Multicomponent LiBH4-LiAlH4-MgH2 hydrogen storage system: in-situ synchrotron radiation powder diffraction studies

In the last 20 years enormous efforts have been devoted to the research and development of materials that can hold sufficient hydrogen in terms of gravimetric and volumetrics densities, and, at the same time, with favourable thermodynamic and kinetic properties. In particular, the quest to find a high capacity reversible hydride has shifted the interest towards amide and borohydride compounds [1]. 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 do not meet the limits that are required for a practical, on board hydrogen carrier. Moreover, all of these materials are plagued by high kinetics barriers to dehydrogenations 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 species into the reaction to stabilize the product reaction [2]. In this study, the sorption properties of the LiAlH4-LiBH4-MgH2 system have been thoroughly investigated. The choice seemed obvious in the light of the fact that the LiAlH4-LiBH4-MgH2 system has a high gravimetric capacity, high volumetric hydrogen density and a rather low dehydrogenation temperature compared to the single compounds. The phase-structural transformations occurring during the desorption process on the powder mixture prepared by ball milling, are provided by in-situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD), coupled manometric – calorimetric analysis and MAS solid-state NMR. Interesting, at 262 °C, MgH2 desorbs hydrogen and reacts with Al to form the Mg17Al12 e Mg2Al3 phases. Moreover, the AlxMg1-xB2 phase is also detected at 400°C. Details of the microstructural parameters, obtained by diffraction patterns refinement according to Rietveld method, will allow to exhaustively explain the desorption mechanism.