Ca(BH4)2-Mg2NiH4: on the pathway to a Ca(BH4)2 system with a reversible hydrogen cycle

Unveiling the destabilization of sp3 and sp2 bonds in transition metal-modified borohydrides to improve reversible dehydrogenation and rehydrogenation

Borohydrides offer promise as potential carriers for hydrogen storage due to their high hydrogen concentration. However, the strong chemical bonding within borohydrides poses challenges for efficient hydrogen release during usage and restricts the re-hydrogenation process when attempting to regenerate the material. These high thermodynamic and kinetic barriers present obstacles in achieving reversible de-hydrogenation and re-hydrogenation of borohydrides, impeding their practical application in hydrogen storage systems. Employing density functional theory calculations, we conduct a comprehensive investigation into the influence of transition metals on both the BH4 cluster, a fundamental building block of borohydrides, and pure boron, which is formed as the end product following hydrogen release. Our research reveals correlations among the d-band center, work function, and surface energy of 3d and 4d transition metals. These correlations are directly linked to the weakening of bonding within the BH4 cluster when adsorbed on catalyst surfaces. On the other hand, we also explore how various intrinsic properties of transition metals influence the formation of boron vacancies and the hydrogen bonding process. By establishing a comprehensive correlation between the weakening of sp3 hybridization in the BH4 cluster and the sp2 hybridization in boron, we facilitate the identification and screening of optimal candidates capable of achieving reversible de-hydrogenation and re-hydrogenation in borohydrides.

A comprehensive study on lithium-based reactive hydride composite (Li-RHC) as a reversible solid-state hydrogen storage system toward potential mobile applications

Reversible solid-state hydrogen storage is one of the key technologies toward pollutant-free and sustainable energy conversion. The composite system LiBH₄–MgH₂ can reversibly store hydrogen with a gravimetric capacity of 13 wt%. However, its dehydrogenation/hydrogenation kinetics is extremely sluggish (∼40 h) which hinders its usage for commercial applications. In this work, the kinetics of this composite system is significantly enhanced (∼96%) by adding a small amount of NbF₅. The catalytic effect of NbF₅ on the dehydrogenation/hydrogenation process of LiBH₄–MgH₂ is systematically investigated using a broad range of experimental techniques such as in situ synchrotron radiation X-ray powder diffraction (in situ SR-XPD), X-ray absorption spectroscopy (XAS), anomalous small angle X-ray scattering (ASAXS), and ultra/small-angle neutron scattering (USANS/SANS). The obtained results are utilized to develop a model that explains the catalytic function of NbF₅ in hydrogen release and uptake in the LiBH₄–MgH₂ composite system.

Superb dehydrogenation/hydrogenation kinetic enhancement of the LiBH₄–MgH₂ reactive hydride composite system by addition of NbB₂ nano-particles as nucleation agents for MgB₂.

Using the Emission of Muonic X-rays as a Spectroscopic Tool for the Investigation of the Local Chemistry of Elements

There are several techniques providing quantitative elemental analysis, but very few capable of identifying both the concentration and chemical state of elements. This study presents a systematic investigation of the properties of the X-rays emitted after the atomic capture of negatively charged muons. The probability rates of the muonic transitions possess sensitivity to the electronic structure of materials, thus making the muonic X-ray Emission Spectroscopy complementary to the X-ray Absorption and Emission techniques for the study of the chemistry of elements, and able of unparalleled analysis in case of elements bearing low atomic numbers. This qualitative method is applied to the characterization of light elements-based, energy-relevant materials involved in the reaction of hydrogen desorption from the reactive hydride composite Ca(BH4)2-Mg2NiH4. The origin of the influence of the band-structure on the muonic atom is discussed and the observed effects are attributed to the contribution of the electronic structure to the screening and to the momentum distribution in the muon cascade.

A Review of the MSCA ITN ECOSTORE—Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity

Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.

Enhanced Stability of Li-RHC Embedded in an Adaptive TPX™ Polymer Scaffold

In this work, the possibility of creating a polymer-based adaptive scaffold for improving the hydrogen storage properties of the system 2LiH+MgB₂+7.5(3TiCl₃·AlCl₃) was studied. Because of its chemical stability toward the hydrogen storage material, poly(4-methyl-1-pentene) or in-short TPX^TM was chosen as the candidate for the scaffolding structure. The composite system was obtained after ball milling of 2LiH+MgB₂+7.5(3TiCl₃·AlCl₃) and a solution of TPX^TM in cyclohexane. The investigations carried out over the span of ten hydrogenation/de-hydrogenation cycles indicate that the material containing TPX^TM possesses a higher degree of hydrogen storage stability.

Exploring Ternary and Quaternary Mixtures in the LiBH4 -NaBH4 -KBH4 -Mg(BH4 )2 -Ca(BH4 )2 System

Binary combinations of borohydrides have been extensivly investigated evidencing the formation of eutectics, bimetallic compounds or solid solutions. In this paper, the investigation has been extended to ternary and quaternary systems in the LiBH₄ -NaBH₄ -KBH₄ -Mg(BH₄ )₂ -Ca(BH₄ )₂ system. Possible interactions among borohydrides in equimolar composition has been explored by mechanochemical treatment. The obtained phases were analysed by X-ray diffraction and the thermal behaviour of the mixtures were analysed by HP-DSC and DTA, defining temperature of transitions and decomposition reactions. The release of hydrogen was detected by MS, showing the role of the presence of solid solutions and multi-cation compounds on the hydrogen desorption reactions. The presence of LiBH₄ generally promotes the release of H₂ at about 200 °C, while KCa(BH₄ )₃ promotes the release in a single-step reaction at higher temperatures.

Controllable decomposition of Ca(BH4)2 for reversible hydrogen storage

The formation of CaB₆ from the thermal decomposition of Ca(BH₄)₂ goes along two distinct routes, i.e. via CaB₂H₆ or elemental boron as a reaction intermediate, depending on temperature.