Thermodynamic and kinetic properties of the LiBH4 – MgH2 and LiBH4 – MgH2 – LiAlH4 systems for solid state hydrogen storage

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 and the ternary LiAlH4-LiBH4-MgH2 composite 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 first system is well known in literature concerning the good reversibility of the sorption reaction and the high gravimetric capacity, while the second one has been explored in one only paper concerning its hydrogen sorption properties. For both the composites, no chemico-physical characterization has been made up to now. For the first system, 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. Concerning the second system, the sequence of phase transitions, melting and decomposition is richer and more complicated. The first hydrogen release is already at 165 °C (due to the alanate decomposition). MgH2 decomposes at 260 °C and LiBH4 at 320 °C, i.e. at temperature appreciably lower than in the binary system. Studies on the reversibility of the system are in progress.