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Thermodynamics and Kinetics Tuning of LiBH4 for Hydrogen Storage

Zhao Ding , Weijie Yang , Kaifu Huo , Leon Shaw

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1586 -1597.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1586-1597 DOI: 10.7536/PC200831
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Thermodynamics and Kinetics Tuning of LiBH4 for Hydrogen Storage
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Abstract

To meet the challenge of energy shortage and climate change, it is required to build the new renewable energy based structure and gradually abandon the conventional fossil fuel based energy structure. Hydrogen energy has attracted more and more attention, due to its high energy density, large calorific value, abundant resource and zero pollution. LiBH4, which has been acknowledged as one of the most promising hydrogen storage alternatives for onboard energy carrier applications, is still not qualified for the industrialization, though it has been studied for years. Herein, a state-of-the-art review on the modification of stable thermodynamics and sluggish kinetics of hydrogen storage in LiBH4, aiming to providing reference and solutions for its promotion and application. Multiple main-stream techniques along with their latest efforts have been discussed, including mechanical milling activation, nanoscaffold confinement, catalyst modification, ions substitution, reactant destabilization and a novel process termed as high-energy ball milling with in-situ aerosol spraying (BMAS). Remarkable, BMAS is the technology of proven ability to overcome the kinetic barriers for thermodynamically favorable systems like LiBH4 + MgH2 mixture and provide thermodynamic driving force to enhance hydrogen release at a lower temperature.

Contents

Contents

1 Introduction

2 Thermodynamical tuning of LiBH4

2.1 Cation/anion substitution

2.2 Reactant destabilization

3 Kinetics tuning of LiBH4

3.1 Mechanical milling activation

3.2 Nanoscaffold confinement by the infiltration approach

3.3 Modification by doping catalysts

4 Dual-tuning thermodynamics and kinetics of LiBH4

5 Conclusion and outlook

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Keywords

hydrogen storage materials / LiBH4 / nanoengineering / thermodynamical tuning / kinetics tuning / BMAS

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Zhao Ding, Weijie Yang, Kaifu Huo, Leon Shaw. Thermodynamics and Kinetics Tuning of LiBH4 for Hydrogen Storage. Progress in Chemistry, 2021, 33(9): 1586-1597 DOI:10.7536/PC200831

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1 Introduction

Hydrogen energy refers to the chemical energy released by the chemical reaction between hydrogen and oxygen, which is a clean secondary energy. It is known as the "ultimate energy" to solve the energy crisis and mitigate climate change in the 21st century because of its advantages such as wide sources, high energy density, high combustion heat value, zero pollution, storability, reproducibility and electrical combustibility. All countries in the world attach great importance to the development and utilization of hydrogen energy, and the United States, Japan and Germany have raised hydrogen energy planning to the height of national energy strategy. According to the forecast of the International Hydrogen Energy Council, by 2050, 20% of the global planned emission reduction of 6 billion tons of carbon dioxide will depend on the application of hydrogen energy[1]. By then, the hydrogen industry will not only meet about 18% of energy demand, but also create a $2.5 trillion market for hydrogen production and fuel cells. Hydrogen vehicles will account for 20% to 25% of global vehicles. In the hydrogen energy plan led by China, the National Energy Group has invested heavily in the research and development of hydrogen energy applications. In the past two years, it has laid out layer by layer in the field of hydrogen energy and fuel cells, and initially established a scientific research system with national laboratories as the main part and universities, research institutes and enterprises as the supplement, hoping to make China a powerful country in hydrogen energy technology[2].
Due to the flammable, explosive and high diffusion characteristics of hydrogen, the potential safety hazards of leakage and explosion in the process of storage and transportation of hydrogen are the primary problems to be solved in the promotion of hydrogen energy[3]. Compared with high-pressure gaseous and low-temperature liquid hydrogen storage, solid hydrogen storage has the advantages of convenient transportation, high volume density, low pressure, low cost and high safety, and is particularly suitable for fuel cell vehicles. The LiBH4 with mass and volume hydrogen storage densities of 18.5 wt% and 121 kg/m3, respectively, is considered to be one of the most promising solid hydrogen storage energy carriers for vehicles.
The strong bonding between the atoms in the LiBH4 determines the high thermodynamic stability, and the high orientation also determines the high kinetic energy barrier of the decomposition reaction[4]. It is reported that the decomposition of LiBH4 to release hydrogen is completed in three steps (as shown in formula (1)): (I) heating to the 105~112℃,LiBH4 causes the isomeric transformation, the orthorhombic crystal stable at room temperature is transformed into the hexagonal crystal stable at high temperature, and about 0.1 wt%H2;(ii) is released at the same time. Heating to 275-278 ℃ causes the solid LiBH4 to melt, accompanied by the release of 0.5~1.0 wt%H2[5⇓⇓⇓~9]; (iii) heating until the 400~680℃,LiBH4 decomposes to form LiH and releases more than 9.0 wt% of H2. In addition, it was proved that the mixture of LiH and B was successfully charged into LiBH4 at 690 ℃ and 200 bar. Therefore, the LiBH4 can release and absorb hydrogen reversibly at high temperature/high pressure.
2LiBH4 ↔ 2LiH+2B+3H 2
According to the latest research report of the U.S. Department of Energy, a qualified on-board hydrogen storage system should release and absorb about 6.5 wt% hydrogen at -40 ~ 85 ℃[10]. In view of this, reducing the temperature of hydrogen release/charging reaction and increasing the rate of hydrogen release/charging reaction are the main technical problems and important development directions to realize the industrialization of gadolinium-based on-board hydrogen storage energy materials.

2 Thermodynamic control of LiBH4 hydrogen storage

2.1 Anion/cation substitution

The main way to overcome the thermodynamic instability is to replace or partially replace the Li+ by other cations with stronger electronegativity to destabilize the structure of the LiBH4. The ionic bond strength is a function of the decomposition temperature and reaction enthalpy of the compound, while the electronegativity is closely related to the bond strength, so theoretically, if the cationic Li+ of the LiBH4 is partially replaced to form a mixed cationic phase, such as a LiK(BH4)2, the thermodynamic stability must be significantly reduced[11].
Nakamori et al studied the thermodynamic stability of a series of metal borohydrides M(BH4)n(M=Li, Na, K, Cu, Mg, Zn, Sc, Zr and Hf, n = 1 ~ 4) by first-principles calculation. The results show that the ionic bond and electron transfer between the metal Mn+ cation and [BH4]- anion in the M(BH4)n are the key to determine the stability of metal borohydrides.The ΔHboro of the formation energy of the M(BH4)n is linearly related to the Pauling electronegativity χP of the cation, that is, ΔHboro=248.7χP-390.8 kJ/mol[BH4]-. The formation energy of the borohydride increases with the increase of the electronegativity of the metal, and the higher the formation energy, the worse the thermodynamic stability[12]. Miwa et al. studied the influence of partial substitution of Li+ by Cu+ on the thermodynamic stability of LiBH4. The ion size of :Cu+ and Li+ is similar.Assuming that (Li1-xCux)BH4(x=0.25, 0.5, 0.75 and 1) have the same orthorhombic crystal structure as the LiBH4,The results of first-principles calculation show that the formation energy of the composite increases with the increase of copper content, but the hydrogen storage capacity also decreases significantly.Considering that X = 0. 3, the (Li0.7Cu0.3)BH4 with hydrogen storage capacity of about 8. 74 wt% is the most suitable for practical application, and its formation energy is -39 kJ/mol H2[13]. Similarly, Nakamori et al. Found that the initial dehydrogenation temperature of the LiNH2 was reduced by 50 K after the partial substitution of Mg ions for Li[14]. Ravnsbæk et al. Successfully prepared new coordination hydride hydrogen storage materials, LiZn2(BH4)5, NaZn2(BH4)5 and NaZn(BH4)3, by high-energy ball milling of mixtures of MBH4(M=Li (or Na) and ZnCl2 in different proportions, and their crystal structures are shown in Figure 1.It is found that these borohydrides will slowly decompose and release hydrogen, as shown in reaction (2), even in the argon protective atmosphere at -35 ℃, and the hydrogen release product is metallic zinc, because the thermodynamic stability of metallic zinc is much higher than that of zinc hydride or zinc boride[15].
2MZn2(BH4)5+M2ZnCl4→5Zn+4MCl+5B2H6+5H2
By comparing the thermodynamic properties of the NaZn2(BH4)5 and the NaZn(BH4)3, it is verified that the stability of metal borohydrides will increase with the increase of alkali metal content[12]. In 2018, Bai et al. Of Beijing University of Technology prepared LiBH4-4LiNH2, Mg(BH4)2-4LiNH2 and Ca(BH4)2-4LiNH2 by liquid ball milling, and the kinetic study confirmed that.Compared with the LiBH4-4LiNH2, the apparent activation energies of the Mg(BH4)2-4LiNH2 and the Ca(BH4)2-4LiNH2 are reduced by 29.3% and 26.2%, respectively[16]. More importantly, compared with the LiBH4,Ca(BH4)2, the [BH4]- group in the Mg(BH4)2 is more likely to have a complex reaction with the [NH2]- group in the LiNH2,O as to inhibit the generation of by-product NH3[16].
While trying to replace cations, scholars have found that replacing anions is also expected to become another new way to improve the thermodynamics of LiBH4. Taking the orthorhombic crystal containing four LiBH4 tetrahedra as the research object, Corno et al. Found in the calculation process that when 2 ≤ the number of substitution atoms ≤ 4, the energy of the orthorhombic crystal obtained by replacing H atoms of the same LiBH4 tetrahedra with F is lower than that of replacing H atoms of different tetrahedra at the same time, and the structure is more stable[17]. From the thermodynamic point of view, the reaction of pure hydrides to fluorinated hydrides with any H/F ratio at room temperature is an endothermic reaction, for example, the higher the energy, the worse the stability, so the substitution of F for H makes the pure LiBH4 thermodynamically unstable. Taking the supercell of Li8B8H32 as the research object, Yin et al. simulated the process of F replacing H in LiBH4 by first-principles calculation, and found that the dehydrogenation reaction thermodynamics of Li8B8H32-xFx was significantly improved compared with that of LiBH4, and the dehydrogenation product LiH1-xFx had better hydrogen charging thermodynamic conditions than LiH[18]. According to the zero-point energy correction and van Hoff equation, the enthalpy change of the dehydrogenation reaction of the Li8B8H30F2 is only 36.5 kJ/mol H2: in the 1 bar H2 environment, and the decomposition temperature of the compound with the mass hydrogen storage density up to 9.6 wt% is only about 100 deg C. However, the feasibility of this method depends on whether the compounds obtained after F- replacement can maintain the original lattice characteristics of LiBH4 and LiH, so conclusive experimental evidence is still needed to clarify the specific existence state of F-.
Huang et al. Of Northwestern University studied the effects of Mg substitution, N substitution and Mg + N co-substitution on the crystal structure and hydrogen dissociation energy of LiBH4[19]. The first-principles calculations show that the interaction between Mg-H bond and N-H bond weakens the B-H bond, which makes the co-substitution of Mg + N more unstable than the substitution of Mg or N alone. The hydrogen dissociation energy (-2. 461 ~ -0. 4 eV) of the co-substitution is much lower than the Mg substitution (-0. 987 ~ 0. 331 eV) and N substitution (1. 531 ~ 2. 350 eV), and the onset temperature of hydrogen desorption is 160. 5. Ley et al. prepared novel compounds LiLa(BH4)3Cl and LiGd(BH4)3Cl by ball milling a mixture of LaCl3-3LiBH4 and GdCl3-3LiBH4, and heated to 200-400 ° C, evolved 5.36 wt% and 4.99 wt% hydrogen, respectively, as shown in reaction equations (3) and (4)[20]. The disorder of Li atoms in this compound directly leads to the high lithium ion conductivity of LiM(BH4)3Cl(M=La, Gd) (Fig. 2).
2LiLa(BH4)3Cl→LaH2+LaB6+11H2+2LiCl
4LiGd(BH4)3Cl→GdH2+3GdB4+23H2+4LiCl
Metal + anion substituted LiCe(BH4)3Cl,Li with insulator characteristics were successfully prepared by ball milling LiBH4 and CeCl3,Frommen with molar ratio of 1:3. The bonds between Cl, Ce and host lattice are all ionic bonds, but the bonds between B and H are ionic + covalent mixed bonds.When heated, the 220℃,LiCe(BH4)3Cl decomposes into CeH2 and amorphous CeB6 or elementary substance B, and releases 6.2 ~ 6.8 wt% of hydrogen. However, the hydrogen adsorption capacity of the dehydrogenation product can only reach 13% of the original hydrogen storage capacity at 340 ℃ and 80 bar hydrogen environment[21]. Similar results have been obtained in the study of new compounds such as Ca((BH4)1-xIx)2, K2Zn(BH4)xCl4-x and K3Zn(BH4)xCl5-x[22][23]. Therefore, the synergistic substitution of cation + anion can improve the hydrogen evolution thermodynamics of LiBH4 more effectively than the substitution of cation (metal) and anion (non-metal) alone.

2.2 Reactant destabilization

According to the "reverse stability" criterion, the introduction of appropriate reactants to react with the LiBH4 to produce a product more stable than the hydrogen-releasing product (LiH + B) of the single-phase LiBH4 decomposition can theoretically make the LiBH4 thermodynamically unstable. Vajo et al. predicted by first-principles calculations that the enthalpy change of the dehydrogenation reaction of the LiBH4+MgH2 system with a theoretical hydrogen storage capacity of up to 11.45 wt% is only 46 kJ/mol H2[7]. In 1 bar hydrogen environment, reaction (5) can occur at 168 ℃, which is about 240 ℃ lower than the decomposition temperature of single-phase LiBH4, because the dehydrogenation product MgB2 of the new system is more thermodynamically stable than B generated by single-phase decomposition of LiBH4 and Mg generated by single-phase decomposition of MgH2, which reduces the energy difference between the hydrogen adsorption state and the hydrogen release state, as shown in Figure 3.
2LiBH4+MgH2 ↔ MgB 2+2LiH+4H2
Many scholars believe that the reaction between LiBH4 and MgH2 is completed in two steps, and the MgH2 is decomposed into Mg first, as shown in reaction formula (6).Then the product Mg reacts with the LiBH4 to form LiH and MgB2, and releases hydrogen, as shown in reaction (7), so the initial hydrogen release temperature of the composite system is actually determined by the decomposition temperature of the MgH2[24⇓~26]. According to the enthalpy change of the decomposition reaction in Fig. 3,MgH2, the theoretical decomposition temperature of the MgH2 is about 311 ° C in a hydrogen atmosphere of 1 bar. Therefore, although the enthalpy change of the hydrogen evolution reaction (5) of the LiBH4+MgH2 system has been as low as 46 kJ/mol H2 and has good thermodynamic conditions, the actual reaction temperature is still higher than 300 ℃ due to the limitation of the initial decomposition temperature of MgH2.
MgH2↔ Mg+H2
2LiBH4+Mg ↔ MgB2+2LiH+3H2
Ohba et al. predicted by density functional theory (DFT) calculation that the hydrogen release process of LiBH4 should involve the formation of intermediate compound Li2B12H12, and the standard reaction enthalpy change 44.4 kJ/mol H2: of reaction (8) in 1 bar and 0.01 bar hydrogen environment, the LiBH4 began to release hydrogen from about 170 ℃ and about 48 ℃, respectively[27]. However, in the actual experiment, the formation of Li2B12H12 can only be detected when heated to 427 ~ 457 ℃, which indicates that although the thermodynamic theory of low temperature formation of Li2B12H12 is feasible, there are obvious kinetic obstacles[28]. Ozolins et al. predicted the reaction of Li2B12H12 with MgH2 by density functional theory. As shown in reaction formula (9), the standard reaction enthalpy is 60.1 kJ/mol H2[29]. At 1 bar and 0.01 bar hydrogen, that reaction can take place at about 214 C. and about 98 C., respectively. However, since the preparation of the intermediate compound Li2B12H12 requires a high temperature of more than 400 ℃, whether the reaction (9) can occur at such a low temperature has never been experimentally verified.
12LiBH4 ↔ Li 2B12H12+10LiH+13H2
Li2B12H12+6MgH2 ↔ 6MgB 2+2LiH+11H2
Inspired by the research of Li-Mg-B-H system, the exploration of other efficient destabilizers has gradually become a research hotspot of LiBH4 based hydrogen storage materials. Pinkerton et al. proposed that the standard enthalpy of the dehydrogenation reaction (10) of the LiBH4+CaH2 system with a hydrogen storage capacity of 11.7 wt% is 11.1 wt% of hydrogen released by the 59 kJ/mol H2,6LiBH4+CaH2+0.25TiCl3 system at 450 ℃ for 30 min. However, Barkhordarian et al. tried to keep the 6LiH+CaB6 mixture in a 35 MPa high-pressure hydrogen environment at the same temperature for 1440 min, and did not obtain any hydrogen-charged compounds (LiBH4 and CaH2)[30][31].
6LiBH4+CaH2 ↔ CaB 6+6LiH+10H2
By analyzing the phase composition and change before and after dehydrogenation by powder X-ray diffraction (PXRD), Liu et al. Found that the dehydrogenation reaction of the LiBH4+CaH2 system is actually a reaction between molten LiBH4·LiCl and CaH2, as shown in reaction formula (11)[32]. 6LiBH4+CaH2+xTiCl3(x=0, 0.05, 0.15 and 0.25) can be divided into four stages, namely "formation-solid solution-molten solvent-precipitation": (I) LiCl is formed during ball milling, (ii) with the increase of temperature, the o-LiBH4 transforms into h-LiBH4 and combines with LiCl to form LiBH4·LiCl solid solution, (III) LiCl is dissolved in the molten solvent to form LiBH4·LiCl solid solution. (iii) the solid solution is melted above 280 deg C, and the viscosity of the molten LiBH4·LiCl is higher than that of the molten LiBH4, so that the aggregation of the LiBH4 and the agglomeration of the CaH2 are prevented, and the nanostructure is retained; (iv) when that temperature is high than 350. degree. C., the molten LiBH4·LiCl reacts with the CaH2 to precipitate LiCl.
3xLiCl+xTi(BH4)3+(6-3x)LiBH4+CaH2↔CaB+ (6-3x)LiH+3xLiCl+(10+0.5x)H2+xTiH2
Lim et al. Found that the enthalpy change of hydrogen desorption reaction of the system with a certain amount of NbF5,6LiBH4+CaH2 was reduced to 56.5 kJ/mol H2, and the reversible hydrogen storage content of 6 wt% could be maintained at 450 ℃ without attenuation[33,34]. Jiang et al. Also found a more pronounced effect than the CaH2,CaHCl destabilizer, as shown in reaction (12)[35]. In fact, compared with the LiBH4+MgH2,LiBH4+CaH2 system, the hydrogen release/charge reaction conditions are more stringent, so it is more dependent on the modification of the catalyst.
6LiBH4+CaHCl ↔ CaB 6+5LiH+LiCl+10H2
Through first-principles calculations, Alapati et al. predicted that the 2LiBH4+ScH2 system could completely react and release 8. 9 wt% hydrogen when heated to 60 ℃ in 1 bar hydrogen environment, but Purewal et al. found that the ScH2 did not participate in the hydrogen release reaction, and its effect as a destabilizer was far lower than the theoretical prediction[36][37].

3 LiBH4 Dynamics Control

Compared with microcrystalline or coarse-grained materials, the proportion of grain boundary disordered regions in nanocrystalline materials is significantly increased, and the grain boundary disordered regions can provide a fast diffusion channel for hydrogen atoms, so nanocrystalline hydrogen storage materials have relatively excellent hydrogen discharge/charge kinetics[38][7,26,39⇓⇓⇓⇓⇓⇓⇓ ~47].

3.1 Ball milling-mechanical activation

Mechanical ball milling is the most common process to prepare nanostructured materials, which has low cost, low energy consumption and is suitable for large-scale application. Coarse-grained materials undergo repeated severe plastic deformation, cold welding and crushing in the process of ball milling, and gradually become nano-powders with uniform particle size[48⇓⇓~51]. Taking the vertical ball mill as an example, during ball milling, the impellers distributed at right angles continuously stir the balls and powder, and strong physical forces up to several GPa can be generated at the collision points between the balls, between the grinding balls and the container wall, and between the grinding balls and the impellers, which can effectively destroy the Van der Waals interaction and reduce the size of the powder. Theoretically, the faster the impeller speed is, the larger the ball-powder ratio is, and the higher the milling energy is. After a long time of high-energy milling, nano-grains with ultra-fine particle size, large surface area and high defect concentration will be obtained.
Pang et al. Of Zhejiang University successfully prepared LiBH4 nanoribbons with a width of only 10 ~ 40 nm by mechanically-driven physical vapor deposition (MFPVD) process. When the nanoribbons were heated to 60 ℃, hydrogen was released obviously. However, the recovery rate of LiBH4 by this method was not ideal, so the hydrogen storage properties of the LiBH4 nanoribbons were not successfully characterized[52]. Agresti et al. Demonstrated that LiH and B can form LiBH4,LiBH4 by high-energy ball milling in hydrogen atmosphere. The recovery rate of LiBH4,LiBH4 increases with the increase of hydrogen pressure, ball milling time and ball hardness. The initial and peak dehydrogenation temperatures of the prepared amorphous LiBH4 are 80 ℃ and 442 ℃, respectively, and about 11wt% of hydrogen is released when heated to 500 ℃[54]. If the bulk LiH and B are charged with hydrogen to form LiBH4, a high temperature of at least 600 ℃ is required. Çakanyıldırım et al. Successfully prepared LiBH4 by ball milling a mixture of Li and B in a high-pressure hydrogen atmosphere of 60 bar, and also proved that the catalytic effect of NiO was better than that of Ni for the dehydrogenation reaction[55]. In general, the nanostructures modulated by ball milling can overcome the kinetic barriers and greatly improve the kinetic conditions of the hydrogen release/charging reaction, thus improving the hydrogen release/charging efficiency of the material.

3.2 Nanoconfinement

It is found that the nanomaterials prepared by simple ball milling are prone to lose their nanostructure due to sintering and agglomeration during high temperature hydrogen release/charging cycles, while the materials prepared by nanoporous or mesoporous materials can maintain their nanostructure stably at high temperature.
Guo et al. Of Yanshan University synthesized carbon nanocages (CNCs) with high specific surface area by porous template method to confine molten LiBH4. The initial dehydrogenation temperature of the LiBH4@CNCs hydrogen storage composite material prepared was reduced to 200 ℃, which was 180 ℃ lower than that of the bulk LiBH4. When heated to 550 ℃, 7.47 wt% hydrogen was released, and the dehydrogenation product could be charged into LiBH4 at 400 ℃ and 50 bar hydrogen atmosphere[56]. However, the LiBH4 in the molten state is very active. During the impregnation process, part of the LiBH4 is oxidized to LiBO2,LiBO2 and reacts with the LiBH4 to form Li3BO3. Therefore, a large part of the hydrogen storage capacity will be lost in the process of preparing the composite. Sun et al. Obtained a LiBH4 with a particle size of about 9 nm through SBA-15 template confinement, and its initial and peak hydrogen release temperatures were reduced to 45 ℃ and 100 ℃, respectively, and it released up to 8. 5 wt% hydrogen at 105 ℃ for 10 min[57]. Unfortunately, the hydrogen desorption products could not adsorb hydrogen at 450 ℃ and 7 MPa. Vajo et al. Obtained nano-sized LiBH4 by impregnation of carbon aerogel with an average pore size of ~ 13 nm. The onset and peak hydrogen release temperatures were reduced to 230 ℃ and 380 ℃, respectively. The hydrogen release of nano-sized particles in the third cycle was 60% of that in the first cycle. Under the same conditions, the bulk LiBH4 had been attenuated to 28% of the original[58,59]. Javadian et al. prepared nano-sized LiBH4-Mg2NiH4 particles by using activated carbon and carbon aerogel with pore sizes of about 4 nm and about 30 nm, respectively, showed excellent single-step dehydrogenation kinetics at 300 ℃, and could completely dehydrogenate to MgNi2.5B2 when heated to 450 ℃[60]. However, the hydrogen storage capacity of the nanoparticle degrades significantly with the release/charge cycles, releasing 3.8 wt%H2 in the first cycle, dropping to 2.3 wt%H2 in the second cycle, and releasing only 1.7 wt%H2 in the fifth cycle. Shao et al. prepared LiBH4@ZTC by using zeolite template carbon (ZTC) with high porosity and stable mechanical properties. Compared with the bulk LiBH4, the initial hydrogen desorption temperature (194 ℃) of the nanoparticles was reduced by 181 ℃, and the apparent activation energy was also reduced from 189. 4 kJ/mol to 129. 0 kJ/mol, which greatly reduced the kinetic barriers[61]. However, the hydrogen storage capacity of the LiBH4@ZTC also declines significantly with the cycle, and the hydrogen release capacity of the second and fifth cycles has dropped to 71% and 54% of the original, respectively. The LiBH4@2Ti3C2 prepared by Zang et al with two-dimensional layered Ti3C2MXene confinement can reduce the initial dehydrogenation temperature to 172.6 ℃, and can release 9.6 wt% hydrogen at 380 ℃ for 1 H[62]. However, the hydrogen storage capacity of the LiBH4@2Ti3C2 composite system degrades significantly with the release/charge cycles, and the hydrogen storage capacity has decreased by 48% after three cycles. Recently, Pan et al of Zhejiang University prepared 7LiBH4@3PHCNS by porous hollow carbon nanospheres (PHCNSs) confinement, the initial hydrogen desorption temperature was reduced to 200 ℃, and the hydrogen released up to 8. 1 wt% at 350 ℃ for 25 min. However, through SEM and TEM characterization, it was found that after five cycles of hydrogen desorption/charging, there was obvious particle agglomeration, which affected the kinetics of hydrogen desorption/charging reaction and led to the decay of hydrogen storage performance with cycles[63]. Liu et al. Found that in the high-pressure hydrogen environment of 100 bar, the original mesoporous carbon hollow spheres (MCHSs) with a diameter of 300 nm and a large number of 2 ~ 8 nm nanochannels, as shown in Figure 4, will be transformed into a unique double-layer carbon nanobowl structure, and the LiBH4(20 wt%)@MCHSs prepared by the nanobowl confined LiBH4 melt.The initial and peak hydrogen release temperatures are 225 ℃ and 353 ℃, respectively, and the hydrogen release rate is as high as 10. 9 wt% when heated to 500 ℃, and the hydrogen release product can be reversibly charged up to 8. 5 wt% at 300 ℃ and 100 bar hydrogen environment for the first time, which also provides more possibilities for the application of nano-light metal composite hydrides in hydrogen fuel cell vehicles[64].

3.3 Catalyst doping modification

The doping modification of catalysts is an important technical approach to reduce the temperature of hydrogen release/charging and accelerate the reaction of hydrogen release/charging, and its effectiveness has been fully verified by the significant improvement of the kinetics of hydrogen desorption/adsorption reaction of NaAlH4 by Ti-based compounds, but the exact catalytic mechanism is still unclear. In the search for catalysts suitable for LiBH4, researchers have found that some metal oxides, such as SiO2, TiO2, or metal halides, such as TiCl3.They can improve the dehydrogenation kinetics of LiBH4, but their catalytic effect on the hydrogenation reaction of dehydrogenation products LiH and B is not ideal, which is far lower than the catalytic effect of Ti system on NaAlH4[8,45,46,65]. This shows that the coordination metal hydride has a strong selectivity for the catalyst.
Zang et al. Of Nankai University doped a mixture of NiO, NiCo2O4 and Co3O4 mesoporous nanorods in a certain proportion during ball milling of LiBH4 to prepare nano-LiBH4-NiCo2O4.During the heating process, the (80℃<T<350℃)LiBH4 reacts with the NiCo2O4 (12) to release hydrogen, and it is found that the actual active catalyst is the CoB+Ni3B of the reaction product.The thermodynamic and kinetic conditions of the hydrogen release reaction of the LiBH4 at high temperature were significantly improved. 5.7 wt% of hydrogen was released at 400 ℃ for 1 min, 12 wt% of hydrogen was released at 400 ℃ for 5 H, and 16.1 wt% hydrogen was released when the LiBH4 was heated to 500 ℃[66].
12LiBH4+3NiCo2O4→4Li3BO3+6CoB+Ni3B+B+24H2
Cai et al. Found that CoB can provide negative electrons for LiBH4 and Li2B12H12 in the process of hydrogen release due to its unique non-compensatory structure, thus promoting the release of hydrogen. After trying different forms of CoB catalysts (rod, sheet, chain, bayberry and mulberry), they found that:The CoB catalyst with the largest specific surface area has the best catalytic performance, the initial and peak desorption temperatures of the LiBH4-CoB system are reduced to 179 ℃ and 367 ℃, respectively, and 10. 4 wt% hydrogen is released within 1 H at 350 ℃, but the hydrogen charging of the desorption product requires a high temperature above 400 ℃[67]. In addition, the SiB4 catalyst (Fig. 5) with the same non-compensation structure can not only destabilize the LiBH4 thermodynamics and improve the hydrogen desorption kinetics, enabling the LiBH4-SiB4 to release 10.3 wt% of hydrogen at 350 ℃, but also improve the hydrogen charging thermodynamics of the dehydrogenation product LiH, enabling 9.3 wt% of hydrogen to be absorbed at 400 ℃ through reaction (13)[68].
SiB4+4LiH+6H2→4LiBH4+Si
Dolotko et al. prepared 2LiBH4-SiS2 nanomaterials by mechanochemical method, and found that the initial hydrogen release temperature was 88 ℃, and when heated to 385 ℃, the hydrogen release amount reached 4.3 wt%. Unfortunately, in the high-pressure hydrogen environment at the same temperature, the dehydrogenation product could only absorb 1.5 wt% of hydrogen[69]. He et al. found that under the catalysis of Fe-B compound, the LiBH4·NH2NH2 composite can release about 13.0 wt% hydrogen at 140 ℃ through the reaction formula (14). However, the hydrogen release products are stable compounds Li3BN2 and BN, which makes it extremely difficult to absorb hydrogen again. The experiment proves that only 0.5 wt% hydrogen can be absorbed even under the high-pressure hydrogen condition of 100 bar[70][71].
3LiBH4·N2H4→Li3BN2+2BN+N2+12H2
Wang et al. Found that adding Li2NH to ball milling for more than 24 H or heating to 147℃,Li2NH would react with LiBH4 to form Li6(BH4)2(NH)2, and the LiBH4-Li2NH complex could begin to adsorb hydrogen at 37 ℃ through reaction (15).It is the lowest operating temperature of the LiBH4/Li2NH system reported so far. However, like the LiBH4·NH2NH2 system, the complex releases a large amount of hydrogen at 250 ℃ while forming stable compounds Li3BN2 and BN[72].
3(LiBH4-Li2NH)+3H2→2LiBH4+Li4(BH4)(NH2)3+3LiH
Accord to that reaction formula (16), Li, etc., is obtained by heat treat a mixture of a Nb(OEt)5 and a LiBH4 ball mill,The LiBH4-0.04(Li3BO3+NbH) prepared by introducing the double catalysts of Li3BO3 and NbH have superior hydrogen release/charge reversibility and hydrogen release/charge kinetic conditions (Fig. 6):(i)Li3BO3 can provide Li+ active cations to weaken the Li-B bond[73]; The (ii)Li3BO3 can also provide the O atom to attract the electron-deficient B atom in the [BH4]- and reduce the thermodynamic stability of the LiBH4; (iii) As an "active valve" in the system, the presence of NbH can accelerate the diffusion of hydrogen.
LiBH4+xNb(C2H5O)5→(1-3x)LiH+xNbH+xLi3BO3+(1-x)B+(1.5-1.5x)H2+3xC2H6+2xC2H6O
It was found that the initial and peak dehydrogenation temperatures of the composite were 190 ℃ and 340 ℃, respectively, and 8. 2 wt% hydrogen was released when the composite was heated to 400 ℃, and 7. 9 wt% hydrogen was reabsorbed by the dehydrogenation product at 500 ℃ and 50 bar. It is worth noting that even after 30 cycles, the composite can still release up to 7. 2 wt% hydrogen, but the high temperature requirement (400 ~ 500 ℃) of the hydrogen release/charge reaction limits the further promotion and application of the system.
Recently, Zhu et al of South China University of Technology found that the milling of LiBH4 doped with surface-modified AlN has obvious advantages: (1) Hδ+ can be generated and react with Hδ- in the LiBH4 to make the LiBH4 thermodynamically unstable[74]; (2) the hydrogen release/charge reaction path is regulated and controlled by the functional groups on the AlN to avoid the generation of intermediate compound Li2B12H12; And (3) provide a fast diffusion channel for Li and H. Therefore, the LiBH4+0.5AlN composite can release up to 11. 8 wt% hydrogen at 400 ℃ for 9 H, and the product can be charged successfully at 400 ℃ and 10 MPa hydrogen, but the hydrogen storage capacity of the system decays to 6 wt% after several charging/discharging cycles.

4 Thermodynamic/kinetic dual control of LiBH4.

Shaw et al. From Illinois University of Technology have proved that the reaction (8) can occur at room temperature through experiments, and the nano-LiBH4 particles prepared by evaporating the LiBH4/THF solvent begin to release hydrogen at about 32 ℃, which further confirms that nanostructure modulation can effectively improve the kinetics of LiBH4 hydrogen release reaction[75,76].
On this basis, Ding et al. tried to spray LiBH4/THF aerosol (AS) into the closed ball milling tank while milling the MgH2(BM) with high energy, referred to AS BMAS process, to prepare nano-LiBH4+MgH2 composite hydrogen storage materials. The experimental equipment diagram is shown in Figure 7[77⇓~79]. The main part of the equipment is a vertical ball mill. There are two air release valves on the top of the ball mill tank, one is connected to the aerosol generator, and the other is connected to the vacuum pump. The ball milling, aerosol spraying and vacuum pumping operations are controlled by a programmed electromechanical module (EASY 512-AC-RC). Before the experiment, the aerosol was sprayed onto the porous PTFE membrane, and observed by scanning electron microscopy (SEM). As shown in Figure 8, the size of the LiBH4 particles in the aerosol was between 20 and 100 nm. During ball milling, the local temperature at the collision point generated by the high-speed collision between the balls, between the grinding ball and the container wall, and between the grinding ball and the impeller is as high as 200 to 250 deg C. Therefore, when the aerosol droplets touch the grinding balls and the MgH2 particles, the THF solvent with the melting point of only 66 deg C is instantaneously vaporized, and a layer of uniform coating is formed on the surface of the MgH2 particles by the remaining nano LiBH4[80⇓~82]. During high-energy ball milling, the MgH2 particles are continuously fractured and cold-welded, and the nano-LiBH4 with smaller particle size is wrapped in the MgH2 particles, and finally the evenly mixed nano-LiBH4+MgH2 particles are obtained. The vacuum pump works intermittently to pump out the argon and THF gas in the tank and reduce the air pressure so as to inject more LiBH4/THF aerosol. During the whole BMAS process, there is flowing water on the outer wall of the ball mill tank to prevent the temperature in the tank from being too high.
Through the new BMAS process, Ding and Shaw et al. Successfully prepared a BMAS(25%LiBH4) powder with a molar ratio of LiBH4 to MgH2 of 1:2. The experiment proved that when heated to 265 ℃, the powder would release hydrogen through two sub-reactions (8) and (9) predicted by the first-principles calculation, creating a new record of reversible hydrogen release (5.7 wt%)/charge (5.3 wt%) capacity of the solid LiBH4+MgH2 (without catalyst) system, as shown in Figure 9[77][27,29][24⇓~26,80,83⇓⇓⇓⇓ ~88]. Only about 0.5 wt% or less of hydrogen is released from single-phase LiBH4 below the melting point, and only 4.0 wt% of hydrogen is released from the nano-2LiBH4+MgH2 composite prepared by high-energy ball milling at liquid nitrogen temperature under the same conditions (265 ℃)[5][80]. The simulation of Shrinking-Core gas-solid reaction kinetics model shows that the hydrogen desorption reaction of BMAS(25%LiBH4) powder has changed from hydrogen diffusion control of 2LiBH4+MgH2 powder prepared by high-energy ball milling to nucleation and growth control of LiH and MgB2 products[89][90]. Therefore, the interface area between the LiBH4 and the MgH2, which plays a key role in the nucleation and growth of LiH and MgB2, is the key factor affecting the dehydrogenation kinetics and charging/dehydrogenation capacity of BMAS(LiBH4+2MgH2) powder.
The BMAS(50%LiBH4) powder prepared by Ding et al. With a molar ratio of LiBH4 to MgH2 of 1:1 by doubling the BMAS cycle could only release about 4.1 wt% hydrogen at 265 ° C[79]. Although that hydrogen storage property is reduce compared with that of the BMAS(25%LiBH4) pow, a Mg(BH4)2 phase is found in the powder,This is the first report of the direct reaction of LiBH4 with MgH2 and the successful formation of Mg(BH4)2, which decomposes into MgH2 and releases hydrogen when heated to 265 C[7,26,39,43,44,78,81,82]. The Fe3B phase was also detected in the products of hydrogen release. It was inferred from the experimental results that the Fe3B directly participated in the hydrogen release/charge reaction cycle of the system and showed some hydrogen storage properties[91]. In fact, Hao et al. Have predicted the hydrogen storage properties of Fe3B by first-principles calculations, but they have never been verified by experiments[92,93]. Different kinetic models were used to analyze the dehydrogenation reaction of solid BMAS(50%LiBH4) powder at 265 ℃. It was found that the rate limiting step of the dehydrogenation reaction of the powder was initially controlled by the nucleation and growth of LiH and MgB2, but with the progress of dehydrogenation/charging cycles, the rate limiting step gradually changed to the control of moving phase boundary, and finally to the control of hydrogen diffusion[94]. As shown in fig. 10, the powder decomposition pressure test shows that the saturated decomposition pressure of the BMAS(50%LiBH4) powder is 19.1 times and 7.0 times higher than that of the nano MgH2 powder and the bulk LiBH4 micropowder prepared by ball milling, respectively; The results of differential scanning calorimetry (DSC) show that the apparent activation energy of BMAS(50%LiBH4) powder is 23. 3 kJ/mol and 30. 6 kJ/mol lower than that of bulk LiBH4 powder and MgH2 powder prepared by ball milling, respectively, which directly proves that BMAS process has unique advantages in the preparation of LiBH4+MgH2 hydrogen storage materials.

5 Conclusion and prospect

Under the guidance of the development concept of innovation, coordination, green, open and sharing, the adjustment of energy structure is an inevitable choice to solve the problem of harmonious coexistence between man and nature, accelerate the construction of a resource-saving and environment-friendly society, and promote the construction of a beautiful China. LiBH4 is one of the known solid materials with the highest mass and volume hydrogen storage density, and is the first choice of hydrogen storage energy materials for vehicles. Therefore, how to reduce its stability and improve its reversible hydrogen storage performance has become a research hotspot in recent years. Through the review of the current research status, the following conclusions can be drawn:
(1) Mechanical ball milling is a simple and effective method to prepare nanocrystals with ultra-fine particle size, large surface area and high defect concentration, which can improve the kinetics of hydrogen release/charging reaction at the lowest cost, but the nanomaterials prepared by mechanical ball milling are prone to lose their nanostructure due to sintering and agglomeration during high temperature hydrogen release/charging cycles;
(2) Although the nanometer-confined Hf-based hydrogen storage materials can stably maintain their nanostructures during the cyclic reaction, the hydrogen storage performance of the nanocomposites can not be prevented from significantly declining with the hydrogen release/charge cycles, and the practical application prospects of this technology largely depend on the progress of the preparation technology of nanoporous materials;
(3) Catalyst doping improves the dehydrogenation kinetics of the LiBH4 to a certain extent and realizes faster hydrogen release at lower temperature, but the catalytic effect of the catalysts developed at present on the hydrogen charging process of dehydrogenation products LiH and B is not obvious, so the technical problem of the current process is to develop compounds that can effectively catalyze the dehydrogenation/hydrogen charging reaction at the same time;
(4) Partial or complete substitution of anions/cations can effectively reduce the thermodynamic stability of LiBH4 and achieve lower dehydrogenation temperature, but this process requires extremely harsh experimental conditions, so that the technology is still mainly in the first-principles theoretical calculation stage, few successful synthesis reports, and part of the hydrogen storage capacity will be lost after substitution;
As the most promising "destabilizer" of LiBH4“, the introduction of (5)MgH2 greatly reduces the enthalpy change of hydrogen release/charge reaction, which theoretically should achieve a lower hydrogen release temperature, but the actual initial hydrogen release temperature of LiBH4+MgH2 system is still higher than 300 ℃ due to the initial decomposition temperature of MgH2 reactant and the kinetic barrier of Li2B12H12 intermediate product formation.
Through the newly developed high-energy ball milling combined with aerosol spraying (BMAS) process, our group has successfully prepared solid (≤265℃)LiBH4+MgH2 composite hydrogen storage materials with a reversible hydrogen release/charge capacity of about 5.7 wt%/about 5.3 wt%. The core of the BMAS process is to introduce nano LiBH4 by spraying LiBH4/THF aerosol, prepare nano MgH2 by high-energy ball milling, and realize the uniform mixing of nano LiBH4 particles and nano MgH2 particles.Based on this, two parallel hydrogen release/charge reaction paths of the MgH2-Mg (reactions (6), (7)) and the LiBH4-Li2B12H12 (reactions (8), (9)) of the LiBH4+MgH2 system become possible at ≤ 265 ° C. The BMAS process can help the LiBH4+MgH2 system with good thermodynamic properties to overcome the kinetic obstacles, and even provide thermodynamic driving force to promote the release of hydrogen at low temperatures, which is the most promising process to meet the requirements of industrial vehicle nano-hydrogen storage materials. At the same time, first-principles calculations show that hydrogen storage systems such as LiBH4+MgH2+Mg(BH4)2, LiBH4+Mg(BH4)2 and LiBH4+Ca(BH4)2 have good thermodynamic conditions, but are greatly limited by their kinetic obstacles.So that these systems show very slow hydrogen desorption kinetics below 300 ℃. The proposal of BMAS technology opens up a new direction for the research and improvement of hydrogen storage performance of these composite systems, and provides a reliable solution[29][47,95⇓⇓ ~98]. In a word, the BMAS process is simple to operate and suitable for large-scale application, which has great potential in realizing the localization of low-cost production of high-quality nanomaterials that even meet the technical standards of on-board hydrogen storage, and is of great significance for ultimately reducing the cost of on-board hydrogen storage materials.

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Funding

U.S. National Science Foundation(CMMI-1261782)

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