Metal Hydrides and Oxyhydrides for Energy-Related Applications

Dear Colleagues,

We are very glad to announce the Special Issue “Metal Hydrides and Oxyhydrides for Energy-Related Applications”, aimed at gathering the most recent advances in the field.

Metal hydrides have become widely studied as solid-state hydrogen-storage media due to their exceptionally high gravimetric and volumetric hydrogen densities, the latter often suppressing that of liquid hydrogen. At the same time, the volumetric energy density of a hydrogen medium can be higher than that of any other materials, including Li-batteries. Whereas particular hydride classes or compounds possess one or several of the key properties for a hydrogen storage material—and find niche applications—none of them has it all. Current research efforts in this field have thus been focused on tuning the kinetics, thermodynamics, and yield of hydrogen release/absorption reactions using a range of different approaches such as, for example, doping with potentially catalytic additives, controlling particle size through nanostructuring and scaffolding, and/or design of novel reactions with systematically selected combinations of reactants. At the same time, the vigorous search for a hydrogen-storage holy grail resulted in the discovery of a range of other fascinating properties, opening up the potential for applications within a wide range of solutions for alternative energy. The most important applications include batteries, systems for thermal energy storage and refrigeration, actuation, sensors and smart windows, semiconductors and, impressively, superconductivity.

The numerous potential applications described above are due, in part, to the astonishing chemical bonding flexibility of hydrogen. Hydrogen can be bound to other elements with ionic (H⁻, H⁺), covalent bonds (H^cov) located at interstitial sites as neutral atoms (H^int) or existing in intermediate states. The balance between the different contributions define the thermodynamic and electric properties of these compounds. Introduction of the ionic H⁻ into inorganic oxides or oxidation of metal hydrides can result in the formation of oxyhydrides.

Contrarily to hydrides, the field of metal oxyhydrides is rather unexplored, presenting a new horizon of challenges and opportunities. Introduction of H⁻ into an oxide can tune binding energy and electronic structure, altering the local symmetry and causing crystal field splitting, changing bonding nature, etc. This can induce unique structural and chemicophysical properties in the materials, including H⁻ conductivity, photochromism, photoelectrochemical water splitting, electrical conductivity, and catalytic properties. Around 50 oxyhydride compounds are currently known to possess unique and interesting properties compared to those of the corresponding oxides or hydrides.

A variety of traditional and new synthesis methods can be employed to synthesise hydrides and oxyhydrides in powder, thin film, and/or nanostructured form. Variation in synthesis conditions can significantly affect and tune the desired properties. The scrupulous characterisation of structure and dynamics of hydrides and oxyhydrides can enable knowledge-guided synthesis and property modifications and is thus vital. In addition, the life cycle assessment (LCA) of systems based on hydrides and oxyhydrides can shed light on the environmental and economic impact of these new technologies.

This Special Issue aims at summarising the recent advances in various aspects of the synthesis, characterisation, energy-related applications, and LCA of these applications of metal hydrides and oxyhydrides and the systems based on these compounds.

Prof. Dr. Olena Zavorotynska
Dr. Smagul Karazhanov
Guest Editors

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• metal hydrides
• hydrogen storage
• batteries
• hydrogen fuel
• ammonia fuel
• cation conductivity
• H− transport
• thermal energy storage
• superconductivity
• oxyhydrides
• electron conductivity
• mixed-anion compounds
• smart windows
• photoelectrochemical cells (PECs)
• photoanodes
• photocatalysts
• water splitting
• life cycle assessment
• recycling