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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.

Environmentally benign synthesis of graphite at low temperatures is a great challenge in the absence of transition metal catalysts. Herein, we report a green and efficient approach of synthesizing graphite from carbon dioxide at ultralow temperatures in the absence of transition metal catalysts. Carbon dioxide is converted into graphite submicroflakes in the seconds timescale via reacting with lithium aluminum hydride as the mixture of carbon dioxide and lithium aluminum hydride is heated to as low as 126 °C. Gas pressure-dependent kinetic barriers for synthesizing graphite is demonstrated to be the major reason for our synthesis of graphite without the graphitization process of amorphous carbon. When serving as lithium storage materials, graphite submicroflakes exhibit excellent rate capability and cycling performance with a reversible capacity of ~320 mAh g –1 after 1500 cycles at 1.0 A g –1 . This study provides an avenue to synthesize graphite from greenhouse gases at low temperatures. Green synthesis of graphite is a great challenge in the absence of the graphitization of amorphous carbon at high temperatures. Here, the authors report a green approach of synthesizing graphite from carbon dioxide at low temperature in seconds timescale.

A systematic structure search in the La–H and Y–H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH₈ a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen–Cooper–Schrieffer (BCS) arguments indicate they might be high-Tc superconductors. In particular, the superconducting transition temperature Tc calculated for LaH10 is 274–286 K at 210 GPa. Similar calculations for the Y–H system predict stability of the sodalite-like fcc YH10 and a Tc above room temperature, reaching 305–326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

Magnesium hydride and selected magnesium-based ternary hydride (Mg2FeH6, Mg2NiH4, and Mg2CoH5) syntheses and modification methods, as well as the properties of the obtained materials, which are modified mostly by mechanical synthesis or milling, are reviewed in this work. The roles of selected additives (oxides, halides, and intermetallics), nanostructurization, polymorphic transformations, and cyclic stability are described. Despite the many years of investigations related to these hydrides and the significant number of different additives used, there are still many unknown factors that affect their hydrogen storage properties, reaction yield, and stability. The described compounds seem to be extremely interesting from a theoretical point of view. However, their practical application still remains debatable.

Storing hydrogen in metals has received much attention due to the advantages of this approach for safely storing. It is a promising method of storing hydrogen and eliminates the challenges associated with storing hydrogen gas at high pressure, which includes material durability, tank safety, and overall weight. Much work has been done for the past decade to bring this approach closer to wide‐scale application. However, much experimental research is needed to improve the volumetric and gravimetric capacity, hydrogen adsorption/desorption kinetics, material life cycle, and reaction thermodynamics of potential materials for hydrogen storage. Other important properties to consider are transient performance, the regeneration process of spent storage materials, effective adsorption temperature associated with activation energy, induced pore sizes in materials, increasing pore volume and surface area, and materials densification. In recent years, this solid‐state storage has progressed at conditions close to normal atmospheric pressure and temperature, with metal hydrides (MHs) emerging as a promising option. Their high storage density per unit volume, volume storage capabilities, and their ability to reverse the process while maintaining stability have qualified the MHs for low‐pressure storage and fulfilling the hydrogen storing requirements. However, understanding the principles of kinetics and thermodynamics is crucial for understanding the reactions of MHs as they absorb and release hydrogen. This review evaluates the current hydrogen storage methods, the different types of MHs, their thermodynamics and kinetics, as well as their applications and challenges. For the advancement of further research in this field of study, suggestions for future work and studies are also provided.

X-ray diffraction studies of rare-earth intermetallic Er sub(1 - x)Tb sub( x)Fe sub(2) compounds (with 0 less than or equal to x less than or equal to 0.6) and their hydrides were performed at room temperature. As the terbium content (x) in Er sub(1 - x)Tb sub(x)Fe sub(2) increases, the sign of the magnetostriction constant lambda sub(111) was shown to alternate from positive to negative. The lattice parameters, amount of incorporated hydrogen, and sizes of interstices were found to increase with increasing terbium content. The sizes of interstices accessible for occupation with hydrogen atoms were calculated for all alloys and their hydrides. Spontaneous anisotropic magnetostriction related to the rare-earth atoms was found to make no determining contribution to the resulting crystal lattice distortion caused by the incorporation of hydrogen atoms.

Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H₂ fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particleswam optimization structural search, we show that in the stoichiometry CaH₆ a body-centered cubic structure with hydrogen that forms unusual \"sodalite\" cages containing enclathrated Ca stabilizes above pressure 150 GPa. The stability of this structure is derived from the acceptance by two H₂ of electrons donated by Ca forming an \"H₄\"unit as the building block in the construction of the three-dimensional sodalite cage. This unique structure has a partial occupation of the degenerated orbitals at the zone center. The resultant dynamic Jahn-Teller effect helps to enhance electron-phonon coupling and leads to superconductivity of CaH₆. A superconducting critical temperature (Tc) of 220-235 K at 150 GPa obtained from the solution of the Eliashberg equations is the highest among all hydrides studied thus far.

Theoretical predictions and subsequent experimental observations of high-temperature superconductivity in dense hydrogen-rich compounds have reinvigorated the field of superconductivity. A systematic computational study of the hydrides of lanthanum and yttrium over a wide composition range reveals hydrogen-rich structures with intriguing electronic properties under pressure. Electron–phonon coupling calculations predict the existence of new superconducting phases, some exhibiting superconductivity in the range of room temperature. Moreover, the calculated stabilities indicate the materials could be synthesized at pressures that are currently accessible in the laboratory. The results open the prospect for the design, synthesis, and recovery of new high-temperature superconductors with potential practical applications. A systematic structure search in the La–H and Y–H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH 10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH 8 a C 2/ m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen–Cooper–Schrieffer (BCS) arguments indicate they might be high- T c superconductors. In particular, the superconducting transition temperature T c calculated for LaH 10 is 274–286 K at 210 GPa. Similar calculations for the Y–H system predict stability of the sodalite-like fcc YH 10 and a T c above room temperature, reaching 305–326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performance.

Renewable energy prices are decreasing, making it easier to make energy systems that are good for the environment. High-density storage for renewable energy is possible with hydrogen. This work focuses on the theoretical study of LiXH 3 (where X = Ti, Mn, and Cu), including their structural, electronic, mechanical, thermoelectric, and hydrogen storage properties, using first-principles calculations. LiCuH 3 is more stable than LiMnH 3 and LiTiH 3 , based on the optimization graph. The electronic properties show the metallic nature of these studied hydrides. Born’s criterion indicates that all studied hydrides are brittle for various mechanical applications. LiTiH 3 , LiMnH 3 , and LiCuH 3 are all thought to be able to store hydrogen with gravimetric storage capacities of 5.22%, 4.66%, and 4.11%, respectively. Based on how their thermoelectric properties change with temperature, all the materials under study can absorb heat energy, which shows that they are both electrically and thermally conductive.