Ca(BH₄)₂ + MgH₂: Desorption Reaction and Role of Mg on Its Reversibility

The Ca(BH 4 ) 2 –MgH 2 composite system represents a promising candidate for mobile hydrogen storage due to a 10.5 wt % theoretical hydrogen storage capacity and an estimated equilibrium temperature lower than 160 °C. For this system, the reversibility was achieved without further addition of additives. In this study, the decomposition path of the Ca(BH 4 ) 2 + MgH 2 composite system is investigated in detail by in situ synchrotron radiation powder X-ray diffraction and differential scanning calorimetry combined with thermogravimetry. The sorption properties are analyzed by volumetric measurements. 11 B{ 1 H} solid state magic angle spinning–nuclear magnetic resonance was employed for the characterization of the final amorphous or nanocrystalline boron-based decomposition products. This study shows that the intermediate formation of Ca 4 Mg 3 H 14 upon dehydrogenation of the Ca(BH 4 ) 2 –MgH 2 composite system is not a necessary step, and its presence can be adjusted modifying the preparation procedure. Moreover, the d -value mismatch calculated for the {111}CaB 6 /{1011}Mg plane pair is the lowest among the other plane pairs considered in the system. The mismatch in the third direction between CaB 6 and Mg is also extremely good. These findings propose Mg as a supporter of the heterogeneous nucleation of CaB 6 during decomposition of the Ca(BH 4 ) 2 + MgH 2 composite system.