Diverse hydrogen chemistry with perspectives for energy storage

The chemistry of hydrogen and its interaction with matter is remarkably diverse with new discoveries and materials continuously being uncovered. New types of chemical bonding and interactions allow for the preparation of new compounds with unusual compositions and properties. For instance, neutral hydrogen molecules may spontaneously form penta-dihydrogen clusters, (H₂)₅, in nanoporous materials with extremely dense packing, similar to metallic hydrogen at high pressure. Hydrides with extreme hydrogen densities - 'superhydrides' - have yielded record critical temperatures under pressure and now guide routes toward low-pressure high-temperature hydride superconductors. A well-known weak interaction identified in biological matter, the hydrogen bond, has an inorganic analogue: the dihydrogen bond. These two interactions have very similar bond lengths and bond strengths that are known to produce flexible and relatively open structures, which often have interesting functionalities. Recently, the di-hydrogen bond has come into focus for development of fast divalent magnesium and calcium cationic conductors. In this review, we highlight key advances in the synthesis and characterisation of novel hydrogen-based materials and illustrate how the compositional and structural versatility of hydrides leads to new functionalities. Hydrides are highly relevant materials with a diversity of energy applications such as solid-state hydrogen storage, solid-state batteries and superconductors, as well as future global hydrogen transportation.