Experimental Evidence of Ca[B₁₂H₁₂] Formation During Decomposition of a Ca(BH₄)₂ + MgH₂ Based Reactive Hydride Composite

The combination of Ca(BH 4 ) 2 and MgH 2 materials results in a composite with remarkable hydrogen storage properties. However, full reversibility upon the (re)hydrogenation reaction has not yet been achieved. The poor reversibility is shown to be linked to the formation of stable intermediate phases or side products upon decomposition. In this work, we show, for the first time, the clear experimental evidence of CaB 12 H 12 among the decomposition products of a Ca(BH 4 ) 2 + MgH 2 composite. A combination of 11 B Magic Angle Spinning–Nuclear Magnetic Resonance ( 11 B{ 1 H} MAS NMR), ex situ X-ray diffraction (XRD) and Rietveld analysis are presented. An assessment of the (de)hydrogenation and (re)hydrogenation reactions of Ca(BH 4 ) 2 + MgH 2 composite is reported. The experimental results provided in this work highlight the reasons for the limited reversibility observed in the Ca(BH 4 ) 2 + MgH 2 composite upon (re)hydrogenation.