Inducing One-Step Dehydrogenation of Magnesium Borohydride via Confinement in Robust Dodecahedral Nitrogen-Doped Porous Carbon Scaffold

A dodecahedral activated N-doped porous carbon scaffold is synthesized and used for the nanoconfinement of Mg(BH4)2. The optimized mesoporous scaffold possesses an accumulated pore width of 2.65 nm, high specific surface area (3955.9 m2 g-1), and large pore volume (2.15 cm3 g-1), providing ample space for the confinement of Mg(BH4)2 particles and numerous surface active sites for interactions with the same. The confined Mg(BH4)2 system features a dehydrogenation onset temperature of 81.5 °C, an extremely high capacity of 10.2 wt% H2, and an almost single-step dehydrogenation profile. Moreover, the system exhibits superior capacity retention of 82.7% after 20 cycles at a moderate temperature of 250 °C. Precise activation control enables a transformation from microporous carbon materials to mesoporous ones, and hence the efficient nanoconfinement of Mg(BH4)2 and realization of one-step dehydrogenation. The evolution of borohydride intermediates is systematically revealed throughout the cycling process. Density functional theory calculations demonstrate defective N heteroatoms within the scaffold are vital in reducing the strength of B─H bonds, and the N-doped carbon can facilitate decomposition of the irreversible MgB12H12 intermediate. This study opens up new avenues for designing robust carbon scaffolds doped with heteroatoms and analyzing intermediate evolution in nanoconfined Mg-based borohydride systems.

Magnesium-Based Hydrogen Storage Alloys: Advances, Strategies, and Future Outlook for Clean Energy Applications

Magnesium-based hydrogen storage alloys have attracted significant attention as promising materials for solid-state hydrogen storage due to their high hydrogen storage capacity, abundant reserves, low cost, and reversibility. However, the widespread application of these alloys is hindered by several challenges, including slow hydrogen absorption/desorption kinetics, high thermodynamic stability of magnesium hydride, and limited cycle life. This comprehensive review provides an in-depth overview of the recent advances in magnesium-based hydrogen storage alloys, covering their fundamental properties, synthesis methods, modification strategies, hydrogen storage performance, and potential applications. The review discusses the thermodynamic and kinetic properties of magnesium-based alloys, as well as the effects of alloying, nanostructuring, and surface modification on their hydrogen storage performance. The hydrogen absorption/desorption properties of different magnesium-based alloy systems are compared, and the influence of various modification strategies on these properties is examined. The review also explores the potential applications of magnesium-based hydrogen storage alloys, including mobile and stationary hydrogen storage, rechargeable batteries, and thermal energy storage. Finally, the current challenges and future research directions in this field are discussed, highlighting the need for fundamental understanding of hydrogen storage mechanisms, development of novel alloy compositions, optimization of modification strategies, integration of magnesium-based alloys into hydrogen storage systems, and collaboration between academia and industry.

First-Principles Elucidation of Initial Dehydrogenation Pathways in Mg(BH4)2

Complex borohydrides such as Mg(BH₄)₂ offer one of highest capacities to chemically store hydrogen for onboard applications; however, it suffers greatly from kinetic constraints that prevent realization of full capacity and reversibility. Understanding these kinetic limitations solely from experiments is extremely challenging due to the unusual complexity of various competing elemental reaction steps involved during the de/rehydrogenation reaction. This work aims to map out the energetics associated with initial dehydrogenation of Mg(BH₄)₂ from first-principles simulations and to identify the preferred reaction pathways. Our calculations suggest the rate-limiting step during BH₄^--B₃H₈^- conversion is the formation of the B₂H₇^- intermediate. We further emphasize and clarify that the B₃H₈^- and H^- intermediates, formed during initial Mg(BH₄)₂ decomposition, appear as molecular species that are embedded in the Mg-BH₄-Mg matrix as evidenced in the nuclear magnetic resonance measurements and not as bulk MgH₂ and Mg(B₃H₈)₂ as previously assumed in theoretical predictions of the thermodynamics.

Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH4)2

The thermodynamic properties of key compounds Mg(B₃H₈)₂, MgB₂H₆, MgB₁₀H₁₀, Mg(B₁₁H₁₄)₂, Mg₃(B₃H₆)₂, and MgB₁₂H₁₂, proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH₄)₂ and the uptake of hydrogen by MgB₂, have been investigated using solid-state density functional theory (DFT) calculations. More accurate tretment of the cell-size effects with respect to the entropies was also investigated in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero-point energy corrections can lower the electronic energies of reaction by 20-30 kJ/(mol H₂) for these intermediates, while adding the thermal and entropy contibutions results in a total decrease of up to ∼50 kJ/(mol H₂). Although our treatment lowers the calculated formation energy of Mg(B₃H₈)₂, it is still too high to explain the experimental observation of B₃H₈^-. We discuss possible reasons for this disparity and propose that the formation of B₃H₈^- and H^- in a disordered amorphous phase has a large energy difference compared to the phase-separated Mg(B₃H₈)₂ and MgH₂ considered in calculations. A comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH₄)₂, Mg(B₃H₈)₂, Mg(B₁₁H₁₄)₂, MgB₁₀H₁₀, and MgB₁₂H₁₂ provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB₂H₆ and the proposed trianion species Mg₃(B₃H₆)₂ that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH₄)₂ without the formation of MgH₂ as a coproduct which could phase separate and inhibit rehydrogenation.

Thermal stability and structural studies on the mixtures of Mg(BH₄)₂ and glymes

Coordination complexes of Mg(BH₄)₂ are of interest for energy storage, ranging from hydrogen storage in BH₄ to electrochemical storage in Mg based batteries. Understanding the stability of these complexes is...

Improved Methods for the Synthesis of KB3H8, NH3B3H7, and N-Alkyl Analogues of NH3B3H7

Improved methods for the synthesis of KB₃H₈, NH₃B₃H₇, and N-alkyl analogues of NH₃B₃H₇ have been developed based on previous works. KB₃H₈ was synthesized by the reaction of metallic potassium (K) with borane dimethyl sulfide ((CH₃)₂S·BH₃) with high yield and atom-economy. In the preparation of NH₃B₃H₇ and its N-alkyl analogues, KB₃H₈ served as a starting material and was converted to THF·B₃H₇ first through reactions with HCl diethyl ether solution or oxidation agent CoCl₂. Then, the formed THF·B₃H₇ in situ reacted with the corresponding ammonia or amines to form the amine borane final products. This work paves an alternative way for preparing organic-inorganic hybrid materials containing B, N, and C atoms.

KB3H8: an environment-friendly reagent for the selective reduction of aldehydes and ketones to alcohols

Selective reduction of aldehydes and ketones to their corresponding alcohols with KB₃H₈, an air- and moisture-stable, nontoxic, and easy-to-handle reagent, in water and THF has been explored under an air atmosphere for the first time. Control experiments illustrated the good selectivity of KB₃H₈ over NaBH₄ for the reduction of 4-acetylbenzaldehyde and aromatic keto esters.

A safe and efficient synthetic method for alkali metal octahydrotriborates, unravelling a general mechanism for constructing the delta B3 unit of polyhedral boranes

A safe and efficient synthetic method for MB₃H₈ (M = Na, K, Rb and Cs) has been developed with excellent yields by directly reacting the corresponding MBH₄ with the dimethyl sulfide borane complex (DMS·BH₃). A general mechanism for constructing B₃H₈^-, a basic unit for building polyhedral boranes, has been unravelled.

Experimental and computational characterization of phase transitions in CsB3H8

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing B₃H₈^- (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH₄^- (3BH₄^-↔ B₃H₈^- + 2H₂), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsB₃H₈ prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsB₃H₈) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named β-CsB₃H₈), and then into a face-centered cubic phase, here named γ-CsB₃H₈, at 88 °C. These phases are not stable at room temperature thus requiring in situ measurements for their characterization. The phase transitions and decomposition pathway of CsB₃H₈ were studied with in situ synchrotron powder X-ray diffraction (SR-PXD), in situ and ex situ vibrational spectroscopies (Raman and FTIR), and differential-scanning calorimetry combined with thermo-gravimetric analysis (DSC-TGA). The structure determination was validated by vibrational spectroscopy analysis and modeling of the periodic structures by density functional methods. In γ-CsB₃H₈, a significant disorder in B₃H₈^- positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.

Effects of Glymes on the Distribution of Mg(B10H10) and Mg(B12H12) from the Thermolysis of Mg(BH4)2

We examined the effects of concentrations and identities of various glymes, from monoglyme up to tetraglyme, on H2 release from the thermolysis of Mg(BH4)2 at 160–200 °C for 8 h. 11B NMR analysis shows major products of Mg(B10H10) and Mg(B12H12); however, their relative ratio is highly dependent both on the identity and concentration of the glyme to Mg(BH4)2. Selective formation of Mg(B10H10) was observed with an equivalent of monoglyme and 0.25 equivalent of tetraglyme. However, thermolysis of Mg(BH4)2 in the presence of stoichiometric or greater equivalent of glymes can lead to unselective formation of Mg(B10H10) and Mg(B12H12) products or inhibition of H2 release.

Thermal Conversion of Unsolvated Mg(B3H8)2 to BH4 - in the Presence of MgH2

In the search for energy storage materials, metal octahydrotriborates, M(B₃H₈) _n , n = 1 and 2, are promising candidates for applications such as stationary hydrogen storage and all-solid-state batteries. Therefore, we studied the thermal conversion of unsolvated Mg(B₃H₈)₂ to BH₄ ^- as-synthesized and in the presence of MgH₂. The conversion of our unsolvated Mg(B₃H₈)₂ starts at ∼100 °C and yields ∼22 wt % of BH₄ ^- along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of activated MgH₂ to unsolvated Mg(B₃H₈)₂ drastically increases the thermal conversion to 85-88 wt % of BH₄ ^- while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH₂ substantially decreases the formation of (closo-hydro)borates and provides the necessary H₂ for the B₃H₈-to-BH₄ conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH₄ ^- under moderate conditions of temperature (200 °C) in less than 1 h, making the MgB₃H₈-MgH₂ system very promising for energy storage applications.

Heterostructures Built in Metal Hydrides for Advanced Hydrogen Storage Reversibility

Hydrogen storage is a vital technology for developing on-board hydrogen fuel cells. While Mg(BH₄ )₂ is widely regarded as a promising hydrogen storage material owing to its extremely high gravimetric and volumetric capacity, its poor reversibility poses a major bottleneck inhibiting its practical applications. Herein, a facile strategy to effectively improve the reversible hydrogen storage performance of Mg(BH₄ )₂ via building heterostructures uniformly inside MgH₂ nanoparticles is reported. The in situ reaction between MgH₂ nanoparticles and B₂ H₆ not only forms homogeneous heterostructures with controllable particle size but also simultaneously decreases the particle size of the MgH₂ nanoparticles inside, which effectively reduces the kinetic barrier that inhibits the reversible hydrogen storage in both Mg(BH₄ )₂ and MgH₂ . More importantly, density functional theory coupled with ab initio molecular dynamics calculations clearly demonstrates that MgH₂ in this heterostructure can act as a hydrogen pump, which drastically changes the enthalpy for the initial formation of BH bonds by breaking stable BB bonds from endothermic to exothermic and hence thermodynamically improves the reversibility of Mg(BH₄ )₂ . It is believed that building heterostructures provides a window of opportunity for discovering high-performance hydrogen storage materials for on-board applications.

Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications

We synthesized and studied the dimensionality of Mg(B₃H₈)₂, a controversial intermediate in the thermal decomposition of Mg(BH₄)₂, furthemore, the high cationic mobility making it a promising candidate as a solid electrolyte in magnesium batteries.

Hydrogen Desorption in Mg(BH4)2-Ca(BH4)2 System

Magnesium borohydride, Mg(BH4)2, and calcium borohydride, Ca(BH4)2, are promising materials for hydrogen storage. Mixtures of different borohydrides have been the subject of numerous researches; however, the whole Mg(BH4)2-Ca(BH4)2 system has not been investigated yet. In this study, the phase stability and the hydrogen desorption were experimentally investigated in the Mg(BH4)2-Ca(BH4)2 system, by means of XRD, ATR-IR, and HP-DSC. Mg(BH4)2 and Ca(BH4)2 are fully immiscible in the solid state. In the mechanical mixtures, thermal decomposition occurs at slightly lower temperatures than for pure compounds. However, they originate products that cannot be identified by XRD, apart from Mg and MgH2. In fact, amorphous phases or mixtures of different poorly crystalline or nanocrystalline phases are formed, leading to a limited reversibility of the system.

11B NMR Chemical Shift Predictions via Density Functional Theory and Gauge-Including Atomic Orbital Approach: Applications to Structural Elucidations of Boron-Containing Molecules

11B nuclear magnetic resonance (NMR) spectroscopy is a useful tool for studies of boron-containing compounds in terms of structural analysis and reaction kinetics monitoring. A computational protocol, which is aimed at an accurate prediction of 11B NMR chemical shifts via linear regression, was proposed based on the density functional theory and the gauge-including atomic orbital approach. Similar to the procedure used for carbon, hydrogen, and nitrogen chemical shift predictions, a database of boron-containing molecules was first compiled. Scaling factors for the linear regression between calculated isotropic shielding constants and experimental chemical shifts were then fitted using eight different levels of theory with both the solvation model based on density and conductor-like polarizable continuum model solvent models. The best method with the two solvent models yields a root-mean-square deviation of about 3.40 and 3.37 ppm, respectively. To explore the capabilities and potential limitations of the developed protocols, classical boron-hydrogen compounds and molecules with representative boron bonding environments were chosen as test cases, and the consistency between experimental values and theoretical predictions was demonstrated.

Quantitative Assessment of B-B-B, B-Hb -B, and B-Ht Bonds: From BH3 to B12 H12 2

We report the thermodynamic stabilities and the intrinsic strengths of three-center-two-electron B-B-B and B-H_b -B bonds ( H b : bridging hydrogen), and two-center-two-electron B-H_t bonds ( H t : terminal hydrogen) which can be served as a new, effective tool to determine the decisive role of the intermediates of hydrogenation/dehydrogenation reactions of borohydride. The calculated heats of formation were obtained with the G4 composite method and the intrinsic strengths of B-B-B, B-H_b -B, and B-H_t bonds were derived from local stretching force constants obtained at the B3LYP-D2/cc-pVTZ level of theory for 21 boron-hydrogen compounds, including 19 intermediates. The Quantum Theory of Atoms in Molecules (QTAIM) was used to deepen the inside into the nature of B-B-B, B-H_b -B, and B-H_t bonds. We found that all of the experimentally identified intermediates hindering the reversibility of the decomposition reactions are thermodynamically stable and possess strong B-B-B, B-H_b -B, and B-H_t bonds. This proves that thermodynamic data and intrinsic B-B-B, B-H_b -B, and B-H_t bond strengths form a new, effective tool to characterize new (potential) intermediates and to predict their role for the reversibility of the hydrogenation/dehydrogenation reactions.

Morphology-Dependent Stability of Complex Metal Hydrides and Their Intermediates Using First-Principles Calculations

Complex light metal hydrides are promising candidates for efficient, compact solid-state hydrogen storage. (De)hydrogenation of these materials often proceeds via multiple reaction intermediates, the energetics of which determine reversibility and kinetics. At the solid-state reaction front, molecular-level chemistry eventually drives the formation of bulk product phases. Therefore, a better understanding of realistic (de)hydrogenation behavior requires considering possible reaction products along all stages of morphological evolution, from molecular to bulk crystalline. Here, we use first-principles calculations to explore the interplay between intermediate morphology and reaction pathways. Employing representative complex metal hydride systems, we investigate the relative energetics of three distinct morphological stages that can be expressed by intermediates during solid-state reactions: i) dispersed molecules; ii) clustered molecular chains; and iii) condensed-phase crystals. Our results verify that the effective reaction energy landscape strongly depends on the morphological features and associated chemical environment, offering a possible explanation for observed discrepancies between X-ray diffraction and nuclear magnetic resonance measurements. Our theoretical understanding also provides physical and chemical insight into phase nucleation kinetics upon (de)hydrogenation of complex metal hydrides.

Kinetic Enhancement of Direct Hydrogenation of MgB2 to Mg(BH4 )2 upon Mechanical Milling with THF, MgH2 , and/or Mg

Modification of magnesium diboride, MgB₂ , by mechanical milling with THF, MgH₂ , and/or Mg results in a lowering of the conditions required for its direct, bulk hydrogenation to magnesium borohydride, Mg(BH₄ )₂ , by 300 bar and 100 °C. Following mechanical milling with MgH₂ or THF and Mg, MgB₂ can be hydrogenated to Mg(BH₄ )₂ at 300 °C under 700 bar of H₂ while achieving ∼54-71 % conversion to the borohydride. The discovery of a means of dramatically lowering the conditions required for the hydrogenation of MgB₂ is an important step towards the development of a practical onboard hydrogen storage system based on hydrogen cycling between Mg(BH₄ )₂ and MgB₂ . We suggest that mechano-milling with THF, Mg, and/or MgH₂ may possibly introduce defects in the MgB₂ structure which enhance hydrogenation. The ability to activate the MgB₂ through the introduction of structural defects transcends its relevance to hydrogen storage, as a method of overcoming its chemical inertness provides the key to harnessing other interesting properties of this material.

Facile Synthesis of Unsolvated Alkali Metal Octahydrotriborate Salts MB3 H8 (M=K, Rb, and Cs), Mechanisms of Formation, and the Crystal Structure of KB3 H8

A facile synthesis of heavy alkali metal octahydrotriborates (MB₃ H₈ ; M=K, Rb, and Cs) has been developed. It is simply based on reactions of the pure alkali metals with THF⋅BH₃ , does not require the use of electron carriers or the addition of other reaction media such as mercury, silica gel, or inert salts as for previous procedures, and delivers the desired products at room temperature in very high yields. However, no reactions were observed when pure Li or Na was used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. The low sublimation energies of K, Rb, and Cs were found to be key for initiation of the reactions. The syntheses can be carried out at room temperature because all of the elementary reaction steps have low energy barriers, whereas reactions of LiBH₄ /NaBH₄ with THF⋅BH₃ have to be carried out under reflux. The high stability and solubility of KB₃ H₈ were examined, and a crystal structure thereof was obtained for the first time.

The interconversion between THF·B3H7 and B3H8-: an efficient synthetic method for MB3H8 (M = Li and Na)

An efficient synthetic method for MB3H8 (M = Li and Na) has been developed with improved yields on the basis of the investigation of the interconversion between THF·B3H7 and corresponding MB3H8. The mechanism was tentatively proposed based on the understanding of the nucleophilicity of the B-H bonding pair electrons.

Potassium octahydridotriborate: diverse polymorphism in a potential hydrogen storage material and potassium ion conductor

Hydrogen storage properties and polymorphism in KB₃H₈. The order–disorder polymorphic transition results in disordered B₃H₈⁻ anions, facilitating cation mobility.

Hydrogenation properties of lithium and sodium hydride – closo-borate, [B10H10]2− and [B12H12]2−, composites

The hydrogen absorption properties of metal closo-borate/metal hydride composites are studied under high hydrogen pressures.

Effect of additives, ball milling and isotopic exchange in porous magnesium borohydride

Magnesium borohydride (Mg(BH₄)₂) is a promising material for solid state hydrogen storage. However, the predicted reversible hydrogen sorption properties at moderate temperatures have not been reached due to sluggish hydrogen sorption kinetics. Hydrogen (H) → deuterium (D) exchange experiments can contribute to the understanding of the stability of the BH₄⁻ anion. Pure γ-Mg(BH₄)₂, ball milled Mg(BH₄)₂ and composites with the additives nickel triboride (Ni₃B) and diniobium pentaoxide (Nb₂O₅) have been investigated. In situ Raman analysis demonstrated that in pure γ-Mg(BH₄)₂ the isotopic exchange reaction during continuous heating started at ∼80 °C, while the ball milled sample did not show any exchange at 3 bar D₂. However, during ex situ exchange reactions investigated by infrared (IR) and thermogravimetric (TG) analyses a comparable H → D exchange during long exposures (23 h) to deuterium atmosphere was observed for as received, ball milled and γ-Mg(BH₄)₂ + Nb₂O₅, while the Ni₃B additive hindered isotopic exchange. The specific surface areas (SSA) were shown to be very different for as received γ-Mg(BH₄)₂, BET area = 900 m² g⁻¹, and ball milled Mg(BH₄)₂, BET area = 30 m² g⁻¹, respectively, and this explains why no gas–solid H(D) diffusion was observed for the ball milled (amorphous) Mg(BH₄)₂ during the short time frames of in situ Raman measurements. The heat treated ball milled sample partially regained the porous γ-Mg(BH₄)₂ structure (BET area = 560 m² g⁻¹). This in combination with the long reaction times allowing for the reaction to approach equilibrium explains the observed gas–solid H(D) diffusion during long exposure. We have also demonstrated that a small amount of D can be substituted in both high surface area and low surface area samples at room temperature proving that the B–H bonds in Mg(BH₄)₂ can be challenged at these mild conditions.

Specific surface area measurements (BET) of as received and ball milled samples showed the collapse of the porous network after milling, while a heat treated ball milled sample regained most of its porous γ-Mg(BH₄)₂ structure.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Elucidating the mechanism of MgB2 initial hydrogenation via a combined experimental–theoretical study

The initial hydrogenation of MgB₂ occurs via a multi-step process, which can result in the direct production of [BH₄]⁻ complexes.

Complex metal borohydrides: multifunctional materials for energy storage and conversion

With the limited supply of fossil fuels and their adverse effect on the climate and the environment, it has become a global priority to seek alternate sources of energy that are clean, abundant, and sustainable. While sources such as solar, wind, and hydrogen can meet the world's energy demand, considerable challenges remain to find materials that can store and/or convert energy efficiently. This topical review focuses on one such class of materials, namely, multi-functional complex metal borohydrides that not only have the ability to store sufficient amount of hydrogen to meet the needs of the transportation industry, but also can be used for a new generation of metal ion batteries and solar cells. We discuss the material challenges in all these areas and review the progress that has been made to address them, the issues that still need to be resolved and the outlook for the future.

In situ characterization of the decomposition behavior of Mg(BH4)2 by X-ray Raman scattering spectroscopy

We present an in situ study of the thermal decomposition of Mg(BH4)2 in a hydrogen atmosphere of up to 4 bar and up to 500 °C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH4)2 can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 °C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB12H12. At temperatures between 300 °C and 400 °C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH2. Above 400 °C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH4)2 is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications.

A novel method for the synthesis of solvent-free Mg(B3H8)2

A novel and solvent-free method for the synthesis of Mg(B₃H₈)₂ by a gas–solid reaction between B₂H₆ and Mg₂NiH₄ is presented.

Structure-dependent vibrational dynamics of Mg(BH4)2 polymorphs probed with neutron vibrational spectroscopy and first-principles calculations

Neutron vibrational spectroscopy and DFT calculations are used in order to gain deeper insights into the structure-dependent vibrational properties of Mg(BH₄)₂ polymorphs.

Stabilization of volatile Ti(BH4)3 by nano-confinement in a metal-organic framework

Volatile Ti(BH₄)₃ molecules stabilized on the surface of a MOF.

Liquid complex hydrides are a new class of hydrogen storage materials with several advantages over solid hydrides, e.g. they are flexible in shape, they are a flowing fluid and their convective properties facilitate heat transport. The physical and chemical properties of a gaseous hydride change when the molecules are adsorbed on a material with a large specific surface area, due to the interaction of the adsorbate with the surface of the host material and the reduced number of collisions between the hydride molecules. In this paper we report the synthesis and stabilization of gaseous Ti(BH₄)₃. The compound was successfully stabilized through adsorption in nanocavities. Ti(BH₄)₃, upon synthesis in its pure form, spontaneously and rapidly decomposes into diborane and titanium hydride at room temperature in an inert gas, e.g. argon. Ti(BH₄)₃ adsorbed in the cavities of a metal organic framework is stable for several months at ambient temperature and remains stable up to 350 K under vacuum. The adsorbed Ti(BH₄)₃ reaches approximately twice the density of the gas phase. The specific surface area (BET, N₂ adsorption) of the MOF decreased from 1200 m² g^–1 to 770 m² g^–1 upon Ti(BH₄)₃ adsorption.

Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.

The role of MgB12H12 in the hydrogen desorption process of Mg(BH4)2

The presence of MgB12H12 has often been considered as the major obstacle for the reversible hydrogen storage in Mg(BH4)2. This communication provides evidence that the MgB12H12 phase (or [B12H12](2-) monomer) does not exist in the decomposition products of Mg(BH4)2 at temperatures ranging from 265 to 400 °C, and thereby it will not act as a dead end.

Superhalogens: A Bridge between Complex Metal Hydrides and Li Ion Batteries

Complex metal hydrides and Li ion batteries play an integral role in the pursuit of clean and sustainable energy. The former stores hydrogen and can provide a clean energy solution for the transportation industry, while the latter can store energy harnessed from the sun and the wind. However, considerable materials challenges remain in both cases, and research for finding solutions has traditionally followed parallel paths. In this Perspective, I show that there is a common link between these two seemingly disparate fields that can be unveiled by studying the electronic structure of the anions in complex metal hydrides and in electrolytes of Li ion batteries; they are both superhalogens. I demonstrate that considerable progress made in our understanding of superhalogens in the past decade can provide solutions to some of the materials challenges in both of these areas.

Facile synthesis of anhydrous alkaline earth metal dodecaborates MB12H12 (M = Mg, Ca) from M(BH4)2

Thermal decomposition of MB₁₂H₁₂ (M = Mg, Ca) forms H-deficient monomers MB₁₂H_12−x containing icosahedral B₁₂ skeletons and is followed by the formation of (MB_yH_z)_n polymers.

Fast carbon dioxide recycling by reaction with γ-Mg(BH4)2

γ-Mg(BH₄)₂ was found to be a promising material for CO₂ recycling (mainly to format and alkoxide-like compounds) with very fast kinetics because of its very large surface area.