleus is stronger than the deshielding effect of lithium vacancy. In addition, its resonance peak is overlapped by a broad peak and a narrow peak, wherein the broad peak corresponds to normal [BH4] group, and the narrow peak corresponds to deformed [BH4] group near the lithium vacancy, which has stronger mobility.

[0090] 3, the morphology of the metal borohydride composite prepared in Example 1 is observed, Figure 4 Fig. (a) and (b) are scanning electron microscope (SEM) image (a) and transmission electron microscope (TEM) image (b) of the metal borohydride composite prepared in Example 1, respectively. It can be seen that LiBH4 particles with a size of 50-150 nm are uniformly dispersed on the fluorinated graphene carrier.

[0091] Figure 4 Fig. (c) and (d) are EDS element distribution map (c) and high-resolution TEM image (d) of the metal borohydride composite prepared in Example 1, respectively. In which, the lattice fringes of LiF (200) plane and the lattice fringes of LiBH4 (101) plane intersecting with the same are clearly visible, and B and F elements show relatively overlapped distribution on C substrate, indicating that LiF and LiBH4 two phases coexist in space. In summary, the lithiumophilic fluorinated graphene carrier is used to induce interfacial chemical reaction in the present application, lithium vacancy is generated in LiBH4, and LiF bridging phase is formed between LiBH4 and the carrier.

[0092] The morphology of the metal borohydride composite prepared in Example 2 is observed, and the scanning electron microscope (SEM) image of Example 2 is shown in Fig. Figure 5 It can be seen that the morphology of Example 2 is similar to that of Example 1, and it can be seen that LiBH4 particles with a size of 50-150 nm are uniformly dispersed on the fluorinated graphene carrier.

[0093] The morphology of the metal borohydride composite prepared in Example 3 is observed, and the scanning electron microscope (SEM) image of Example 3 is shown in Fig. Figure 6 It can be seen that most of the LiBH4 particles are uniformly dispersed on the fluorinated graphene carrier, but because the loading amount of LiBH4 is too large, agglomerated large particles also begin to appear.

[0094] 4, the hydrogen absorption and desorption properties of the metal borohydride composite prepared in Example 1 are tested by volume method. The temperature-programmed hydrogen desorption test procedure is to heat to 400 ℃ at a rate of 2 ℃ / min under vacuum. The isothermal hydrogen desorption test procedure is to heat to 350 ℃ at a rate of 10 ℃ / min under vacuum and keep the temperature. The isothermal hydrogen absorption test procedure is to heat to 450 ℃ at a rate of 10 ℃ / min under 100 bar hydrogen pressure and keep the temperature.

[0095] Figure 7 The hydrogen desorption curves (a) and (b) of the metal borohydride composite material prepared in Example 1 are shown. Figure 7 As shown in Figure (a), this material begins to release hydrogen at room temperature, releasing 2.6 wt% hydrogen gas by 250°C, while single LiBH4 releases no hydrogen gas at this temperature. When the temperature is raised to 400°C, the hydrogen release of this material reaches 8.4 wt%, far exceeding the 2.6 wt% of single LiBH4. From... Figure 7 As shown in Figure (b), the material exhibits significantly improved hydrogen release kinetics. It rapidly releases 7.0 wt% hydrogen within the initial 60 minutes at 350°C. After holding at this temperature for another 60 minutes, the hydrogen release is essentially complete, with a total release of 8.0 wt%. In contrast, under the same conditions, a single LiBH4 only releases 1.7 wt%.

[0096] Figure 8 The hydrogen desorption curve of the metal borohydride composite material prepared in Example 2 is shown in the attached figure. Figure 8 As can be seen, similar to Example 1, this material begins to release hydrogen at room temperature. This further illustrates the positive effect of Li vacancies on reducing thermodynamic stability and hydrogen release temperature. Furthermore, it is evident that the metal borohydride composite material of Example 2 released 1.9 wt% hydrogen at 250°C, and the hydrogen release reached 6.6 wt% when the temperature was raised to 400°C. The hydrogen release of Example 2 is slightly lower than that of Example 1, possibly because the LiBH4 loading in the metal borohydride composite material of Example 2 is relatively low. However, the hydrogen release performance of the metal borohydride composite material of Example 2 is still significantly higher than that of the control group with only LiBH4.

[0097] Figure 9 The hydrogen desorption curve of the metal borohydride composite material prepared in Example 3 is shown in the attached figure. Figure 9 As can be seen, similar to Example 1, this material begins to release hydrogen at room temperature. This further illustrates the positive effect of Li vacancies on reducing thermodynamic stability and hydrogen release temperature. The metal borohydride composite material of Example 3 released 2.0 wt% hydrogen at 250°C, and the hydrogen release reached 7.8 wt% when the temperature was raised to 400°C. The hydrogen release of Example 3 is slightly lower than that of Example 1, possibly because some LiBH4 aggregates in the metal borohydride composite material of Example 3 lead to performance degradation. However, the hydrogen release performance of the metal borohydride composite material of Example 3 is still significantly higher than that of the control group with single LiBH4.

[0098] 5. The thermodynamic properties of the metal borohydride composite material prepared in Example 1 were tested. Figure 10 Differential scanning calorimetry (a) and Kissinger fitting plot (b) of the metal borohydride composite material prepared in Example 1. By analyzing... Figure 10Integral fitting of the hydrogen desorption peak in Figure (a) yields a hydrogen desorption enthalpy of 62.4 kJ / mol, which is 9% lower than the standard hydrogen desorption enthalpy of single LiBH4 (69 kJ / mol), indicating a decrease in thermodynamic stability. The Kissinger method was used to further analyze the hydrogen desorption peak. Figure 10 Peak fitting of the mass spectrometry curves obtained at different heating rates in Figure (b) shows that the apparent activation energy for hydrogen desorption of this material is 65.7 kJ / mol, which is 63.3% lower than the apparent activation energy of LiBH4 alone (179.1 kJ / mol), indicating a significant improvement in kinetics.

[0099] 6. The mechanical stability of the metal borohydride composite material prepared in Example 1 was tested. Figure 11 The images show the metal borohydride composite material prepared in Example 1 after mechanical compression and its isothermal hydrogen release curve. It can be seen that the material maintains good hydrogen release kinetics under compression, with a corresponding volumetric hydrogen release density as high as 76.2 g / L, even superior to that of a liquefied hydrogen tank (70 g / L).

[0100] 7. The isothermal hydrogen absorption performance and cycling performance of the metal borohydride composite material prepared in Example 1 were tested. Figure 12 Isothermal hydrogen absorption curves (a) and cycling curves (b) of the metal borohydride composite material prepared in Example 1. From... Figure 12 Figure (a) shows that this material exhibits excellent hydrogen absorption kinetics, rapidly absorbing hydrogen to reach saturation of 7.1 wt% within 60 minutes, with an initial reversibility of 88.8%. From... Figure 12 Figure (b) shows that the material has excellent cycle life, with a reversible hydrogen capacity of 5.7 wt% after 30 cycles and a capacity retention of 70%. 8. X-ray diffraction and nuclear magnetic resonance tests were performed on the first hydrogen release product of the metal borohydride composite material prepared in Example 1. Figure 13 X-ray diffraction pattern (a) of the first hydrogen release product of the metal borohydride composite material prepared in Example 1 and 11 b-NMR spectrum (b). From Figure 13 As shown in Figure (a), the material generates LiH after hydrogen release, and the diffraction peaks of LiH are slightly shifted at higher angles compared to the standard PDF card, indicating a decrease in lattice spacing. This is because the radius of a lithium atom is larger than that of a hydrogen atom, and the presence of lithium vacancies leads to lattice contraction in LiH. Figure 13 Figure (b) shows a broad resonance peak at -11 ppm, which belongs to amorphous elemental B. This indicates that the hydrogen desorption products of this material are lithium-rich vacant LiH and B.

[0101] 9. The morphology of the products after hydrogen desorption from the metal borohydride composite material prepared in Example 1 was tested. Figure 14Scanning electron microscope image of the dehydrogenated product of the metal borohydride composite prepared in Example 1. It can be seen that the material has transformed from the nanoparticle morphology before dehydrogenation to nanoflower morphology, but is still uniformly dispersed on the support, indicating that the loading structure is stable.

[0102] 10. Infrared test of the metal borohydride composite prepared in Example 1 after 10 and 30 hydrogen absorption times, Figure 15 Fourier infrared spectrum of the metal borohydride composite prepared in Example 1 after 10 and 30 hydrogen absorption times. The vibration peak of the [BH4] group is clearly visible, again demonstrating excellent cycle reversibility.

[0103] 11. Observation of the morphology of the metal borohydride composite prepared in Comparative Example 1, Figure 16 Scanning electron microscope image of the metal borohydride composite prepared in Comparative Example 1, which is different from Example 1, showing a bulk morphology of agglomeration. This shows that the KOH activation process promotes the exfoliation of graphene sheets and the exposure of surface nucleation sites, which is the key to achieving high content and high dispersion of LiBH4 loading.

[0104] 12. Observation of the morphology of the metal borohydride composite prepared in Comparative Example 2, Figure 17 Scanning electron microscope image of the metal borohydride composite prepared in Comparative Example 2, which is similar to Example 1, showing a loose and dispersed two-dimensional loading morphology. This again shows that the KOH activation process helps to expose surface nucleation sites and promote uniform loading of LiBH4.

[0105] 13. Test of the thermodynamic properties of the metal borohydride composite prepared in Comparative Example 2, Figure 18 Temperature-dependent dehydrogenation curve (a) and differential scanning calorimetry curve (b) of the prepared material. From Figure 18 the (a) figure, it can be seen that the material has almost no hydrogen release below 250°C, while Example 1 has obvious dehydrogenation at this stage. By integrating and fitting the dehydrogenation peak in the (b) figure, Figure 18 the dehydrogenation enthalpy of the material is 65.4 kJ / mol, higher than the 62.4 kJ / mol of Example 1. The above results show that the carrier fluorination-induced interfacial reaction to generate lithium vacancies is the key reason for the thermodynamic destabilization and low-temperature dehydrogenation of LiBH4.

[0106] 14. Test of the mechanical stability of the metal borohydride composite prepared in Comparative Example 2 and Comparative Example 3, Figure 19 Isothermal dehydrogenation curves of the metal borohydride composite prepared in Comparative Example 2 before and after mechanical compression. Figure 20Isothermal hydrogen desorption curves of the metal borohydride composite prepared in Comparative Example 3 before and after mechanical compression. The metal borohydride composite prepared in Comparative Example 3 shows a significant degradation in kinetics due to the restriction of mass transfer caused by the compression of the interlayer stacking. In comparison with Comparative Example 3, the metal borohydride composite prepared in Comparative Example 2 has a faster hydrogen desorption rate, indicating that the creation of a porous structure in the support by KOH activation can improve the hydrogen desorption kinetics of LiBH4. This is because the presence of pores can provide additional hydrogen diffusion channels. Even after mechanical compression, the material can still maintain good hydrogen desorption kinetics, while Comparative Example 3 shows a significant performance degradation. This is because the porous channels open the inner layer space of the stack, which is beneficial to the interlayer diffusion and desorption of hydrogen. In addition, the hydrogen desorption rate of the material is slightly lower than that of Example 1 before and after compression, indicating that the fluorination of the support synergistically promotes the kinetic performance of LiBH4. This is related to the lithium vacancies generated by the fluorination-induced interfacial reaction, which weakens the B-H bond and improves the ion mobility.

[0107] 15. The cyclic hydrogen desorption performance of the metal borohydride composite prepared in Comparative Example 2 was tested, Figure 21 The cyclic hydrogen desorption curve of the metal borohydride composite prepared in Comparative Example 2. The hydrogen desorption amount of the material decreases from 7.4 wt% to 4.4 wt% after 5 cycles, with a capacity retention rate of 59.5%, which is lower than the 70% of Example 1. This indicates that the generation of LiF interfacial bridges helps to improve the cyclic stability of LiBH4 hydrogen storage.

[0108] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for producing a metal borohydride composite material, characterized by, The method comprises the following steps: obtaining a carbon-based carrier, activating the carbon-based carrier by using an alkaline substance to obtain an activated carbon-based carrier; carrying out gas-phase fluorination treatment on the activated carbon-based carrier to obtain a fluorinated carbon-based carrier; mixing the fluorinated carbon-based carrier with a metal source, a boron source and a solvent, and in-situ synthesizing metal borohydride on the surface of the fluorinated carbon-based carrier by using hydrogen to drive a solvothermal reaction, and forming a metal-F bridging phase by reacting metal atoms in the metal borohydride with F atoms in the fluorinated carbon-based carrier, and generating metal vacancies in the metal borohydride to obtain a metal borohydride composite material.

2. The method of producing a metal borohydride composite material according to claim 1, characterized by, The molar content of metal vacancies in 1 mol of the metal borohydride is 0.1 mol to 0.2 mol; and / or, the average particle size of the loaded metal borohydride in the metal borohydride composite material is 50 nm to 150 nm.

3. The method of claim 2, wherein the metal borohydride composite is prepared by the steps of: (a) mixing a metal borohydride with a metal salt; (b) adding a reducing agent to the mixture; and (c) adding a solvent to the mixture. The mass percentage content of the loaded metal borohydride in the metal borohydride composite material is 50% to 80%; and / or, the molar ratio of the metal borohydride to the metal-F bridging phase in the metal borohydride composite material is (0.8 to 0.9) mol:(0.1 to 0.2) mol.

4. The method of producing a metal borohydride composite material according to any one of claims 1 to 3, characterized by, The carbon-based carrier comprises at least one of oxidized graphite, activated carbon, carbon nanotubes and biomass-derived carbon; and / or, the alkaline substance comprises at least one of KOH and NaOH; and / or, the step of activation treatment comprises: placing the carbon-based carrier in a solution of the alkaline substance with a concentration of 5 mol / L to 7 mol / L, stirring for 3 h to 5 h, standing for 10 h to 15 h, then separating the precipitate, drying and treating in an inert atmosphere at a temperature of 700°C to 900°C for 1 h to 3 h, and then washing and drying to obtain the activated carbon-based carrier; and / or, the average size of the pore size in the activated carbon-based carrier is 1 nm to 5 nm.

5. The method of claim 4, wherein the metal borohydride composite is prepared by the steps of: (a) mixing a metal borohydride with a metal salt; (b) adding a reducing agent to the mixture; and (c) adding a solvent to the mixture. The step of gas-phase fluorination treatment comprises: carrying out gas-phase fluorination treatment on the activated carbon-based carrier in a fluorine-containing atmosphere with a temperature of 25°C to 180°C and a fluorine volume percentage content of 5% to 8% for 15 min to 60 min to obtain the fluorinated carbon-based carrier; and / or, the content of fluorine atoms in the fluorinated carbon-based carrier is 20% to 30%.

6. The method of producing a metal borohydride composite material according to any one of claims 1 to 3 or 5, characterized by, The metal source comprises at least one of a lithium source, a potassium source, a magnesium source and a calcium source; and / or, the boron source comprises triethylamine borane; and / or, the solvent comprises at least one of n-hexane, cyclohexane and tetrahydrofuran; and / or, the mass ratio of the metal source to the boron source is (1 to 3):(0.5 to 0.7).

7. The method of producing a metal borohydride composite material according to claim 6, wherein The conditions of the solvothermal reaction driven by hydrogen include: reacting for 10 h to 20 h under the conditions of a hydrogen pressure of 35 bar to 55 bar and a temperature of 80°C to 100°C.

8. A metal borohydride composite material, characterized by, The metal borohydride composite material comprises a fluorinated carbon-based carrier with a porous structure and metal borohydride loaded in the fluorinated carbon-based carrier, the metal borohydride has metal vacancies, and metal atoms in the metal borohydride form a metal-F bridging phase with fluorine atoms in the fluorinated carbon-based carrier.

9. The metal borohydride composite of claim 8, wherein, The fluorinated carbon-based carrier comprises fluorinated graphene. And / or, the metal borohydride includes LiBH4, the molar content of lithium vacancy in 1 mol of LiBH4 is 0.1 mol~0.2 mol; the metal-F bridging phase includes LiF bridging phase; And / or, in the metal borohydride composite material, the average particle size of the loaded metal borohydride is 50 nm~150 nm; And / or, in the metal borohydride composite material, the mass percentage content of the loaded metal borohydride is 50%~80%; the molar ratio of the metal borohydride to the metal-F bridging phase is (0.8~0.9) mol:(0.1~0.2) mol.

10. A hydrogen storage material, characterized by, The metal borohydride composite material prepared by the method of any one of claims 1~7 and / or the metal borohydride composite material of any one of claims 8~9.

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