A future hydrogen economy requires dense, safe, efficient and reversible hydrogen storage materials. Hydrogen as an energy carrier is difficult to store because of the low critical temperature of 33K, i.e. hydrogen is a gas at room temperature. For mobile and in many cases also for stationary applications the volumetric and gravimetric density of hydrogen in a storage material is crucial. Hydrogen can be stored by six different methods and phenomena: high pressure gas cylinders (up to 800 bar), liquid hydrogen in cryogenic tanks (at 21 K), adsorbed hydrogen on materials with a large specific surface area (at T< 100 K), absorbed on interstitial sites in a host metal (at ambient pressure and temperature), chemically bond in covalent and ionic compounds (at ambient pressure), or through oxidation of reactive metals e.g. Li, Na, Mg, Al, Zn with water.
Metal hydrides and complex hydrides exhibit a great volumetric hydrogen density up to 150 kg m-3, which corresponds to more than twice the hydrogen density of liquid hydrogen. Metal hydrides host the hydrogen atoms on interstitial sites. Physically, the hydrogen is described as a lattice gas in the intercalation compound. However, metal hydrides only remain metallic upon hydrogen absorption when the compound consists of a significant amount of transition metals. Therefore, the gravimetric hydrogen density in metallic hydrides is limited to less than 3 mass%. The p-element complex hydrides e.g. M[AlH4]x and M [BH4]x exhibit a gravimetric hydrogen density of up to 20 mass%. Although many such compounds are well known for there chemical behavior very little is known about there physical properties, e.g. structure, thermodynamic stability, electronic structure and hydrogen diffusion.
In this project we propose to investigate the structure and the thermodynamic parameters of a series of pelement complex hydrides boranates (e.g. LiBH4, NaBH4, KBH4, Mg(BH4)2, Ca(BH4)2 and Ti(BH4)3.
Furthermore, the hydrogen sorption mechanism will be investigated. We will combine experimental investigations with theoretical modelling in order to understand the local environment of the hydrogen in the lattice (type of bonding of hydrogen) and to develop a model to describe the stability of the hydrides. The main results will be: 1) The structure and the electronic charge density of the p-element complex hydrides, 2) the stability (Van’t Hoff plot) of the complexes, 3) the hydrogen sorption kinetics (activation energy) 4) the hydrogen sorption mechanism in complex hydrides.
In order to achieve the results we will perform neutron diffraction measurements at SINQ (accepted proposal), X-ray diffraction measurements at SLS (submitted proposal), differential-scanning calorimetry under hydrogen pressure, thermal desorption spectroscopy and X-ray photoelectron spectroscopy. The experimental results will be compared with the theoretical ab-initio calculation on the relaxed structures.
The p-element complex hydrides represent a new and very promising class of hydrides with a gravimetric hydrogen density one order of magnitude greater than what we know from metallic hydrides. The knowledge of the properties of the complex hydrides and the understanding of the hydrogen sorption mechanism will greatly improve the scientific basis of hydrogen storage in solids.