H2 sorption performance of NaBH4 – MgH2 composites prepared by mechanical activation

The current research on solid state hydrogen storage materials for on-board applications is focused on reactive hydrides composites (RHC), i.e. systems based on the improvement of the dehydrogenation thermodynamic of a complex hydride when one (generally the light hydride MgH2) or more hydrides take part to the reaction. The extent of the destabilization, as well as the sorption characteristics of the composites, strongly depends on the structural and nanostructural properties of the constituent hydrides, which are in turn affected by the preparation route. The aim of this work is to evaluate the influence of different mechanical activation conditions on the storage properties of NaBH4 –MgH2 composites, up to now scarcely explored in literature. The first results regard composites with 2:1 and 1:2 stoichiometry milled under different atmosphere (Ar or H2). X-ray powders diffraction analysis shows that milling does not lead to the formation of any new phase, but it reduces the average crystallite size of the powders down to nanometric scale. All the mixtures release an H2 amount close to the theoretical value expected for the full dissociation of both the hydrides and much higher than the target fixed by the US Department of Energy for on-board application. The thermal programmed desorption profiles of the mixtures clearly show two steps, with MgH2 dissociating first and with higher rate and NaBH4 gradually dehydrogenating at temperatures close to 400°C. Concerning the 2:1 stoichiometry, when the samples are processed under Ar the two dehydrogenation processes are characterized by a lower starting temperature but also by a lower average rate with respect to the sample milled in H2. The 1:2 sample milled under Ar shows the best kinetic performance. Unfortunately, also for this mixture more than 10 h are required to obtain full desorption at a temperature as high as 450°C.