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Hydrogen has a very diverse chemistry and reacts with most other elements to form compounds, which have fascinating structures, compositions and properties. Complex metal hydrides are a rapidly expanding class of materials, approaching multi-functionality, in particular within the energy storage field. This review illustrates that complex metal hydrides may store hydrogen in the solid state, act as novel battery materials, both as electrolytes and electrode materials, or store solar heat in a more efficient manner as compared to traditional heat storage materials. Furthermore, it is highlighted how complex metal hydrides may act in an integrated setup with a fuel cell. This review focuses on the unique properties of light element complex metal hydrides mainly based on boron, nitrogen and aluminum, e.g., metal borohydrides and metal alanates. Our hope is that this review can provide new inspiration to solve the great challenge of our time: efficient conversion and large-scale storage of renewable energy.

Rare earth (RE) borohydrides have received considerable attention during the past ten years as possible hydrogen storage materials due to their relatively high gravimetric hydrogen density. This review illustrates the rich chemistry, structural diversity and thermal properties of borohydrides containing RE elements. In addition, it highlights the decomposition and rehydrogenation properties of composites containing RE-borohydrides, light-weight metal borohydrides such as LiBH4 and additives such as LiH.

Energy density and safety are the main factors that govern the development of the rechargeable battery technology. Currently, batteries beyond typical Li-ion batteries such as those based on solid-state electrolytes (SSEs) or other active elements (e.g., Na or Mg) are being examined as alternatives. For example, SSEs that would enable stable and reliable operation of all-solid-state Li-, Na-, and Mg-based batteries, with preferably improved capacity, are considered to be one of the most desired inventions. Lightweight complex metal hydrides are a family of solid compounds that were recently discovered to have extraordinary ionic conductivities and, in some cases, electrochemical properties that enabled battery reversibility. Consequently, they have become one of the promising electrolyte materials for future development of electrochemical storage devices. In this work, we present an overview of a wide range of lightweight hydride-based materials that could be used as electrolytes and/or anodes for mono-/divalent batteries and have a pivotal role in the implementation of new technological solutions in the field of electrochemistry.

Metal hydrides are feasible for energy storage applications as they are able to decompose with hydrogen gas release. In this work, gaseous complex sodium hydrides, NaMH 4 and Na 2 MH 5 (M = B or Al), have been investigated using DFT/B3P86 and MP2 methods with 6-311++G(d,p) basis set; the optimized geometry, vibrational spectra and thermodynamic (TD) properties have been determined. Based on TD approach, a stability of the hydrides to different dissociation channels is analysed; the enthalpies of formation ∆ f H °(0) of gaseous species have been obtained: − 1 ± 17 kJ mol −1 (NaBH 4 ), 91 ± 14 kJ mol −1 (NaAlH 4 ), − 13 ± 16 kJ mol −1 (Na 2 BH 5 ), and 71 ± 16 kJ mol −1 (Na 2 AlH 5 ). The complex hydrides are confirmed to produce gaseous products with hydrogen gas release at elevated temperature, whereas heterophase reactions, with NaH and B/Al products in condensed state, are predicted to occur spontaneously at lower temperature. Graphical abstract

This work aims at investigating the effect that the energy transferred during particle collisions in a milling process entails on solid-gas reactions. For this purpose, the synthesis of Mg(BH4)2 from MgB2 in a pressurized hydrogen atmosphere was chosen as a model reaction. MgB2 was milled under a broad set of milling parameters (i.e. milling times and rotation regimes) and the obtained product thoroughly characterized. By proving the partial formation of Mg(BH4)2, the results of this investigation indicate that the energy transferred to the powder bed by the powder particles during milling is not negligible, in particular when the milling process is protracted for a long period.

Hirshfeld atom refinement (HAR) is a method which enables the user to obtain more accurate positions of hydrogen atoms bonded to light chemical elements using X-ray data. When data quality permits, this method can be extended to hydrogen-bonded transition metals (TMs), as in hydride complexes. However, addressing hydrogen thermal motions with HAR, particularly in TM hydrides, presents a challenge. At the same time, proper description of thermal vibrations can be vital for determining hydrogen positions correctly. In this study, we employ tools such as SHADE3 and Normal Mode Refinement (NoMoRe) to estimate anisotropic displacement parameters (ADPs) for hydrogen atoms during HAR and IAM refinements performed for seven structures of TM (Fe, Ni, Cr, Nb, Rh and Os) and metalloid (Sb) hydride complexes for which both the neutron and the X-ray structures have been determined. A direct comparison between neutron and HAR/SHADE3/NoMoRe ADPs reveals that the similarity between neutron hydrogen ADPs and those estimated with NoMoRe or SHADE3 is significantly higher than when hydrogen ADPs are refined with HAR. Regarding TM—H bond lengths, traditional HAR exhibits a slight advantage over the other methods. However, combining NoMoRe/SHADE3 with HAR results in a minor decrease in agreement with neutron TM—H bond lengths. For the Cr complex, for which high-resolution X-ray data were collected, an investigation of resolution-related effects was possible.

The insatiable—and ever-growing—demand of both the developed and the developing countries for power continues to be met largely by the carbonaceous fuels comprising coal, and the hydrocarbons natural gas and liquid petroleum. We review the properties of the chemical elements, overlaid with trends in the periodic table, which can help explain the historical—and present—dominance of hydrocarbons as fuels for power generation. However, the continued use of hydrocarbons as fuel/power sources to meet our economic and social needs is now recognized as a major driver of dangerous global environmental changes, including climate change, acid deposition, urban smog and the release of many toxic materials. This has resulted in an unprecedented interest in and focus on alternative, renewable or sustainable energy sources. A major area of interest to emerge is in hydrogen energy as a sustainable vector for our future energy needs. In that vision, the issue of hydrogen storage is now a key challenge in support of hydrogen-fuelled transportation using fuel cells. The chemistry of hydrogen is itself beautifully diverse through a variety of different types of chemical interactions and bonds forming compounds with most other elements in the periodic table. In terms of their hydrogen storage and production properties, we outline various relationships among hydride compounds and materials of the chemical elements to provide some qualitative and quantitative insights. These encompass thermodynamic and polarizing strength properties to provide such background information. We provide an overview of the fundamental nature of hydrides particularly in relation to the key operating parameters of hydrogen gravimetric storage density and the desorption/operating temperature at which the requisite amount of hydrogen is released for use in the fuel cell. While we await the global transition to a completely renewable and sustainable future, it is also necessary to seek CO 2 mitigation technologies applied to the use of fossil fuels. We review recent advances in the strategy of using hydrocarbon fossil fuels themselves as compounds for the high capacity storage and production of hydrogen without any CO 2 emissions. Based on these advances, the world may end up with a hydrogen economy completely different from the one it had expected to develop; remarkably, with ‘Green hydrogen' being derived directly from the hydrogen-stripping of fossil fuels. This article is part of the theme issue ‘Mendeleev and the periodic table'.

LiAlH4 is a potential solid-state H2 storage material, where safe and efficient H2 storage is of critical importance for the transition towards a sustainable emission-free economy. To improve the H2 release and storage properties of LiAlH4, confinement in porous media decreases the temperature of H2 release and improves the kinetics, where considerably improved H2 release properties are accompanied by a loss in the total amount of H2 released. The capability of mesoporous carbon black to improve the H2 storage properties of confined LiAlH4 is investigated with temperature-programmed desorption and time-stability measurements using X-ray diffraction and N2 gas adsorption measurements to characterize the composite materials’ composition and structure. Here, we present the capability of commercial carbon black to effectively lower the onset temperature of H2 release to that of near-ambient, ≥295 K. In addition, the confinement in mesoporous carbon black destabilized LiAlH4 to a degree that during ≤14 days in storage, under Ar atmosphere and at ambient temperature, 40% of the theoretically contained H2 was lost due to decomposition. Thus, we present the possibility of destabilizing LiAlH4 to a very high degree and, thus, avoiding the melting step before H2 release at around 440 K using scaffold materials with fine-tuned porosities.

LiAlH4 is a potential solid-state H2 storage material, where safe and efficient H2 storage is of critical importance for the transition towards a sustainable emission-free economy. To improve the H2 release and storage properties of LiAlH4, confinement in porous media decreases the temperature of H2 release and improves the kinetics, where considerably improved H2 release properties are accompanied by a loss in the total amount of H2 released. The capability of mesoporous carbon black to improve the H2 storage properties of confined LiAlH4 is investigated with temperature-programmed desorption and time-stability measurements using X-ray diffraction and N2 gas adsorption measurements to characterize the composite materials’ composition and structure. Here, we present the capability of commercial carbon black to effectively lower the onset temperature of H2 release to that of near-ambient, ≥295 K. In addition, the confinement in mesoporous carbon black destabilized LiAlH4 to a degree that during ≤14 days in storage, under Ar atmosphere and at ambient temperature, 40% of the theoretically contained H2 was lost due to decomposition. Thus, we present the possibility of destabilizing LiAlH4 to a very high degree and, thus, avoiding the melting step before H2 release at around 440 K using scaffold materials with fine-tuned porosities.

Ammine metal borohydrides show large compositional and structural diversity, and have been proposed as candidates for solid-state ammonia and hydrogen storage as well as fast cationic conductors. Here, we report the synthesis method of ammine barium borohydrides, Ba(BH4)2·xNH3 (x = 1, 2). The two new compounds were investigated with time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction, thermal analysis, infrared spectroscopy and photographic analysis. The compound Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, and is isostructural to Sr(BH4)2·2NH3, forming octahedral [Ba(NH3)2(BH4)4] complexes, which are connected into a two-dimensional layered structure, where the layers are interconnected by dihydrogen bonds, N–Hδ+⋯−δH–B. A new structure type is observed for Ba(BH4)2·NH3, which crystallizes in an orthorhombic unit cell with space group symmetry P212121, forming a three-dimensional framework structure of [Ba(NH3)(BH4)6] complexes. The structure is built from distorted hexagonal chains, where NH3 groups form dihydrogen bonds to the nearby BH4−-groups within the chain. Ba(BH4)2·2NH3 is unstable at room temperature and releases NH3 in two subsequent endothermic reactions with maxima at 49 and 117 °C, eventually reforming Ba(BH4)2. We demonstrate that the thermal stability and composition of the gas release for the ammine alkaline earth metal borohydrides can be correlated to the charge density of the metal cation, but are also influenced by other effects.