Reversible hydrogen storage in LiBH4/Ca(AlH4)2 systems

To improve the hydrogen storage property of LiBH₄, the LiBH₄/Ca(AlH₄)₂ reactive systems with various ratios were constructed, and their de-/hydrogenation properties as well as the reaction mechanisms were investigated experimentally. It was found that the sample with the LiBH₄ to Ca(AlH₄)₂ molar ratio of 6:1 exhibits the best comprehensive hydrogen storage properties, desorbing hydrogen completely (8.2 wt.%) within 35 min at 450 °C and reversibly absorbing 4.5 wt.% of hydrogen at 450 °C under a hydrogen pressure as low as 4.0 MPa. During the first dehydrogenation process of the LiBH₄/Ca(AlH₄)₂ systems, the CaH₂ and Al particles were in situ precipitated via the self-decomposition of Ca(AlH₄)₂, and then reacted with LiBH₄ to form CaB₆, AlB₂ and LiH. Whereafter, the sample can cycle between LiBH₄ + Ca(BH₄)₂ + Al in the hydrogenated state and CaB₆ + AlB₂ + LiH in the dehydrogenated state.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Kinetics and modeling study of a Mg(BH4)2/Ca(BH4)2 destabilized system

Borohydrides such as Mg(BH₄)₂ and Ca(BH₄)₂ continue to attract attention as potential hydrogen storage materials because of their high hydrogen content. In this study the desorption kinetics of Mg(BH₄)₂, Ca(BH₄)₂ and a 5Mg(BH₄)₂/Ca(BH₄)₂ mixture of the two were compared in the two-phase region at the same temperature and at a constant pressure thermodynamic driving force. The rate of hydrogen desorption from the two-phase region in the 5Mg(BH₄)₂/Ca(BH₄)₂ mixture was faster than that from either of the constituents. This indicated that Ca(BH₄)₂ was able to serve as a destabilizing agent for Mg(BH₄)₂. The results also showed that the desorption rates from the two-phase region were much faster than those from the single phase region. Modeling studies showed that the rate of hydrogen release from Mg(BH₄)₂, during the first 80% of the reaction, is diffusion controlled while in Ca(BH₄)₂ the reaction rate is phase boundary controlled. In the mixture the rate appears to be under the mixed control of both processes.     

Effect of the presence of chlorides on the synthesis and decomposition of Ca(BH4)2

Calcium borohydride is one of the most interesting compounds for solid-state hydrogen storage, in particular because of its high hydrogen capacity. In this paper, the synthesis of Ca(BH₄)₂ by metathesis reaction via ball milling of a mixture of LiBH₄ and CaCl₂ is described. The effectiveness of this synthesis technique and the possible substitution of Cl ions in the borohydride phases is analysed depending on the back-pressure used for milling. When performed by ball milling under Ar, the metathesis reaction is not successful. A large quantity of a solid solution Li(BH₄)_1−xCl_x remains in the sample and CaHCl is formed rather than Ca(BH₄)₂. In contrast, the use of H₂ back-pressure during milling favours the borohydride phases rather than CaHCl and leads to the formation of a solid solution Ca(BH₄)_2-yCl_y where [BH₄]⁻ groups are partially substituted by Cl ions. This compound has a similar structure as β-Ca(BH₄)₂ but with smaller lattice parameters. It is present in the as-milled sample together with LiCl and Li(BH₄)_1−xCl_x. The decomposition of the mixture occurs at lower temperature than for pure LiBH₄ but higher than for pure Ca(BH₄)₂. The presence of chlorides in the structure of borohydride compounds changes dramatically the thermal properties of the material prepared and should be considered each time a metathesis reaction is used for synthesis.     

Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4

A two-step ball-milling method has been provided to synthesize Mg(BH₄)₂ using NaBH₄ and MgCl₂ as starting materials. The method offers high yield and high purity (96%) of the compound. The as-synthesized Mg(BH₄)₂ is then combined with LiAlH₄ by ball-milling in order to form new multi-hydride systems with high hydrogen storage properties. The structure, the dehydrogenation and the reversibility of the combined systems are studied. Analyses show that a metathesis reaction takes place between Mg(BH₄)₂ and LiAlH₄ during milling, forming Mg(AlH₄)₂ and LiBH₄. Mg(BH₄)₂ is excessive and remains in the ball-milled product when the molar ratio of Mg(BH₄)₂ to LiAlH₄ is over 0.5. The onset dehydrogenation temperature of the combined systems is lowered to ca. 120 °C, which is much lower than that of either Mg(BH₄)₂ or LiAlH₄. The dehydrogenation capacities of the combined systems below 300 °C are all higher than that of both Mg(BH₄)₂ and LiAlH₄. The combined systems are reversible for hydrogen storage at moderate hydrogenation condition, and rapid hydrogenation occurred within the initial 30 min. Moreover, the remained Mg(BH₄)₂ in the combined systems is found also partially reversible. The mechanism of the enhancement of the hydrogen storage properties and the dehydrogenation/hydrogenation process of the combined systems were discussed.     

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.     

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 of solid-state NaBH4/Co-based catalyst composite for hydrogen storage through a high-energy ball-milling process

Solid-state composites of NaBH₄ and Co-based catalyst have been fabricated for hydrogen generation via a novel mechanochemical technique, i.e. the high-energy ball milling, in which the gravimetric storage capacity of hydrogen has reached 6.7 wt%, meeting the 2010 target of at least 0.06 kg H₂/kg set by the U.S. Department of Energy (DOE). The active catalysts used in the hydrolysis reaction of sodium borohydride for hydrogen generation are mainly cobalt oxides. Controlled addition of water, namely water used as a limiting agent, to the solid composites of NaBH₄ and Co-based catalyst greatly improves the H₂ storage capacity and resolved the issues of low gravimetric H₂ storage in conventional aqueous system of sodium borohydride. Factors influencing the performance of hydrogen production such as amounts of H₂O added, catalyst loadings and durations of ball-milling processes are investigated. Moreover the hydrolyzed products of NaBH₄ and spent catalysts are analyzed as well.     

Structural and reaction pathway analyses of Mg(BH4)2·2NH3 for hydrogen storage : A first-principles study

Mg(BH₄)₂·2NH₃ is a relatively new compound considered for hydrogen storage. The fundamental properties of the compound were comprehensively studied using first-principles calculations, such as crystal structure and electronic structure, reaction Gibbs free energy and possible reaction pathway. The calculated crystal structure is in good agreement with the experimental and other theoretical results. Results from electronic density of states (DOS) and electron localization function (ELF) show the covalent characteristics of the N–H and the B–H bonds, and the weak ionic interactions between the Mg atom and the NH₃ ligands or the (BH₄)⁻ ligands. The reaction Gibbs free energies of several possible decomposition reactions were calculated between 0 and 700 K. All the reactions are exothermic. The most likely reaction pathway of the dehydrogenation reaction was clarified to show five distinct steps.     

Enhanced hydrogen sorption kinetics of Mg50Ni-LiBH4 composite by CeCl3 addition

Mg₅₀Ni-LiBH₄ and Mg₅₀Ni-LiBH₄-CeCl₃ composites have been prepared by short times of ball milling under argon atmosphere. Combination of HP-DSC and volumetric techniques show that Mg₅₀Ni-LiBH₄-CeCl₃ composite not only uptakes hydrogen faster than Mg₅₀Ni-LiBH₄, but also releases hydrogen at a lower temperature (225 °C). The presence of CeCl₃ has a catalytic role, but it does not modify the thermodynamic properties of the composite which corresponds to MgH₂. Experimental studies on the hydriding/dehydriding mechanisms demonstrate that LiBH₄ and Ni lead to the formation of MgNi₃B₂ in both composites. In addition, XRD/DSC analysis and thermodynamic calculations demonstrate that the addition of CeCl₃ accounts for the enhancement of the hydrogen absorption/desorption kinetics through the interaction with LiBH₄. The in situ formation and subsequent decomposition of Ce(BH₄)₃ provides a uniform distribution of nanosize CeB₄ compound, which plays an important role in improving the kinetic properties of MgH₂.     

Convenient storage of concentrated hydrogen peroxide as a CaO2·2H2O2(s)/H2O2(aq) slurry for energy storage applications

Prior investigations have proposed, and successfully implemented, a stand-alone supply of aqueous hydrogen peroxide for use in fuel cells. An apparent obstacle for considering the use of aqueous hydrogen peroxide as an energy storage compound is the corrosive nature of the nominally required 50 wt.% maximum concentration. Here we propose storage of concentrated hydrogen peroxide in a high weight percent solid slurry, namely the equilibrium system of CaO₂·2H₂O₂(s)/H₂O₂(aq), that mitigates much of the risk associated with the storage of such high concentrations. We have prepared and studied surrogate slurries of calcium hydroxide/water that are assumed to resemble the peroxo compound slurries. These slurries have the consistency of a paste rather than a distinct two-phase (liquid plus solid) system. This paste-like property of the prepared surrogates enable them to be contained within a 200 lines-per-inch. (LPI) nickel mesh screen (33.6% open area) with no solids leakage, and only liquid transport driven by an adsorbent material is placed in physical contact on the exterior of the screen. This hydrogen peroxide slurry approach suggests a convenient and safe mechanism of storing hydrogen peroxide for use in, say, vehicle applications. This is because fuel cell design requires only aqueous hydrogen peroxide use, that can be achieved using the separation approach utilizing the screen material here. This proposed method of storage should mitigate hazards associated with unintentional spills and leakage issues arising from aqueous solution use.     

Direct synthesis of Mg(AlH4)2 and CaAlH5 crystalline compounds by ball milling and their potential as hydrogen storage materials

Mg(AlH₄)₂ and CaAlH₅ were synthesized by direct ball milling of AlH₃ and MgH₂ or AlH₃ and CaH₂ hydrides. The XRD profiles indicated crystalline compounds. Several ball-milling conditions were studied and the optimum parameters were found. Among these, the key parameter is the pause used to cool down the milling device, which allows reducing the temperature rise during milling. Thus, the maximum yield of complex hydrides was obtained by minimizing the desorbed alane amount. The decomposition properties were studied and were in agreement with those reported for different synthesis methods. Mg(AlH₄)₂ with a good hydrogen capacity and a decomposition reaction enthalpy close to 0 kJ/mol H₂ can be a candidate for one-way storage systems. As for CaAlH₅, it might be suitable for reversible hydrogen storage thanks to its dehydrogenation reaction enthalpy (26 kJ/mol H₂). However, rather high activation energy values were evaluated for both compounds (119 and 161 kJ/mol, respectively).     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

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.     

Thermal properties of Y(BH4)3 synthesized via two different methods

Y(BH₄)₃ is one of the candidates for solid-state hydrogen storage, which contains 9.06 wt% of hydrogen. In this study, the thermal properties of Y(BH₄)₃ synthesized via two different methods are extensively examined. One method relies on the solid–solid metathesis reaction between LiBH₄ and YCl₃, and the other method is the gas–solid reaction between B₂H₆ and YH₃. The two samples are studied by differential scanning calorimetry, thermogravimetry, and X-ray diffraction. They exhibit distinctly different polymorphic phase transformation and melting. It turns out that the side product LiCl in the metathesis reaction, which has been regarded as being inert, shifts the melting point and promotes the formation of YB₄ during decomposition. Differential scanning calorimetry and in situ X-ray diffraction data indicate that the addition of LiBH₄ to Y(BH₄)₃ induces co-melting as is found in the cases of LiBH₄–Ca(BH₄)₂ or LiBH₄–Mg(BH₄)₂. Melt infiltration of Y(BH₄)₃ into mesoporous carbon cage confirms such melting behavior.     

Effects of catalysts doping on the thermal decomposition behavior of Zn(BH4)2

Transition metal complex borohydrides, Zn(BH₄)₂ have been synthesized by solid-state mechanochemical process. Various catalysts such as TiCl₃, TiF₃, nanoNi, nanoFe, Ti, nanoTi, and Zn were used to dope the borohydride in order to lower the decomposition temperature in the range of , without a significant reduction in the hydrogen content per total weight of the sample. In addition, the structural, bonding and thermal characteristics of undoped and catalysts doped Zn(BH₄)₂ were compared and analyzed to find out the optimum catalyst and dopant concentration. Among the different catalysts, 1.5 mol% of nanoNi obtained from Quantum Sphere Inc. was found to possess the optimum behavior in terms of fast kinetics and lowering the melting and decomposition temperature of Zn(BH₄)₂.     

Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

A novel dual-cation/anion complex hydride (Li₂Mg(BH₄)₂(NH₂)₂), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH₄NH₂ and Li₂BH₄NH₂. The prepared Li₂Mg(BH₄)₂(NH₂)₂ crystallizes in a triclinic structure, and the [NH₂] and [BH₄] groups remain intact within the structure. Upon heating, the prepared Li₂Mg(BH₄)₂(NH₂)₂ decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li₂Mg(BH₄)₂(NH₂)₂ first decomposes to LiMgBN₂, LiBH₄, BN, LiH and MgBNH₈ at 100–250 °C, and then, the newly formed MgBNH₈ reacts with LiH to form Mg, LiBH₄ and BN at 250–340 °C. Finally, the decomposition of LiBH₄ releases hydrogen and generates LiH and B at 340–450 °C.     

Enhanced hydrogen desorption from the Co-catalyzed LiBH4–Mg(BH4)2 eutectic composite

Co-based catalyst can significantly improve the dehydrogenation kinetics of the eutectic composite of LiBH₄–Mg(BH₄)₂ (1/1 M ratio). The onset hydrogen desorption temperature of the composite is at about 155 °C, which is ca. 245, 110 or 27 °C lower than that of LiBH₄, Mg(BH₄)₂ or pristine LiBH₄–Mg(BH₄)₂, respectively. Upon holding the samples at 270 °C, the Co catalyzed composite can release hydrogen at a rate 1.6 times faster than that of the pristine one. Electron Paramagnetic Resonance (EPR) characterization evidenced that Co was in a reduced state of Co⁺ which may serve as the functional species in catalyzing the dehydrogenation of the composite.     

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.     

Hydrogenation of LiH/Al catalyzed with TiN,TiMn2 and LaNi5

In order to investigate the catalytic effect of TiN, TiMn₂ and LaNi₅ on the hydrogen storage capacity of LiAlH₄, 2 mol% of the catalyst was milled with LiH/Al and then hydrogenated in Me₂O. Doping with TiN, TiMn₂ or LaNi₅ led to substantial hydrogenation of LiH/Al in accordance with the formation of LiAlH₄. In each case the amount of hydrogen absorbed was dependent on the catalyst and the ball-to-powder ratio used during milling. A high ball-to-powder ratio results in an improvement in the hydrogen storage capacity of LiAlH₄. For each ball-to-powder ratio the highest hydrogen storage capacity was recorded for the TiN-catalyzed sample; hydrogen storage capacity increased from 3.2 to 4.8 to 6.0 wt.% H as the ball to-powder ratio increased from 10:1 to 20:1 to 40:1. The high levels of hydrogenation of LiH/Al catalyzed with TiN, TiMn₂ and LaNi₅ are remarkable because for the LiAlH₄ system only a TiCl₃ catalyst has previously been shown to result in rehydrogenation of the dehydrogenated products to LiAlH₄.