Chemico-physical characterization and sorption properties of reactive hydride composites for hydrogen storage

Borohydrides are promising materials for safe and efficient solid state hydrogen storage applications, due to their high theoretical hydrogen content. For this reason, they are intensely studied all over the world, in order to solve their drawbacks, which still hind their use, especially for on-board applications. In particular, operating temperatures, kinetics and reactions reversibility must be improved to make borohydride-based storage systems compatible with fuel cells technology. The addition of a second and/or a third hydride to these compounds, in the so-called reactive hydride composite (RHC) strategy, seems to help in both improving the reversibility of the systems and the sorption kinetics as well as decreasing the working sorption temperatures. In this view, the activity of our Hydrogen Lab in this period is focused on the preparation, characterization and optimization of binary and ternary borohydrides based RHC. In this work, we present our synthetic and characterization activities and we will focus in particular on two systems, i.e. the catalysed LiBH4-MgH2 and the ternary LiAlH4-LiBH4-MgH2 composite. These systems have been investigated in detail by combined manometric – calorimetric measurements, in-situ and ex-situ X-Ray Powder Diffraction analysis and in-situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD). Moreover, for the former system, combined manometric – optical microscope measurements have been performed on compacted samples to study the morphological evolution of the pellets surface upon heating, for the first time in literature. The former system is well known concerning the good reversibility of the sorption reactions and the high gravimetric capacity, while the latter 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 former system, 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 a hydrogen storage tank. The already described sorption mechanism has been proved by in situ optical microscope investigations. The influence of the samples density on the sorption kinetics 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. Studies on the sorption reaction mechanism and on the reversibility of the system are in progress.