Preparation and dehydrogenation properties of two cobalt-based ammine borohydrides: CoCl3·3NH3/3LiBH4 and CoCl2·3NH3/2LiBH4

Two new cobalt-based ammine borohydrides were prepared via ball milling of LiBH₄ and CoCl_n·3NH₃ (n = 3, 2) with molar ratios of 3:1 and 2:1, respectively. X-ray diffraction (XRD) results revealed the as-prepared composites having amorphous state. Thermogravimetric analysis-mass spectrometry (TG-MS) measurements showed that the two composites mainly release H₂, concurrent with the evolution of a small amount of NH₃. Further results showed that the excessive addition of LiBH₄ can suppress the liberation of NH₃, resulting in the release of H₂ with a high purity (>99 mol.%). By combination with the temperature-programmed-desorption (TPD) results, the CoCl₃·3NH₃/4LiBH₄ and CoCl₂·3NH₃/3LiBH₄composites can release 7.3 wt.% (4.2 wt.% including LiCl) and 4.2 wt.% (2.0 wt.% including LiCl) pure hydrogen, respectively, in the temperature range of 25–300 °C. Isothermal dehydrogenation results reveal that CoCl₃·3NH₃/3LiBH₄ shows favorable dehydrogenation rate at low temperatures, releasing about 5.2 wt.% (2.9 wt.% including LiCl) of hydrogen within 45 min at 80 °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.     

Dehydriding and re-hydriding properties of high-energy ball milled LiBH4 + MgH2 mixtures

Here we report the first investigation of the dehydriding and re-hydriding properties of 2LiBH₄ + MgH₂ mixtures in the solid state. Such a study is made possible by high-energy ball milling of 2LiBH₄ + MgH₂ mixtures at liquid nitrogen temperature with the addition of graphite. The 2LiBH₄ + MgH₂ mixture ball milled under this condition exhibits a 5-fold increase in the released hydrogen at 265 °C when compared with ineffectively ball milled counterparts. Furthermore, both LiBH₄ and MgH₂ contribute to hydrogen release in the solid state. The isothermal dehydriding/re-hydriding cycles at 265 °C reveal that re-hydriding is dominated by re-hydriding of Mg. These unusual phenomena are explained based on the formation of nanocrystalline and amorphous phases, the increased defect concentration in crystalline compounds, and possible catalytic effects of Mg, MgH₂ and LiBH₄ on their dehydriding and re-hydriding properties.     

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.     

Cycleable graphite/FeSi6 alloy composite as a high capacity anode material for Li-ion batteries

FeSi₆/graphite composite was prepared by mechanical ball milling. The FeSi₆ alloy particles consist of an electrochemically active silicon phase and inactive phases FeSi₂, distributed uniformly in the graphite matrix. The composite anode offers a large reversible capacity (about 800 mAh g⁻¹) and good cycleability, due to the buffering effect of the inactive FeSi₂ phase and graphite layers on the volumetric changes of Si phase during lithium–Si alloying reaction. Since FeSi₆ alloy is a low-cost industrial material, this alloy compound provides a possible alternative for development of high capacity lithium-ion batteries.     

On the dehydrogenation of LiAlH4 enhanced by Ti salts and cryogenic ball-milling

Several mixtures of LiAlH₄ and Ti salts (TiH₂, TiF₃, and TiCl₄) were produced using short milling times and cryogenic (liquid nitrogen) cooling. The stoichiometric (2:1) and 5 mol% mixtures LiAlH₄/TiH₂ demonstrated minor improvements on the dehydrogenation temperature of LiAlH₄. Conversely, an enhancement of the dehydrogenation reaction was observed in the LiAlH₄ added with 5 mol% of TiCl₄ and in the stoichiometric mixture 3LiAlH₄ + TiF₃. In these mixtures, an important reduction of the dehydrogenation temperature was observed (37 °C and 55 °C on-set temperature, respectively). This improvement was promoted by the use of cryogenic ball milling and careful control of the energy added to the mixtures during ball milling.     

Reversibility aspect of lithium borohydrides

Reversibility is one of the key features for any hydrogen storage material. Borohydrides such as LiBH₄ have been studied or proposed as candidates for hydrogen storage because of their high hydrogen contents (18.4 wt% for LiBH₄). Limited success has been made in reducing the dehydrogenation temperature. However, full reversibility has not been realized. It is found that the dehydrogenation mechanism of metal borohydrides differs signicantly from the well-known metal hydrides such as LaNi₅H₆ and MgH₂ that release hydrogen in a single decomposition step through a solid state transformation of crystalline structure. The dehydrogenation of lithium borohydrides involves solid–liquid–gas reactions. Some of the steps in the multiple step decomposition processes of metal borohydrides are not reversible. Furthermore, the decomposition also produces stable intermediate compounds that cannot be rehydrided easily. Lastly, the volatile gases, such as BH₃ and B₂H₆, evolved in decomposition of the transition metal borohydrides cause unrecoverable boron loss. Although our experiments show the partial reversibility of the doped LiBH₄, it was not sustainable during dehydriding–rehydriding cycles because of the accumulation of hydrogen inert species and boron loss. Doping with additives reduces the stability of LiBH₄, but it also makes LiBH₄ less reversible. It raises reasonable doubt on the feasibility of making metal borohydrides suitable for reversible hydrogen storage.     

What's the appropriate precondition for ammine metallic borohydrides to generate pure hydrogen?

Multiple-cation compounds always show more desirable controllability than that of mono-metallic compounds in the field of the hydrogen-storage materials. As in the ammine metallic borohydrides (AMB), Li and Mg appears similar chemical properties, but they show different appearances in preventing the impurities in the hydrogen releasing process. We applied Car–Parrinello molecular dynamic (CPMD) method to investigate the dehydrogenation pathway for LiMg(BH₄)₃(NH₃)₂ (AMLB) on the basis of our previous study of Mg(BH₄)₂(NH₃)₂ (AMgB) and LiBH₄NH₃ (ALB). Interestingly, the purity of hydrogen gradually improves from ALB to AMgB to AMLB caused by the effects of the corresponding mixed metals. Furthermore, the two metals in AMLB are in close cooperation to control the motion of [NH₃] and [BH₄] groups, which suppress almost all the impurities and show enhanced properties in the decomposition. From our results, the improved dehydrogenation performance should depend on the large polarization of the centre metal(s). Furthermore, the purity of the hydrogen is depending on the quantities of the free hydrogen ions.     

Mechano-chemical synthesis of manganese borohydride (Mn(BH4)2) and inverse cubic spinel (Li2MnCl4) in the (nLiBH4 + MnCl2) (n = 1, 2, 3, 5, 9 and 23) mixtures and their dehydrogenation behavior

Manganese borohydride (Mn(BH₄)₂) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH₄) and manganese chloride (MnCl₂) by applying high energy ball milling for 30 min. For the first time a wide range of molar ratios n = 1, 2, 3, 5, 9 and 23 in the (nLiBH₄ + MnCl₂) mixture was investigated. During ball milling for 30 min the mixtures release only a very small quantity of H₂ that increases with the molar ratio n but does not exceed ∼0.2 wt.% for n = 23. However, longer milling duration leads to more H₂ released. For the equimolar ratio n = 1 the principal phases synthesized are Li₂MnCl₄, an inverse cubic spinel phase, and the Mn(BH₄)₂ borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH₄)₂. With the n increasing from 3 to 23 LiBH₄ is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH₄)₂. Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 °C shows the release of hydrogen as a principal gas with a maximum intensity around 130–150 °C accompanied by a miniscule quantity of borane B₂H₆. The intensity of the B₂H₆ peak is 200–600 times smaller than the intensity of the corresponding H₂ peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH₄)₂ up to about 270–280 °C. At 100 °C under 1 bar H₂ pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly ∼4 wt.% H₂ which is a very large amount of H₂ considering that the mixture also contains 2 mol of LiCl salt. The H₂ quantities experimentally desorbed at 100 and 200 °C do not exceed the maximum theoretical quantities of H₂ expected to be desorbed from Mn(BH₄)₂ for various molar ratios n. It clearly confirms that the contribution from B₂H₆ evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100–200 °C. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant k and the lowest apparent activation energy for dehydrogenation, E_A ∼ 102 kJ/mol. Decreasing or increasing the molar ratio n below or above 3 increases the apparent activation energy. Ball milled mixtures with the molar ratio n = 2 and 3 discharge slowly H₂ during storage at room temperature and 40 °C. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m²/g substantially enhances the rate of discharge at 40 °C.     

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.     

Hydrogen desorption from LiBH4 destabilized by chlorides of transition metal Fe,Co, and Ni

The dehydrogenation temperature of LiBH₄ was considerably decreased to 230 °C–300 °C when it was manually mixed with FeCl₂, CoCl₂ and NiCl₂ in a molar ratio of 2:1. Mixing LiBH₄ with NiCl₂ or FeCl₂ led to complete hydrogen desorption from LiBH₄, i.e. 18.3 wt% hydrogen was achieved with respect to the weight of LiBH₄. However, the CoCl₂ addition resulted in less hydrogen release due to the formation of diborane. Ball milling treatment for the mixtures of LiBH₄ and these chlorides further decreased hydrogen desorption temperatures. The results indicate that LiBH₄ could be effectively destabilized by the chlorides of Fe, Co and Ni. Small doping of these chlorides into LiBH₄ was found to be effective in enhancing dehydrogenation kinetics of the remaining LiBH₄ due to the formation of metal borides.     

LiBH4–VCl3 and LiAlH4–VCl3 mixtures prepared in soft milling conditions for hydrogen release at low temperature

Mixtures of LiBH₄/VCl₃ and LiAlH₄/VCl₃ in 5:1, 3:1, and 5% mol stoichiometries were prepared and tested for hydrogen release. The mixtures were prepared in 10 min of ball milling at room temperature or with cryogenic (N₂-liquid) cooling. The mixtures demonstrated diverse hydrogen release levels, but all of them started releasing hydrogen at low temperatures (33–66 °C) with a change in the reaction pathway as compared to pure LiBH₄ or LiAlH₄. The driving force for that is the formation of the stable salt LiCl. The best material was the 5% mol VCl₃ + LiAlH₄ cryogenic mixture because of the low-temperature dehydrogenation onset of 34 °C; and the dehydrogenation level of 5.1 wt.%, and 6.4 wt.% that was achieved upon heating at 100 °C and 250 °C, respectively.     

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.     

Borohydride-induced destabilization of hydrazine borane

We investigated the destabilization of 4 mol of hydrazine borane N₂H₄BH₃ in the presence of 1 mol of an alkaline borohydride (LiBH₄ or NaBH₄) and, in a second step, of 1 mol of NH₃BH₃ in addition. The destabilization was followed by TGA, DSC and μGC. The solid residues were analyzed by solid-state ¹¹B NMR, IR and XRD. The presence of the borohydride effectively destabilizes N₂H₄BH₃ which is thus able to liberate H₂ from 50 °C. Seeing the results from the other side, one could consider that the alkaline borohydrides are destabilized by N₂H₄BH₃. Such destabilization approach is attractive as it involves boron-based materials only. The best decomposition results were obtained with the sample containing 4 mol of N₂H₄BH₃ and 1 mol of LiBH₄ (containing 16 equiv. H₂). Upon heating up to 300 °C at 5 °C min⁻¹, this sample releases 12.1 mol of H₂ (dehydrogenation extent of 76%) and 1.1 mol of N₂H₄. A solid residue of empirical formulae LiB₅N_5.8H_3.4 is formed. It is composed of polyborazylene- and/or boron nitride-like materials. This is an attractive feature as it implies recyclability of the polymer and elaboration of inorganic ceramics at relatively low temperatures. Our main results are reported herein.     

Hydrogen generation from sodium borohydride solution using a ruthenium supported on graphite catalyst

The catalyst with high activity and durability plays a crucial role in the hydrogen generation systems for the portable fuel cell generators. In the present study, a ruthenium supported on graphite catalyst (Ru/G) for hydrogen generation from sodium borohydride (NaBH₄) solution is prepared by a modified impregnation method. This is done by surface pretreatment with NH₂ functionalization via silanization, followed by adsorption of Ru (III) ion onto the surface, and then reduced by a reducing agent. The obtained catalyst is characterized by transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Very uniform Ru nanoparticles with sizes of about 10 nm are chemically bonded on the graphite surface. The hydrolysis kinetics measurements show that the concentrations of NaBH₄ and NaOH all exert considerable influence on the catalytic activity of Ru/G catalyst towards the hydrolysis reaction of NaBH₄. A hydrogen generation rate of 32.3 L min⁻¹ g⁻¹ (Ru) in a 10 wt.% NaBH₄ + 5 wt.% NaOH solution has been achieved, which is comparable to other noble catalysts that have been reported.     

The effects of ball milling and nanometric nickel additive on the hydrogen desorption from lithium borohydride and manganese chloride (3LiBH4 + MnCl2) mixture

A mixture of [3LiBH₄ + MnCl₂] was processed by high energy ball milling in ultra-high purity hydrogen gas for 0.5 and 1 h. The XRD patterns of milled powders show the sole diffraction peaks of LiCl. The reaction occurring during milling of [3LiBH₄ + MnCl₂] seems to have all characteristics of the metathesis-type reactions occurring between borohydrides (LiBH₄ and NaBH₄) and metal chlorides (MCl_n) induced in a solid state by a mechano-chemical activation synthesis (MCAS). Under pressure of 0.1 MPa H₂ (atmospheric) the ball milled [3LiBH₄ + MnCl₂] mixture is able to desorb ∼4.0 wt.% H₂ at 100 °C within 21,000 s and ∼4.5 wt.% H₂ at 120 and 200 °C within 8000 s and 4000 s, respectively. The addition of n-Ni with SSA = 60.5 m²/g allows desorption of ∼3.7wt.%H₂ within 8,700 s at 100 °C. This is one of the highest H₂ desorption capacities obtained for a complex hydride at 100 °C under atmospheric pressure of H₂ taking into account the fact that the microstructure contains some amount of a useless LiCl constituent. The activation energy of hydrogen desorption for a ball milled undoped [3LiBH₄ + MnCl₂] is ∼102 kJ/mol and ∼98 and 92 kJ/mol after doping with 5 wt.% of nanometric Ni having specific surface area (SSA) of 9.5 and 60.5 m²/g, respectively. After volumetric desorption from 100 to 450 °C the XRD patterns show only LiCl. The n-Ni additive slightly lowers the total quantity of desorbed H₂. Re-absorption tests, under pressure of 10 MPa H₂ at 200 °C, show that the system is, most likely, irreversible. Flammability studies show that the ball milled [3LiBH₄ + MnCl₂] mixture can be ignited by scraping the cylinder walls with a metal tool as well when it is thrown and dispersed in air in a powder form. It also reacts violently in contact with water and a nitric acid.     

Long-term stability of sodium borohydrides for hydrogen generation

As a source of the high purity hydrogen, sodium and potassium borohydrides are investigated in terms of long-term stability in the form of the concentrated solutions, heterogeneous mixtures and in the solid state corresponding to NaBH₄ or KBH₄ crystal hydrates. In order to improve their stability during the long-term storage sodium and potassium hydroxides were added to the initial borohydride compositions. The effect of temperature, concentration of the borohydride and the alkaline solution, and the nature of the cation in the alkaline solution on the rate of borohydride hydrolysis was investigated. The differential technique developed for evaluation of the rate of borohydride hydrolysis was successfully applied for the determination of the long-term stability of the water-alkaline solutions containing NaBH₄(KBH₄)·5H₂O with 1–10 wt.% of NaOH or KOH at 30 °C and 50 °C.     

Improved hydrogen desorption properties of magnesium hydride with TiFe0.8Mn0.2, graphite and iron addition

The high dehydrogenation temperature of magnesium-based hydride (MgH₂) is still a challenge as a potential hydrogen storage material in automobile applications. To improve the hydrogen desorption properties of MgH₂; we selected TiFe_0.8Mn_0.2, graphite and Fe as additives. We prepared the Mg–graphite, Mg–TiFe_0.8Mn_0.2–Fe, Mg–TiFe_0.8Mn_0.2–graphite, Mg–Fe–graphite and Mg–TiFe_0.8Mn_0.2–Fe–graphite composites with high-energy ball milling under argon atmosphere. We investigated the effects of graphite and Fe addition to the desorption mechanism of TiFe_0.8Mn_0.2 using X-ray diffractometer (XRD), scanning electron microscope, differential scanning calorimeter and pressure-composition-temperature measurements using Sievert apparatus. We observed MgH₂ in Mg–TiFe_0.8Mn_0.2–graphite, Mg–Fe–graphite and Mg–TiFe_0.8Mn_0.2–Fe–graphite with XRD analyses after hydrogenation at 200 °C under a hydrogen pressure of 2.5–2.6 MPa. As compared to pure milled MgH₂ powder, we found that the dehydrogenation peak temperatures are decreased by 90, 160 and 165 °Cfor Mg–TiFe_0.8Mn_0.2–graphite, Mg–Fe–graphite, and Mg–TiFe_0.8Mn_0.2–Fe–graphite composites, respectively. The co-addition of TiFe_0.8Mn_0.2, graphite, and Fe exhibit the synergistic effects in improving the hydrogen desorption properties of MgH₂.     

Synthesis and hydrogen storage properties of lithium borohydride amidoborane complex

A series of mixtures of LiAB/LiBH₄ with different molar ratios were prepared and their hydrogen storage properties were investigated in this study. Among them, a new structure was found in the LiAB/LiBH₄ sample with a molar ratio of 1/1. It is of orthorhombic structure and composed of alternative layers of LiAB and LiBH₄. It shows similar hydrogen desorption behaviors of LiAB–LiBH₄ and LiAB–0.5LiBH₄. For use in hydrogen storage, high hydrogen capacity and low operation temperature are demanded, thus, the dehydrogenation properties of LiAB–0.5LiBH₄ were subsequently measured. Three steps of desorption were observed during the heating process, with a total release of 11.5 wt% H₂ at 500 °C. The reaction path was identified using a combined investigation of XRD and ¹¹B solid state NMR. Dehydrogenation kinetic analyses show that the complex has lower activation energy (61 ± 4 kJ mol⁻¹ H₂) than that of LiAB (71 ± 5 kJ mol⁻¹ H₂). It is likely that dehydrogenation process was promoted due to the presence of LiBH₄.