Ball-milling and AlB2 addition effects on the hydrogen sorption properties of the CaH2 + MgB2 system

Among the borohydrides proposed for solid state hydrogen storage, Ca(BH4)2 is particularly interesting because of its favourable thermodynamics and relatively cheap price. Composite systems, where other species are present in addition to the borohydride, show some advantages in hydrogen sorption properties with respect to the borohydrides alone, despite a reduction of the theoretical storage capacity. We have investigated the milling time influence on the sorption properties of the CaH2 + MgB2 system from which Ca(BH4)2 and MgH2 can be synthesized by hydrogen absorption process. Manometric and calorimetric measurements showed better kinetics for long time milled samples. We found that the total substitution of MgB2 with AlB2 in the starting material can improve the sorption properties significantly, while the co-existence of both magnesium and aluminum borides in the starting mixture did not cause any improvement. Rietveld refinements of the X-ray powder diffraction spectra were used to confirm the hypothesized reactions.

Research highlights▶ Calcium hydride + magnesium-aluminum borides as candidates for hydrogen storage. ▶ Long time ball milling improves hydrogen sorption kinetics of the CaH2+MgB2 system. ▶ Coexistence of MgB2 and AlB2 does not improve hydrogen sorption performances. ▶ Total substitution of MgB2 with AlB2 improves the system kinetics and reversibility. ▶ Below 400 °C almost the full hydrogen capacity of the CaH2 + AlB2 system is reached.