Physico-chemical properties and sorption behaviour of the catalysed LiBH4 - MgH2 reactive hydride composite

In the last 20 years, enormous efforts have been devoted to the research and development of materials having, at the same time, high hydrogen gravimetric and volumetric capacities and favourable thermodynamic and kinetic sorption properties for practical applications as hydrogen tank. In particular, the quest to find a high capacity reversible hydride has shifted the interest towards alanate, amide and borohydride compounds. Currently, alkaline and alkaline-earth metal borohydrides are considered the most attractive materials for automotive applications. Despite a high gravimetric and volumetric capacity, the thermodynamics of the reversible dehydrogenation of many complex borohydrides doesn't meet the targets required for a practical on board hydrogen carrier. Moreover, all of these materials are plagued by high kinetics barriers to dehydrogenation and/or rehydrogenation in the solid state. Therefore, a valid approach was established in order to tune the dehydrogenation thermodynamic properties (i.e. reducing decomposition enthalpy) of the borohydrides, by incorporating a second or third specie into the reaction to stabilize the reaction products. In this study, the kinetic and thermodynamic properties of the sorption steps characterizing the binary LiBH4-MgH2 system have been investigated in detail by combined manometric – calorimetric measurements, in situ and ex situ X-Ray powder diffraction analyses and in-situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD). The system is well known in literature concerning the good reversibility of the sorption reaction and the high gravimetric capacity, but no chemico-physical characterization has been made up to now. By DSC measurements it has been shown that LiBH4 phase transition and melting take place (at 100 °C and 240 °C respectively) before any dehydrogenation step. Subsequently, MgH2 decomposes (at around 320 °C) and free Mg reacts with the borohydride to give MgB2 (380 °C). This last compound re-hydrogenates in one step giving, through the simultaneous reaction with LiH, the two starting hydrides. The total gravimetric capacity is 9 wt %. Absorption and desorption enthalpies and activation energies have been determined, together with the heat capacity and the thermal conductivity, fundamental data for the sketching and the realization of the hydrogen storage tank. The influence of the density of the samples on the hydrogen sorption properties and on the thermal conductivity has been evaluated too.