Magnesium Borohydride Research Articles

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 2.65nm, high specific surface area (3955.9 m2g-1), and large pore volume (2.15 cm3g-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 borohydride monoammine as hydrogen storage: Structure and pathway analysis for the thermal decomposition reaction

A perspective compound for the chemical storage of hydrogen is the magnesium borohydride monoammine complex containing 15.61 wt% hydrogen. Mg(BH4)2·NH3 was synthesized by co-heating of Mg(BH4)2·(NH3)6 and Mg(BH4)2 and studied by powder X-ray diffraction, FTIR, X-ray fluorescence spectroscopy, synchronous thermal analysis and mass spectrometry. Quantum-chemical calculations data demonstrate that in a monoammine crystal individual molecules are combined into parallel zig-zag chains in which magnesium atoms are connected by bridging fragments of [−BH4−] groups. The assignment of absorption bands in the IR spectrum of Mg(BH4)2·NH3 has been refined. There is an incomplete release of H2 as a result of thermolysis (3.12 wt%) and the formation of stable reaction products, which contained hydrogen was found. The thermal decomposition reaction pathway was proposed.

The Influence of Reorientational and Vibrational Dynamics on the Mg2+ Conductivity in Mg(BH4)2·CH3NH2.

Reorientational dynamics in solid electrolytes can significantly enhance the ionic conductivity, and understanding these dynamics can facilitate the rational design of improved solid electrolytes. Additionally, recent investigations on metal hydridoborate-based solid electrolytes have shown that the addition of a neutral ligand can also have a positive effect on the ionic conductivity. In this study, we investigate the dynamics in monomethylamine magnesium borohydride (Mg(BH4)2·CH3NH2) with quasielastic and inelastic neutron scattering, density functional theory calculations, and molecular dynamics simulations. The results suggest that the addition of methylamine significantly speeds up the reorientational frequency of the BH4 - anion compared to Mg(BH4)2. This is likely part of the explanation for the high Mg-ion transport observed for Mg(BH4)2·CH3NH2. Furthermore, while the dynamics of both the BH4 - anion and the CH3 group of the methylamine ligand is rapid, the NH2 group of the methylamine ligand exhibits much slower reorientations, as confirmed by both experimental and computational investigations. Notably, molecular dynamics calculations reveal mean square displacements of 0.387 Å2 for NH2, 1.503 Å2 for CH3, and 1.856 Å2 for BH4 - using a trajectory of 10 ps. This study confirms the simultaneous presence of fast dynamics and high ionic conductivity in a metal borohydride-based system and can function as an experimental foundation for future studies on dynamics in hydrogen-rich solid electrolytes.

Mxene-supported NbVH nanoparticles as efficient catalysts for reversible hydrogen storage in magnesium borohydride

Bimetallic niobium vanadium hydride nanoparticles (NbVH NPs) anchored on Ti3C2 were synthesized using a facile ball milling method as an effective catalyst for the dehydrogenation kinetics and reversibility of Mg(BH4)2. It was found that the onset dehydrogenation temperature of the Mg(BH4)2 + 30NbVH NPs@Ti3C2 composite was 105.7 °C and released 9.07 wt% of hydrogen at 330 °C, while undoped Mg(BH4)2 only released 4.2 wt% of H2 from 248 °C to 330 °C. Additionally, the Mg(BH4)2 + 30NbVH NPs@Ti3C2 composite achieved remarkable hydrogen release of over 9.44 wt% at a temperature as low as 230 °C, indicating unexpected dehydrogenation kinetics. In the reversible hydrogen desorption tests, Mg(BH4)2 + 30NbVH NPs@Ti3C2 released 5.98 wt% of H2 in the 2nd cycle at 250 °C and maintained a reversible capacity of 4.35 wt% after 10 cycles, demonstrating a remarkable enhancement in reversibility. The enhancement in the dehydrogenation kinetics of Mg(BH4)2 could be attributed to the greatly reduced activation energies from 343 kJ·mol−1 and 138.3 kJ·mol−1 to 103.7 kJ·mol−1 and 124 kJ·mol−1. Investigations on the evaluation of catalyst and Mg(BH4)2 during reversible hydrogen storage have revealed that NbVH NPs actually acted as a “bidirectional hydrogen pump”, facilitating both the hydrogen release and uptake from Mg(BH4)2. This strategy of multi-component hydrides may show a new way to design effective catalysts for solid-state hydrogen storage materials.

Advancing Catalysts by Nanoconfinement and Catalysis for Enhanced Hydrogen Production from Magnesium Borohydride: A Review.

Hydrogen storage in solid-state materials represents a promising avenue for advancing hydrogen storage technologies, driven by their potential for high efficiency, reduced risk, and cost-effectiveness. Among the employed materials, magnesium borohydride (Mg(BH4)2) stands out for its exceptional characteristics, with a gravimetric capacity of 14.9 wt% and a volumetric hydrogen density capacity of 146 kg/m3. However, the practical application of Mg(BH4)2 is impeded by challenges such as high desorption temperatures (≥ 270 °C), sluggish kinetics, poor reversibility, and the formation of unexpected byproducts like diborane. To address these limitations, extensive research efforts have been directed towards enhancing the hydrogen storage properties of Mg(BH4)2. Various strategies have been explored, including incorporating catalysts or additives, nanoconfinement of Mg(BH4)2 within porous supports, and modifications involving metal alloys and compositional adjustments. These approaches are actively under investigation for improving the performance of Mg(BH4)2-based hydrogen storage systems. This review provides a comprehensive survey of recent advancements in Mg(BH4)2 research, focusing on experimental findings related to nanoconfined Mg(BH4)2 and modified thermodynamic processes aimed at enabling hydrogen release at lower temperatures by mitigating sluggish kinetics. Precisely, nanostructuring techniques, catalyst-mediated nanoconfinement methodologies, and alloy/compositional modifications will be elucidated, highlighting their potential to enhance hydrogen storage properties and overcome existing limitations. Furthermore, this review also discusses the challenges encountered in utilizing Mg(BH4)2 for hydrogen storage applications and offers insights into the prospects of this material. By synthesizing the latest research findings and identifying areas for further exploration, this review aims to contribute to the ongoing efforts toward realizing the full potential of Mg(BH4)2 as a viable solution for hydrogen storage in diverse applications.

Adsorption and storage of hydrogen- A computational model approach

Due to the imperative global energy transition crisis, hydrogen storage and adsorption technologies are becoming popular with the growing hydrogen economy. Recently, complex hydrides have been one of the most reliable materials for storing and transporting hydrogen gas to various fuel cells to generate clean energy with zero carbon emissions. With the ever-increasing carbon emissions, it is necessary to substitute the current energy sources with green hydrogen-based efficient energy-integrated systems. Herein, we propose an input-output model that comprehends complex hydrides such as lithium and magnesium alanates, amides and borohydrides to predict, estimate, and directly analyse hydrogen storage and adsorption. A critical and thorough comparative analysis of the respective complex hydrides for hydrogen adsorption and storage is discussed, elucidating the storage applications in water bodies. Several industrial scale-up processes, economic analysis, and plant design of hydrogen storage and adsorption approaches are suggested through volumetric and gravimetric calculations.

Synthesis, clarifying properties and refining structures of single-crystal magnesium borohydride di- and triammine complexes

Synthesis, clarifying properties and refining structures of single-crystal magnesium borohydride di- and triammine complexes

Small-pore hydridic frameworks store densely packed hydrogen

Nanoporous materials have attracted great attention for gas storage, but achieving high volumetric storage capacity remains a challenge. Here, by using neutron powder diffraction, volumetric gas adsorption, inelastic neutron scattering and first-principles calculations, we investigate a magnesium borohydride framework that has small pores and a partially negatively charged non-flat interior for hydrogen and nitrogen uptake. Hydrogen and nitrogen occupy distinctly different adsorption sites in the pores, with very different limiting capacities of 2.33 H2 and 0.66 N2 per Mg(BH4)2. Molecular hydrogen is packed extremely densely, with about twice the density of liquid hydrogen (144 g H2 per litre of pore volume). We found a penta-dihydrogen cluster where H2 molecules in one position have rotational freedom, whereas H2 molecules in another position have a well-defined orientation and a directional interaction with the framework. This study reveals that densely packed hydrogen can be stabilized in small-pore materials at ambient pressures.

Calcium Borohydride Ca(BH4)2: Fundamentals, Prediction and Probing for High-Capacity Energy Storage Applications, Organic Synthesis and Catalysis

Calcium borohydride (Ca(BH4)2) is a complex hydride that has been less investigated compared to its lighter counterpart, magnesium borohydride. While offering slightly lower hydrogen storage capacity (11.5 wt% theoretical maximum, 9.6 wt% under actual dehydrogenation conditions), there are many improvement avenues for maximizing the reversible hydrogen storage that have been explored recently, from DFT calculations and polymorph investigations to reactive hydride composites (RHCs) and catalytic and nanosizing effects. The stability of Ca(BH4)2, the possibility of regeneration from spent products, and the relatively mild dehydrogenation conditions make calcium borohydride an attractive compound for hydrogen storage purposes. The ionic conductivity enhancements brought about by the rich speciation of borohydride anions can extend the use of Ca(BH4)2 to battery applications, considering the abundance of Ca relative to alkali metal borohydrides typically used for this purpose. The current work aims to review the synthetic strategies, structural considerations of various polymorphs and adducts, and hydrogen storage capacity of composites based on calcium borohydrides and related complex hydrides (mixed anions, mixed cations, additives, catalysts, etc.). Additional applications related to batteries, organic and organometallic chemistry, and catalysis have been briefly described.

Magnesium borohydride Mg(BH4)2 for energy applications: A review

Mg(BH4)2 with several polymorphs, known as a high capacity (14.9 wt.%) hydrogen storage material, has become more intriguing due to the recently found new functions of gas physisorption and ionic conductivity. Here we review the state-of-the-art on the energy related functions of Mg(BH4)2. Mg(BH4)2 tends to form the stable intermediate [B12H12]2− when the dehydrogenation temperature is above 400 °C, the strong B-B bonding of which makes the rehydrogenation condition very harsh. In contrast, lower borane intermediate [B3H8]2− facilitates the rehydrogenation even at a mild condition of 100 °C, suggesting the possibility of reversible hydrogen storage in Mg(BH4)2. The porous polymorph γ-Mg(BH4)2 shows attractive gas adsorption properties in view of its unique hydridic surface and pore shape, and potentially can be applied in hydrogen adsorption and Kr/Xe selectivity. A new diffraction-based adsorption methodology was developed to characterize adsorption thermodynamics and kinetics of γ-Mg(BH4)2, providing a novel idea for the characterization of crystalline porous materials. Moreover, the potential of Mg(BH4)2 as an electrolyte is discussed in the last part. Mg(BH4)2·THF/DME acts as a liquid electrolyte in Mg-batteries, while anion substituted or neutral molecule derivatives of Mg(BH4)2 can act as solid-state electrolyte.

Dynamical properties of the magnesium borohydride – ethylenediamine compound Mg(en)1.2(BH4)2: A nuclear magnetic resonance study

The magnesium borohydride – ethylenediamine (en) compound Mg(en)1.2(BH4)2 exhibits high Mg-ion conductivity above room temperature. To study the dynamical properties of this compound and its partially deuterium-substituted counterpart, we have measured the 1H, 2D, and 11B nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over a wide temperature range (6–324 K). Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. For the fastest of these processes characterized by the activation energy of 47(3) meV, the unusually high reorientational mobility is retained down to low temperatures, so that the corresponding H jump rate is of the order of 108 s−1 near 70 K. The presence of fast reorientational motion of [BH4]− anions in Mg(en)1.2(BH4)2 supports the idea that anion reorientations may facilitate the cation diffusion. In the studied temperature range, we have not found any distinct signs of localized motion of the ethylenediamine molecules or their fragments at the NMR frequency scale.

Structural and Spectroscopic Studies of Steric and Electronic Substituent Effects in a Series of Magnesium Aminodiboranates Mg[(BH3)2NMeR]2.

The magnesium N,N-dimethylaminodiboranate compound Mg[(BH3)2NMe2]2, the most volatile compound of magnesium known, serves as an excellent chemical vapor deposition (CVD) precursor for the growth of thin films such as the dielectric material MgO. To explore how the thermal stability and physical properties of magnesium aminodiboranates depend on the steric and electronic properties of the nitrogen-bound substituents, we have made a series of analogues of Mg[(BH3)2NMe2]2, in which one of the two methyl substituents on nitrogen is replaced with an ethyl, iso-propyl, or tert-butyl group. In the crystal structure of Mg[(BH3)2NMe(t-Bu)]2, the magnesium center is coordinated to two chelating (BH3)2NMe(t-Bu) ligands, each of which binds in a κ2,κ2 fashion so that the magnesium center forms eight Mg-H contacts. Unlike Mg[(BH3)2NMe2]2, however, which has a linear N···Mg···N angle and is an isolated molecule in the solid state, the N···Mg···N angle in Mg[(BH3)2NMe(t-Bu)]2 is distinctly nonlinear (149.9°) because hydrogen atoms of BH3 groups of nearby molecules form two additional Mg-H contacts with the magnesium center. When the complexes are heated in toluene solution, the (BH3)2NMeR- groups reversibly undergo B-N bond cleavage (with concomitant migration of a hydrogen atom) to release the aminoborane BH2═NMeR and form magnesium borohydride, Mg(BH4)2. For the methyl, ethyl, and iso-propyl derivatives, the equilibrium strongly favors Mg[(BH3)2NMeR]2. In contrast, for the tert-butyl derivative, the equilibrium strongly favors BH2═NMe(t-Bu) and Mg(BH4)2. The results suggest that more strongly electron-donating groups slightly strengthen the B-N bonds and disfavor B-N bond cleavage, provided that the groups are not too large. In contrast, sterically bulky ligands disfavor B-N bond reformation, thus promoting the dissociative equilibrium that involves B-N bond cleavage. Interestingly, the rates at which the complexes approach equilibrium depend only weakly on the nature of the R group, at least within the series studied. These findings are relevant to the potential use of magnesium aminodiboranates as CVD precursors to the superconducting phase MgB2.

Methylamine Magnesium Borohydrides as Electrolytes for All-Solid-State Magnesium Batteries

Solid-state magnesium electrolytes may pave the way for novel types of rechargeable, sustainable, and cheap batteries with high volumetric and gravimetric capacities. There are, however, currently no solid-state magnesium electrolytes that fulfill the requirements for solid-state battery applications. Here, we present the synthesis, structure, and properties of six new methylamine magnesium borohydride compounds, α- and β-Mg(BH4)2·6CH3NH2, Mg(BH4)2·3CH3NH2, and α-, α′- and β-Mg(BH4)2·CH3NH2. The β-Mg(BH4)2·CH3NH2 polymorph displays a record high Mg2+ ionic conductivity of σ(Mg2+) = 1.50 × 10–4 S cm–1 at room temperature. The high Mg2+ conductivity of β-Mg(BH4)·CH3NH2 is facilitated by a one-dimensional chain-like structure interconnected by weak dihydrogen bonds and dispersion interactions, forming a migration pathway across the chains. The oxidative stability of Mg(BH4)2·CH3NH2 is ∼1.2 V vs Mg/Mg2+, and the reversible plating and stripping were confirmed by cyclic voltammetry and symmetric cell cycling, revealing high stability toward magnesium electrodes for at least 50 cycles at 60 °C.

Enhanced interface stability of ammine magnesium borohydride by in situ decoration of MgBr2·2NH3 nanoparticles.

A novel Mg(BH4)2·1.9NH3-MgBr2·2NH3 composite was demonstrated as a solid-state Mg2+ electrolyte. The in situ decoration of MgBr2·2NH3 nanoparticles with an average size of 3.7 nm on the surface of Mg(BH4)2·1.9NH3 improves the Mg2+ conductivity to 2.1 × 10-4 S cm-1 at 50 °C and offers long-term stability towards the Mg metal anode.

Synthesis, Structure and Mg2+ Ionic Conductivity of Isopropylamine Magnesium Borohydride

The discovery of new inorganic magnesium electrolytes may act as a foundation for the rational design of novel types of solid-state batteries. Here we investigated a new type of organic-inorganic metal hydride, isopropylamine magnesium borohydride, Mg(BH4)2∙(CH3)2CHNH2, with hydrophobic domains in the solid state, which appear to promote fast Mg2+ ionic conductivity. A new synthetic strategy was designed by combination of solvent-based methods and mechanochemistry. The orthorhombic structure of Mg(BH4)2∙(CH3)2CHNH2 was solved ab initio by the Rietveld refinement of synchrotron X-ray powder diffraction data and density functional theory (DFT) structural optimization in space group I212121 (unit cell, a = 9.8019(1) Å, b = 12.1799(2) Å and c = 17.3386(2) Å). The DFT calculations reveal that the three-dimensional structure may be stabilized by weak dispersive interactions between apolar moieties and that these may be disordered. Nanoparticles and heat treatment (at T > 56 °C) produce a highly conductive composite, σ(Mg2+) = 2.86 × 10−7, and 2.85 × 10−5 S cm−1 at −10 and 40 °C, respectively, with a low activation energy, Ea = 0.65 eV. Nanoparticles stabilize the partially eutectic molten state and prevent recrystallization even at low temperatures and provide a high mechanical stability of the composite.

Thermal Decomposition of Magnesium Borohydride: New Insights from Synchrotron X-ray Scattering

Magnesium borohydride [Mg(BH4)2] has been extensively investigated as a promising material with applications in energy storage, having properties favorable for both hydrogen and electrochemical storage processes. Recent efforts regarding hydrogen storage to determine conditions optimal for dehydrogenation and hydrogenation have focused on identifying environmental factors that promote various polyborane intermediate pathways. In the present study, we demonstrate the impact of the synthesis route, residual impurities, sample processing, and starting crystalline phase on the structural transformations and decomposition pathways of Mg(BH4)2. Using synchrotron powder X-ray diffraction (PXRD) and small-angle X-ray scattering (SAXS), we provide evidence of the residual dimethyl sulfide solvent coordinated inside the pores of γ-Mg(BH4)2 even after post-synthetic de-solvation. In situ temperature-resolved PXRD and temperature programed desorption (TPD) indicate that these impurity molecules are removed by heating to 100 °C in vacuo. Furthermore, when γ-Mg(BH4)2 is heated slowly under vacuum, we observe a γ- to α-phase transformation in place of the ε-Mg(BH4)2 “intermediate” structure, which provides a direct connection between the α- and γ-Mg(BH4)2 decomposition pathways. At higher temperatures (∼150 °C), Mg(BH4)2 transitions to the well-known β-phase structure. Although XRD results suggest that the crystalline structures of β-Mg(BH4)2 are identical regardless of the starting material, SAXS and transmission electron microscopy indicate that when the γ-phase is used as the starting structure, the resulting β-Mg(BH4)2 material exhibits intergranular (i.e., skeletal) porosity, not observed when annealed from the α-phase. These variations of the microstructure may contribute to differences in dehydrogenation mechanisms observed in TPD data among the three currently existing “as-synthesized” Mg(BH4)2 structural phases.

X-ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

Magnesium borohydride (Mg(BH4)2) is an attractive compound for solid-state hydrogen storage due to its lucratively high hydrogen densities and theoretically low operational temperature. Hydrogen release from Mg(BH4)2 occurs through several steps. The reaction intermediates formed at these steps have been extensively studied for a decade. In this work, we apply spectroscopic methods that have rarely been used in such studies to provide alternative insights into the nature of the reaction intermediates. The commercially obtained sample was decomposed in argon flow during thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) to differentiate between the H2-desorption reaction steps. The reaction products were analyzed by powder X-ray diffraction (PXRD), near edge soft X-ray absorption spectroscopy at boron K-edge (NEXAFS), and synchrotron infrared (IR) spectroscopy in mid- and far-IR ranges (SR-FTIR). Up to 12 wt% of H2 desorption was observed in the gravimetric measurements. PXRD showed no crystalline decomposition products when heated at 260–280 °C, the formation of MgH2 above 300 °C, and Mg above 320 °C. The qualitative analysis of the NEXAFS data showed the presence of boron in lower oxidation states than in (BH4)−. The NEXAFS data also indicated the presence of amorphous boron at and above 340 °C. This study provides additional insights into the decomposition reaction of Mg(BH4)2.

An NMR relaxation study of magnesium borohydride in solid

AbstractThe spin–lattice relaxation phenomenon is a good tool for obtaining the thermodynamical parameters of solid metal borohydrides. In the process of deriving the motional parameters from the NMR experimental data, it is very important to adequately interpret the spin–lattice relaxation data. By more precisely considering the quadrupolar interaction of 11B and intermolecular 1H‐1H dipole–dipole interactions, the activation energies and correlation times of the rotational motions of BH4 group in solid magnesium borohydride could be more accurately determined, and the spin–lattice relaxation phenomenon of the quadrupolar nucleus could be more appropriately understood.

Synthesis, properties and thermal decomposition particularities of magnesium borohydride ammoniates

An elemental analysis, as well as XPD, IR, XPS and TGA-DSC/TPD-MS methods are used to compare composition and properties of Mg(BH4)2(NH3)n (n = 1, 2, 3) complexes obtained by following methods: (1) mechanochemical treatment and (2) heating of magnesium borohydride hexaammoniate and unsolvated magnesium borohydride mixtures. A systematic study of the thermal decomposition features and gases composition released during the thermolysis of the synthesized compounds is carried out. The differences found are discussed.

Towards Solid‐State Magnesium Batteries: Ligand‐Assisted Superionic Conductivity

AbstractSolid‐state inorganic magnesium batteries are considered as potential high energy storage devices of the future. Here we present a series of magnesium borohydride tetrahydrofuran (THF) composites, Mg(BH4)2 ⋅ xTHF(−MgO), 0≤x≤3, as solid‐state electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH4)2 ⋅ 2/3THF (Cc), α‐Mg(BH4)2 ⋅ 2THF (P21/c) and β‐Mg(BH4)2 ⋅ 2THF (C2), and the detailed structures of α‐ and β‐Mg(BH4)2 ⋅ 2THF are presented. The magnesium ionic conductivity of composites formed by these compounds were several orders of magnitude higher than that of the distinct compounds, x=0, 2/3, 2, and 3. The nanocomposite stabilized by MgO nanoparticles (∼50 nm), Mg(BH4)2 ⋅ 1.5THF−MgO(75 wt%), displayed the highest Mg2+ conductivity, σ(Mg2+)∼10−4 S cm−1 at 70 °C, a high ionic transport number of tion=0.99, and cyclic voltammetry revealed an oxidative stability of ∼1.2 V vs. Mg/Mg2+. The electrolyte was stable towards magnesium electrodes, which allowed for stable Mg plating/stripping for at least 100 cycles at 55 °C with a current density of 0.1 mA cm−2. Finally, a proof‐of‐concept rechargeable solid‐state magnesium battery was assembled with a magnesium metal anode and a TiS2 cathode. A maximum discharge capacity of 94.2 mAh g−1 was displayed, which corresponds to y=0.2 in MgyTiS2.