Developing sustainable means of energy-transport and storage is globally recognized as the principal challenge of the 21st century. One of the bottle-necks on the way towards a “green” society concerns the automobile. Hydrogen is a prime energy vector hosting many advantages and a efficient and safe storage is required to use it in fuel-cell powered automobiles. Today, the leading car manufacturers focus on on-board storage where hydrogen is stored in the gas phase at 700 bar in large volume high pressure tanks, mainly due to the fast refueling times (3 min) and the fact the it presents quite a mature technology. Hydrogen storage in the solid state would present both a more efficient and safer means of storing the gas.
The exploration of solid state materials that allow to absorb and desorb hydrogen reversibly has been going on for many decades. The Laboratory of Crystallography has traditionally been a metal-hydrides research lab and heavily involved in the characterization of intermetallic compounds, theirs structure and hydrogen sorption properties. Thanks to their favorable sorption kinetics, different intermetallics are being employed as stationary hydrogen reservoirs in large-scale tanks (e.g. LaNi5). Their low gravimetric capacity however renders them useless for on-board applications in vehicles. Metal borohydrides, on the other hand, show exceptionally large hydrogen contents, both concerning the hydrogen per mass and volume unit.
Since the first report in 2001 on LiBH4 as a solid state hydrogen storage material many different approaches have been discovered to tailor hydrogen sorption in different complex metal hydrides. Currently the main efforts focus on improving the slow sorption kinetics and reversibility, which is opposed to metal-hydrides present a major hurdle still unsolved. The decomposition of metal borohydrides is related to the electronegativity (EN) of the metal cation. Thermolysis (hydrogen release) is promoted by charge-transfer from the electron-deficient BH4 group to the metal. The combination of different EN in mixed-metal borohydrides can thus be considered a means of controlling the hydrogen release temperature. Metal boroyhdrides are used also as host for proton-hydride interactions in anion-mixed materials, where the BH4 group is substituted for by nitrogen bearing molecules such as the imide and amide anions or the ammonium cation. Due to the difference in electronegativity of the element covalently bound to hydrogen, the hydrogen carries partial negative (boron) and positive (nitrogen) charges, which create a chemical predisposition to eliminate the H2 molecule which can be made use of structurally by creating close heteropolar dihydrogen contacts δ+…δ–.