Li2(NH2BH3)(BH4)/LiNH2BH3: The first metal amidoborane borohydride complex with inseparable amidoborane precursor for hydrogen storage

A novel lithium amidoborane borohydride complex, Li₂(NH₂BH₃)(BH₄), was synthesized using mechanochemical method and its crystal structure was successfully determined by a combination of X-ray diffraction (XRD) analysis and first-principles calculations. Interestingly, this compound does not exist as a pure phase, but requires almost equivalent amount of amorphous LiAB as a stabilizing agent. In this paper, we report a careful study of the structure, property, and dehydrogenation mechanism of the 1:1 Li₂(NH₂BH₃)(BH₄)/LiAB composite. This composite can release ∼8 wt% H₂ at 100 °C via a two-step dehydrogenation process, with dehydrogenation kinetics better than the parenting phases. The composite and its dehydrogenation products were characterized by the combined XRD, Fourier transformation infrared (FTIR) spectroscopy, and solid-state ¹¹B MAS NMR techniques. Selective deuterium labeling was performed to elucidate a reaction sequence for the hydrogen release by analyzing the released gases.     

LiBH4·NH3BH3: A new lithium borohydride ammonia borane compound with a novel structure and favorable hydrogen storage properties

Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio results in the formation of a new complex, LiBH₄·NH₃BH₃. Its structure was successfully determined using combined X-ray diffraction and first-principles calculations. LiBH₄·NH₃BH₃ was carefully studied in terms of its decomposition behavior and reversible dehydrogenation property, particularly in comparison with the component phases. In parallel to the property examination, X-ray diffraction and Fourier transformation infrared spectroscopy techniques were employed to monitor the phase evolution and bonding structure changes in the reaction process. Our study found that LiBH₄·NH₃BH₃ first disproportionates into (LiBH₄)₂·NH₃BH₃ and NH₃BH₃, and the resulting mixture exhibits a three-step decomposition behavior upon heating to 450 °C, totally yielding ∼15.7 wt% hydrogen. Interestingly, it was found that h-BN was formed at such a moderate temperature. And owing to the in situ formation of h-BN, LiBH₄·NH₃BH₃ exhibits significantly improved reversible dehydrogenation properties in comparison with the LiBH₄ phase.     

Experimental investigation on lithium borohydride hydrolysis

Lithium borohydride, one of the highest energy density chemical energy carriers, is considered as an attractive potential hydrogen storage material due to its high gravimetric hydrogen density (19.6%). Belonging to borohydride compounds, it presents a real issue to overcome aims fixed by the U.S. Department of Energy in the field of energy, and so crystallizes currently attention and effort to use this material for large scale civil and military applications. However, due to its important hygroscopicity, lithium borohydride is a hazardous material which requires specific handling conditions for industrial aspects.     

Nanoconfined mixed Li and Mg borohydrides as materials for solid state hydrogen storage

Several mixtures of LiBH₄ and Mg(BH₄)₂ borohydrides in different stoichiometric ratios (1:0, 2:1, 1:1, 1:2, 0:1), prepared by high energy ball milling, have been investigated with X-ray powder diffraction and thermal programmed desorption (TPD) volumetric analysis to test the dehydrogenation kinetics in correlation with the physical mixture composition. Afterwards mixed and unmixed borohydrides were dispersed on high specific surface area ball milled graphite by means of the solvent infiltration technique. BET and statistical thickness methods were used to characterize the support surface properties, and SEM micrographs gave a better understanding of the preparation techniques. It has been observed by TPD volumetric measurements that the confinement of the reactive borohydrides on the nanoporous supports leads to a lower dehydrogenation temperature compared to unsupported borohydrides. Moreover, a further decrease of the dehydrogenation temperature has been observed by increasing the specific surface area of the support and the pores volume and by using the prepared mixtures instead of pure materials. The dehydrogenation process seems to be favoured by the heterogeneous nucleation on the graphite surface of decomposition products or intermediate phases from melted liquid borohydrides.     

Addition of transition metals to lithium intercalated fullerides enhances hydrogen storage properties

We report an innovative synthetic strategy based on the solid state reaction of fullerene C₆₀ with lithium-transition metals alloys (platinum and palladium), which provides transition metal-decorated lithium intercalated fullerides, with improved hydrogen storage properties. Compounds with Li₆Pt_0.11C₆₀ and Li₆Pd_0.07C₆₀ stoichiometry were obtained and investigated with manometric/calorimetric techniques which showed an 18% increase of the final H₂ absorbed amount with respect to pure Li₆C₆₀ (5.9 wt% H₂) and an improved absorption process kinetic. The absorption mechanism was investigated with X-rays diffraction which allowed to identify the formation of the hydrofullerides. Scanning Electron Microscopy was applied to gain information on transition metal distribution and detected the presence of platinum and palladium aggregates which are shown to perform a surface catalytic activity towards hydrogen molecule dissociation process.     

NbF5 additive improves hydrogen release from magnesium borohydride

The dehydrogenation properties of Mg(BH₄)₂ with various additives (SiO₂, VCl₃, CoCl₂ and NbF₅) were investigated. The addition of NbF₅ significantly improved the extent of hydrogen release as well as the kinetics. While neat Mg(BH₄)₂ starts to release hydrogen >270 °C, Mg(BH₄)₂ with NbF₅ begins hydrogen release ∼75 °C, as confirmed by mass spectrometry and thermogravimetry. The maximum hydrogen yield of Mg(BH₄)₂, obtained in the presence of 15 wt% NbF₅, was 3.7, 7.4, 10.0, 11.4 wt% for 150, 250, 300 and 350 °C, respectively. Using pXRD, we confirmed that the final crystalline product at 300 °C from Mg(BH₄)₂ + 15 wt% NbF₅ was Mg, while it was MgH₂ for neat Mg(BH₄)₂. Solid state ¹¹B NMR analysis of Mg(BH₄)₂ with 15 wt% NbF₅ at 300 °C showed significant selectivity toward the formation of Mg(B₁₂H₁₂) as intermediate, while neat Mg(BH₄)₂ showed β-Mg(BH₄)₂, Mg(B₂H₆) as well as some Mg(B₁₂H₁₂). Our results demonstrate that NbF₅ is a promising additive to provide high hydrogen yield values from Mg(BH₄)₂ at moderate temperatures <300 °C.     

Synthesis and advanced dehydrogenation properties of niobium-based ammine borohydride

In this paper, niobium-based ammine borohydride has been synthesized via a simple ball milling of NbCl₅·5NH₃ and MBH₄ (M = Li, Na) with a molar ratio of 1:5. Thermogravimetric analysis–mass spectrometry (TGA–MS) and temperature-programmed-desorption (TPD) results revealed that the dehydrogenation of NbCl₅·5NH₃/5LiBH₄ and NbCl₅·5NH₃/5NaBH₄ mixtures showed a two-step decomposition process with a total of 8.1 wt.% and 11.2 wt.% pure hydrogen evolution upon heating to 250 °C, respectively. Isothermal TPD results showed that over 6 wt.% and 10.4 wt.% pure hydrogen were liberated from NbCl₅·5NH₃/5NaBH₄ within 60 min at 150 °C and 220 °C, respectively. Fourier transform infrared spectroscopy (FTIR) and isotope tagging measurements demonstrated that the dehydrogenation mechanism of niobium-based ammine borohydride is not only based on the combination reaction of BH and NH groups, but the BH?HB and NH?HN homo-polar interactions also contribute to the H₂ formation.     

Improved hydrogen storage properties of LiBH4 by mechanical milling with various carbon additives

Various LiBH₄/carbon (graphite (G), purified single-walled carbon nanotubes (SWNTs) and activated carbon (AC)) composites were prepared by mechanical milling method and further examined with respect to their hydrogen storage properties. It was found that all the carbon additives can improve the H-exchange kinetics and H-capacity of LiBH₄ to some extents. Compared with G, SWNTs and AC exhibited better promoting effect on the hydrogen storage properties of LiBH₄. Based on combined property/phase/structure analysis results, the promoting effect of the carbon additives was largely attributed to their heterogeneous nucleation and micro-confinement effect on the reversible dehydrogenation of LiBH₄.     

Magnesium borohydride as a hydrogen storage material: Synthesis of unsolvated Mg(BH4)2

Different methods for preparation of unsolvated magnesium borohydride, a promising material for hydrogen storage, based on exchange reaction of MgCl₂ with lithium and sodium borohydride in different solvents have been evaluated. A convenient scalable method for synthesis of pure Mg(BH₄)₂ by ball milling a mixture of MgCl₂ and NaBH₄ in diethyl ether has been developed. Crystalline stable low and high temperature phases, as well as a new metastable phase of unsolvated magnesium borohydride have been prepared.     

Synthesis and characterisation of a mesoporous carbon/calcium borohydride nanocomposite for hydrogen storage

Borohydrides with high hydrogen content are being extensively studied as potential hydrogen storage systems placing particular emphasis on upturning their unfavourable kinetic and thermodynamic properties which give rise to significantly high dehydrogenation temperatures and slow hydrogen release far away from the desired application window. In this work the encapsulation of Ca(BH₄)₂ particles in the pores of a CMK-3 type ordered mesoporous carbon scaffold by wet chemistry routes, also employing the use of TiCl₃ as a catalyst, is shown to have a beneficial effect on the hydrogen desorption profile of the bulk hydride by shifting its decomposition to noticeable lower temperatures.     

Enhanced hydrogen storage properties of LiBH4-MgH2 composite by the catalytic effect of MoCl3

Though LiBH₄-MgH₂ system exhibits an excellent hydrogen storage property, it still presents high decomposition temperature over 350 °C and sluggish hydrogen absorption/desorption kinetics. In order to improve the hydrogen storage properties, the influence of MoCl₃ as an additive on the hydrogenation and dehydrogenation properties of LiBH₄-MgH₂ system is investigated. The reversible hydrogen storage performance is significantly improved, which leads to a capacity of about 7 wt.% hydrogen at 300 °C. XRD analysis reveals that the metallic Mo is formed by the reaction between LiBH₄ and MoCl₃, which is highly dispersed in the sample and results in improved dehydrogenation and hydrogenation performance of LiBH₄-MgH₂ system. From Kissinger plot, the activation energy for hydrogen desorption of LiBH₄-MgH₂ system with additive MoCl₃ is estimated to be ∼43 kJ mol⁻¹ H₂, 10 kJ mol⁻¹ lower than that for the pure LiBH₄-MgH₂ system indicating that the kinetics of LiBH₄-MgH₂ composite is significantly improved by the introduction of Mo.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Progress in sodium borohydride as a hydrogen storage material: Development of hydrolysis catalysts and reaction systems

Over the past decade, sodium borohydride (NaBH₄) has been extensively investigated as a potential hydrogen storage material. The development of catalyst materials for on demand NaBH₄ hydrolysis, and the design of practical reaction systems for hydrogen storage based on NaBH₄ are key research areas. Progress in the former area has been promising, with many non-noble catalysts being reported with activities comparable to those of higher-cost noble metal catalysts. However, the design of practical hydrogen storage systems remains a critical issue, as identified by the U.S. Department of Energy (DOE) in their “No-Go” recommendation in 2007. The problems of by-product precipitation and catalyst blockage at high NaBH₄ concentrations must be addressed in order to produce a hydrogen storage system capable of meeting the DOE target of 5.5 wt% H₂ (2015). It is likely that a new, novel reaction system design will be required to achieve these targets, given the limitations identified in conventional systems. Moreover, a new process for regenerating spent NaBH₄ will need to be developed, in order to lower its cost to a viable level for use as a transportation fuel.     

Dust cloud combustion characterization of a mixture of LiBH4 destabilized with MgH2 for reversible H2 storage in mobile applications

This study discusses results of an experimental program to determine dust cloud combustion characteristics of 2LiBH₄ + MgH₂ binary system in air. The determined parameters of hydrided and partially-dehydrided states of this system include: maximum deflagration pressure rise (P_MAX), maximum rate of pressure rise (dP/dt)_MAX, minimum ignition temperature (T_C), minimum explosible concentration (MEC), minimum ignition energy (MIE), and explosion severity index (K_St). Impact of dust particle size on the measured parameters is evaluated for the partially-dehydrided state. For dust of same mean particle size, results show the hydrided state to be more explosible in air compared to its partially-dehydrided state. Moreover, MIE of the partially-dehydrided mixture is identified in the test with lowest ignition delay time (IDT) and highest dust cloud concentration (DC). Taguchi's mixed-levels design of experiments (DoE) methodology is employed to calculate dust's MIE response surface as a function of DC and IDT. The one-at-a-time effect and interaction effect between DC and IDT on dust MIE are determined. The core insights of this contribution are useful for quantifying risks in mobile and stationary H₂ storage applications, informing H₂ safety standards, and augmenting property databases of H₂ storage materials.     

Hydrogen generation and storage from hydrolysis of sodium borohydride in batch reactors

The catalytic hydrolysis of alkaline sodium borohydride (NaBH₄) solution was studied using a non-noble; nickel-based powered catalyst exhibiting strong activity even after long time storage. This easy-to-prepare catalyst showed an enhanced activity after being recovered from previous use. The effects of temperature, NaBH₄ concentration, NaOH concentration and pressure on the hydrogen generation rate were investigated. Particular importance has the effect of pressure, since the maximum reached pressure of hydrogen is always substantially lower than predictions (considering 100% conversion) due to solubility effects. The solubility of hydrogen is greatly enhanced by the rising pressure during reaction, leading to storage of hydrogen in the liquid phase. This effect can induce new ways of using this type of catalyst and reactor for the construction of hydrogen generators and even containers for portable and in situ applications.     

Mn-based borohydride synthesized by ball-milling KBH4 and MnCl2 for hydrogen storage

In this work, a mixed-cation borohydride (K₂Mn(BH₄)₄) with P2₁/n structure was successfully synthesized by mechanochemical milling of the 2KBH₄–MnCl₂ sample under argon. The structural and thermal decomposition properties of the borohydride compounds were investigated using XRD, Raman spectroscopy, FTIR, TGA-MS and DSC. Apart from K₂Mn(BH₄)₄, the KMnCl₃ and unreacted KBH₄ compounds were present in the milled 2KBH₄–MnCl₂. The two mass loss regions were observed for the milled sample: one was from 100 to 160 °C with a 1.6 ± 0.1 wt% loss (a release of majority hydrogen and trace diborane), which was associated with the decomposition of K₂Mn(BH₄)₄ to form KBH₄, boron, and finely dispersed manganese; the other was from 165 to 260 °C with a 1.9 ± 0.1 wt% loss (only hydrogen release), which was due to the reaction of KBH₄ with KMnCl₃ to give KCl, boron, finely dispersed manganese. Simultaneously, the formed KCl could dissolve in KBH₄ to yield a K(BH₄)_xCl_1−x solid solution, and also react with KMnCl₃ to form a new compound K₄MnCl₆.     

Synthesis and hydrogen storage properties of ultrafine Mg-Zn particles

Mg-6.9 at.% Zn ultrafine particles (UFPs) were prepared by hydrogen plasma-metal reaction (HPMR) method. The electron microscopy study revealed that they were spherical in shape with particle size in the range 100-700 nm. Each fine particle was composed of single crystal structure of α-Mg(Zn) solid solution and amorphous structure of Mg-Zn alloy. After one absorption and desorption cycle, these UFPs transformed from the single crystal into the nanocrystalline structure and the mean particle size changed from 400 to 250 nm. It was found that the Mg-Zn UFPs could absorb 5.0 wt.% hydrogen in 20 min at 573 K and accomplish a high hydrogen storage capacity of 6.1 wt.% at 573 K. The fine particle size, nanocrystalline structure and the low oxide content of the obtained sample promoted the hydrogen sorption process with low hydrogen absorption activation energy of 56.3 kJ/mol. The enhanced hydrogen sorption properties of high absorbing rate and high storage capacity were due to the improved kinetics rather than the change in enthalpy.     

Theoretical study of the synthesis,characterization and hydrogen storage properties of a high-density hydrogen storage material: (CH3NH3)BH4

Inspired by both alkaline metal borohydrides and organic-inorganic hybrid perovskite, we predict a pair of complex structures of (CH₃NH₃)BH₄ with tremendous high hydrogen capacity (21.27 wt.%). Through comparison and analysis of the electronic structures of alkali metal atoms, CH₃NH₃, NH₄, and NH₃BH₃ molecule, it is concluded that similar spatial and electronic structures show the feasibility of synthesizing (CH₃NH₃)BH₄ by a substitution reaction. Firstly, theoretical structures (S₁ and S₂ in P₁) with stable configurations have been reconstructed by cation substitution followed by a series of restrictive structural optimizations, and both the lattice parameters and the position coordinate information of (CH₃NH₃)BH₄ are obtained. Ignoring the relatively mobile hydrogen, the structural symmetries of S₁ and S₂ are I4mm and P4/nmm, respectively. X-ray diffraction characterizations of S₁ and S₂ are consistent with the experimental results. Secondly, the calculated elastic constants of (CH₃NH₃)BH₄ (S₁ and S₂) with P₁ symmetry indicate that angles α, β and γ oscillate at right angles due to the influence of the cation orientation. The calculated spatial dependence of bulk (B), Young's (E), and shear (G) modulus obviously show that the two P₁ phases all have strong elastic anisotropy. Thirdly, the calculated electronic properties show that the protonic amine-H, hydridic borane-H, and neutral methane-H are widely distributed in (CH₃NH₃)BH₄, which allow for weaving in a planar dihydrogen bonding network, which in turn influences the dehydrogenation reaction. Last and most important, we propose the following dehydrogenation process of (CH₃NH₃)BH₄ via the intermediate compounds: 2(CH₃NH₃)BH₄ → CH₃NH₂BH₂NHCH₃BH₃+3H₂. For each dehydrogenation step, the free energy change is negative, which means (CH₃NH₃)BH₄ can decompose spontaneously, similar to ammonium borohydride, which is strongly related to the planar dihydrogen bonding network.