Hydrogenation Ability of Mg-Li Alloys

The Mg-Li binary system is characterized by the presence of α-Mg(Li) and β-Li(Mg) phases, where magnesium exists in ordered and disordered forms that may affect the hydrogenation properties of magnesium. Therefore, the hydrogenation properties of an AZ31 alloy modified by the addition of 4.0 wt.%, 7.5 wt.% and 15.0 wt.% lithium were studied. The morphology (scanning electron microscopy (SEM)), structure, phase composition (X-ray diffraction (XRD)) and hydrogenation properties (differential scanning calorimetry (DSC)) of AZ31 with various lithium contents were investigated. It was found that the susceptibility of magnesium in the form of α-Mg(Li) to hydrogenation was higher than that for the magnesium occupying a disordered position in β-Li(Mg) solid solutions. Magnesium hydride was obtained as a result of hydrogenation of the AZ31 alloy that was modified with 4.0 wt.%, 7.5 wt.% and 15.0 wt.% additions of lithium, and was characterized by high hydrogen desorption activation energies of 250, 187 and 224 kJ/mol, respectively.

Solid–gas reactions in synthetic chemistry: what can we learn from reaction pathways?

The knowledge of reaction pathways in the preparation of solids is usually rather scarce, which hinders synthesis planning and process control. This is particularly true for metastable compounds, which are a challenge for chemical synthesis, especially in the solid state. In situ studies can help in exploring the energy landscape around their local minimum by investigating formation and decomposition. Screening the multi-parameter space in synthetic chemistry is much more efficient using in as compared to ex situ methods. Studying solid–gas reactions in situ is demanding due to the oftentimes harsh conditions as for temperature and gas pressure. Examples are given for a variety of solids and applications, e.g., metal hydrides (hydrogen storage, hydrogenation – decomposition – desorption – recombination), intermetallics (heterogeneous catalysis), metal nitrides, nitride oxides and oxides (magnetic materials, photocatalysts). Many new metastable compounds with intriguing properties were discovered by such in situ studies in flowing or static gas atmosphere (H2, Ar, NH3, air) at elevated pressures and temperatures using a variety of in situ methods such as X-ray and neutron powder diffraction, thermal analysis, environmental scanning electron microscopy, Raman, NMR, UV-VIS and X-ray absorption fine structure spectroscopy. The potential of unravelling reaction pathways of solid–gas reactions for improving syntheses and controlling chemical processes is demonstrated.

The bibliography includes 48 references.

Based on a talk given at the 5th EUCHEMS Inorganic Chemistry Conference (EICC-5, Moscow, Russia, 2019).

Alanates, a Comprehensive Review

Hydrogen storage is widely recognized as one of the biggest not solved problem within hydrogen technologies. The slow development of the materials and systems for hydrogen storage has resulted in a slow spread of hydrogen applications. There are many families of materials that can store hydrogen; among them, the alanate family can be of interest. Basic research papers and reviews have been focused on alanates of group 1 and 2. However, there are many alanates of transition metals, main group, and lanthanides that deserve attention in a review. This work is a comprehensive compilation of all known alanates. The approaches towards tuning the kinetics and thermodynamics of alanates are also covered in this review. These approaches are the formation of reactive composites, double cation alanates, or anion substitution. The crystallographic and X-ray diffraction characteristics of each alanate are presented along with this review. In the final sections, a discussion of the infrared, Raman, and thermodynamics was included.

In Situ Synthesis of 3D Flower-Like Nanocrystalline Ni/C and its Effect on Hydrogen Storage Properties of LiAlH4

Lithium alanate (LiAlH₄ ) is of particular interest as one of the most promising candidates for solid-state hydrogen storage. Unfortunately, high dehydrogenation temperatures and relatively slow kinetics limit its practical applications. Herein, 3D flower-like nanocrystalline Ni/C, composed of highly dispersed Ni nanoparticles and interlaced carbon flakes, was synthesized in situ. The as-synthesized nanocrystalline Ni/C significantly decreased the dehydrogenation temperature and dramatically improved the dehydrogenation kinetics of LiAlH₄ . It was found that the LiAlH₄ sample with 10 wt % Ni/C (LiAlH₄ -10 wt %Ni/C) began hydrogen desorption at approximately 48 °C, which is very close to ambient temperature. Approximately 6.3 wt % H₂ was released from LiAlH₄ -10 wt %Ni/C within 60 min at 140 °C, whereas pristine LiAlH₄ only released 0.52 wt % H₂ under identical conditions. More importantly, the dehydrogenated products can partially rehydrogenate at 300 °C under 4 MPa H₂ . The synergetic effect of the flower-like carbon substrate and Ni active species contributes to the significantly reduced dehydrogenation temperatures and improved kinetics.

Boric acid-destabilized lithium borohydride with a 5.6 wt% dehydrogenation capacity at moderate temperatures

Boric acid effectively promotes the dehydrogenation of lithium borohydride due to the interactions between protonic and hydridic hydrogen.

Synthesis and decomposition of Li3Na(NH2)4 and investigations of Li-Na-N-H based systems for hydrogen storage

Previous studies have shown modified thermodynamics of amide-hydride composites by cation substitution, while this work systematically investigates lithium-sodium-amide, Li-Na-N-H, based systems. Li3Na(NH2)4 has been synthesized by combined ball milling and annealing of 3LiNH2-NaNH2 with LiNa2(NH2)3 as a minor by-product. Li3+xNa1-x(NH2)4 releases NaNH2 and forms non-stoichiometric Li3+xNa1-x(NH2)4 before it melts at 234 °C, as observed by in situ powder X-ray diffraction. Above 234 °C, Li3+xNa1-x(NH2)4 releases a mixture of NH3, N2 and H2 while a bi-metallic lithium sodium imide is not observed during decomposition. Hydrogen storage performances have been investigated for the composites Li3Na(NH2)4-4LiH, LiNH2-NaH and NaNH2-LiH. Li3Na(NH2)4-4LiH converts into 4LiNH2-NaH-3LiH during mechanochemical treatment and releases 4.2 wt% of H2 in multiple steps between 25 and 340 °C as revealed by Sievert's measurements. All three investigated composites have a lower peak temperature for H2 release as compared to LiNH2-LiH, possibly owing to modified kinetics and thermodynamics, due to the formation of Li3Na(NH2)4 and LiNa2(NH2)3.

Enhancement of the initial hydrogenation of Mg by ball milling with alkali metal amides MNH2(M = Li or Na)

The addition of 4 wt% of MNH₂(M = Li, Na) to pure Mg by ball milling greatly enhances the first hydrogenation (activation). Under 2 MPa of H₂at 608 K, the best activation performance was achieved with the NaNH₂additive.