The milled LiBH4/h-BN composites exhibiting unexpected hydrogen storage kinetics and reversibility

The effect of nanoscale h-BN addition by milling on the de-/re-hydrogenation of LiBH₄ was investigated. With the increasing h-BN ratio, the milled LiBH₄/h-BN composites showed lower dehydrogenation temperature. For the LiBH₄-3BN composite (mole ratio 1:3), the on-set dehydrogenation temperature was reduced from 290 °C for the milled pure LiBH₄ down to 175 °C, and the initial dehydrogenation capacity could reach 3.1 wt.% (equivalent to 13.7 wt.% of the component LiBH₄) within ～2 h at 400 °C. Under moderate rehydrogenation conditions of 400 °C and 10 MPa H₂ pressure, the 2nd and 3th cyclic dehydrogenation capacity of LiBH₄-3BN composite almost remained unchanged, indicating remarkably improved rehydrogenation reversibility in comparison to milled pure LiBH₄. FTIR analysis reveals specific interaction between h-BN and LiBH₄ probably originating from the polar mechanism between polarizable B–H bond and B–N bond, which should be responsible for the enhanced dehydrogenation kinetics and reversibility. This work demonstrates the specific catalytic role of nanoscale h-BN and its potential for reversible hydrogen storage by compositing with high-capacity borohydrides.     

Enhanced hydrogen storage properties of LiBH4 modified by NbF5

In this work, the hydriding–dehydriding properties of the LiBH₄–NbF₅ mixtures were investigated. It was found that the dehydrogenation and reversibility properties of LiBH₄ were significantly improved by NbF₅. Temperature-programed dehydrogenation (TPD) showed that 5LiBH₄–NbF₅ sample started releasing hydrogen from as low as 60 °C, and 4 wt.% hydrogen could be obtained below 255 °C. Meanwhile, ∼7 wt.% H₂ could be reached at 400 °C in 20LiBH₄–NbF₅ sample, whereas pristine LiBH₄ only released ∼0.7 wt.% H₂. In addition, reversibility measurement demonstrated that over 4.4 wt.% H₂ could still be released even during the fifth dehydrogenation in 20LiBH₄–NbF₅ sample. The experimental results suggested that a new borohydride possibly formed during ball milling the LiBH₄–NbF₅ mixtures might be the source of the active effect of NbF₅ on LiBH₄.     

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.     

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.     

Hydrogen sorption in MgH2-based composites: The role of Ni and LiBH4 additives

In the present work we investigate the hydrogen sorption properties of composites in the MgH₂–Ni, MgH₂–Ni–LiH and MgH₂–Ni–LiBH₄ systems and analyze why Ni addition improve hydrogen sorption rates while LiBH₄ enhance the hydrogen storage capacity. Although all composites with Ni addition showed significantly improved hydrogen storage kinetics compared with the pure MgH₂, the fastest hydrogen sorption kinetics is obtained for Ni-doped MgH₂. The formation of Mg₂Ni/Mg₂NiH₄ in Ni-doped MgH₂ composite and its microstructure allows to uptake 5.0 wt% of hydrogen in 25 s and to release it in 8 min at 275 °C. In the MgH₂–Ni–LiBH₄ composite, decomposition of LiBH₄ occurs during the first dehydriding leading to the formation of diborane, which has a Ni catalyst poison effect via the formation of a passivating boron layer. A combination of FTIR, XRD and volumetric measurements demonstrate that the formation of MgNi₃B₂ in the MgH₂–Ni–LiBH₄ composite happens in the subsequent hydriding cycle from the reaction between Mg₂Ni/Mg₂NiH₄ and B. Activation energy analysis demonstrates that the presence of Ni particles has a catalytic effect in MgH₂–Ni and MgH₂–Ni–LiH systems, but it is practically nullified by the addition of LiBH₄. The beneficial role of LiBH₄ on the hydrogen storage capacity of the MgH₂–Ni–LiBH₄ composite is discussed.     

An alternative platform of solid-state hydrides with polymers as composite/encapsulation for hydrogen storage applications: Effects in intermetallic and complex hydrides

Hydrogen energy proved the remarkable benefits from the perspective of future energy needs and environmental concerns. However, the existing challenges in hydrogen energy technologies, mainly hydrogen storage need to be overcome. In hydrogen storage systems, metal hydrides are highly suffered by the protic species and oxygen because of their reactivity in ambient conditions, leading to surface contamination and affecting the hydrogen sorption behaviors. Moreover, the hydrogen storage in metal hydrides can be hindered by impurities such as CO and CO₂ in hydrogen gas. The new concepts of hydrides with polymers are vital for overcoming the hydrides’ contamination-related issues. This comprehensive review spotlight primarily on the alternative ways to avoid the contamination of hydrides with polymers. The researchers worldwide developed different concepts of polymer-hydrides (intermetallic and complex hydrides) systems, mainly composites and polymer encapsulation methods, to avoid contamination from the air/moisture/gas impurities and improve hydrogen sorption properties in hydrides. Interestingly, selective gas properties of polymers with hydrides show tremendous advantages because of their selective permeability of hydrogen, which influences the easier path for hydrogen diffusion to form the hydrides through the polymer matrix and prohibit the contamination of hydride materials from the other gases/protic species. Notably, this review bridges the understanding between the polymers and hydrides (intermetallic, complex and reactive hydride composite) for hydrogen storage systems.     

Destabilization of LiBH4 by SrF2 for reversible hydrogen storage

The de-/rehydrogenation features of the 6LiBH₄/SrF₂ reactive hydride system have been systematically investigated. It was found that the thermal stability of LiBH₄ can be reduced markedly by combining it with SrF₂. Dehydrogenation of the 6LiBH₄/SrF₂ system proceeds via the 6LiBH₄ + SrF₂ → SrB₆ + 2LiF + 4LiH + 10H₂ reaction, which involves SrH₂ as the intermediate product. The dehydrogenation enthalpy change was experimentally determined to be 52 kJ/mol H₂ based on the P–C isotherm analysis. For rehydrogenation, LiBH₄ and SrF₂ were regenerated along with LiSrH₃ at 450 °C under ～8 MPa hydrogen pressure; thus, approximately 5.2 wt% of hydrogen can be released during the second dehydrogenation process.     

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.     

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.     

Magnesium-based alloys for solid-state hydrogen storage applications: A review

Magnesium hydrides (MgH₂) have attracted extensive attention as solid-state H₂ storage, owing to their low cost, abundance, excellent reversibility, and high H₂ storage capacity. This review comprehensively explores the synthesis and performance of Mg-based alloys. Several factors affecting their hydrogen storage performance were also reviewed. The metals addition led to destabilization of Mg–H bonds to boost the dehydrogenation process. A unique morphology could provide more available active sites for the dissociation/recombination of H₂ and allow the activation energy reduction. Also, an appropriate support prevent the agglomeration of Mg particles, hence, sustains their nanosize particles. Moreover, the perspective of avenues for future research presented in this review is expected to act as a guide for the development of novel Mg-based H₂ storage systems. New morphological shape of catalysts, more unexplored and highly potential waste materials, and numerous synthesis procedures should be explored to further enhance the H₂ storage of Mg-based alloys.     

Dehydrogenation and rehydrogenation of a 0.62LiBH4-0.38NaBH4 mixture with nano-sized Ni

The dehydrogenation reaction pathway of a 0.91 (0.62LiBH₄-0.38NaBH₄)-0.09Ni mixture in the temperature range of 25–650 °C in flowing Ar and the cycling stability in H₂ are presented. No H₂ is released immediately after melting at 225 °C. The major dehydrogenation occurs above 350 °C. Adding nano-sized Ni reduces the dehydrogenation peak temperatures by 20–25 °C, leading to three decomposition steps where Ni₄B₃ and Li_1.2Ni_2.5B₂ are found in the major dehydrogenation products for the 1^st and the 3^rd step; whilst the Ni-free mixture decomposes through a two-step decomposition pathway. A total of 8.1 wt% of hydrogen release from the 0.91 (0.62LiBH₄-0.38NaBH₄)-0.09Ni mixture is achieved at 650 °C in Ar. This mixture has a poor hydrogen cycling stability as its reversible hydrogen content decreases from 5.1 wt% to 1.1 wt% and 0.6 wt% during three complete desorption-absorption-cycles. However, the addition of nano-sized Ni facilitates the reformation of LiBH₄.     

A comparative study of the role of additive in the MgH2 vs. the LiBH4-MgH2 hydrogen storage system

The objective of the present work is the comparative study of the behaviour of the Nb- and Ti-based additives in the MgH₂ single hydride and the MgH₂ + 2LiBH₄ reactive hydride composite. The selected additives have been previously demonstrated to significantly improve the sorption reaction kinetics in the corresponding materials. X-Ray Diffraction (XRD), X-Ray Absorption Spectroscopy (XAS), X-Ray Photoelectron Spectroscopy (XPS) and Electron Microscopy (TEM) analysis were carried out for the milled and cycled samples in absence or presence of the additives. It has been shown that although the evolution of the oxidation state for both Nb- and Ti-species are similar in both systems, the Nb additive is performing its activity at the surface while the Ti active species migrate to the bulk. The Nb-based additive is forming pathways that facilitate the diffusion of hydrogen through the diffusion barriers both in desorption and absorption. For the Ti-based additive in the reactive hydride composite, the active species are working in the bulk, enhancing the heterogeneous nucleation of MgB₂ phases during desorption and producing a distinct grain refinement that favours both sorption kinetics. The results are discussed in regards to possible kinetic models for both systems.     

Pellets of MgH2-based composites as practical material for solid state hydrogen storage

Hydrogenation properties and mechanical stability of pellets made starting from compressed ball-milled MgH₂ powders mixed with catalysts (Nb₂O₅ and graphite) and a binding agent (aluminium powder) have been investigated. Structural characterization with X-ray diffraction and gas–solid reaction kinetic and thermodynamic tests with a Sievert's apparatus have been done on the samples up to 50 hydrogen absorption/desorption (a/d) cycles. The best cycling behaviour and mechanical strength stability have been observed for pellets of catalysed MgH₂ powders added with 5 wt% aluminium annealed in vacuum at 450 °C before starting the a/d cycles. This mechanical stability to cycles has been attributed to the formation of a solid solution of aluminium in magnesium.     

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.     

Novel hydrogen generator/storage based on metal hydrides

A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and higher efficiency than a classical electrolyser. The prototype comprises a six-electrode cell assembly using an AB₅ type metal hydride and Ni plates as counter electrodes, in a KOH solution. Metal hydride electrodes with chemical composition LaNi₄.₃Co_0.4Al₀.₃ have been prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study of the alloy has been performed in a Sievert-type apparatus produced by Labtech Ltd. In the present prototype during charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging the hydrogen stored in the metal hydride it is released to an external H₂ storage. Released hydrogen is delivered into the hydrogen storage up to a pressure of 15 bar. It is anticipated that the device will be integrated as a combined hydrogen generator in a stand-alone system associated to a 1 kW fuel cell.     

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.