Formation of CaB6 in the thermal decomposition of the hydrogen storage material Ca(BH4)2

Abstract

Using a combination of high resolution X-ray powder diffraction and X-ray Raman scattering spectroscopy at the B K- and Ca L_2,3-edges, we analyzed the reaction products of Ca(BH₄)₂ after annealing at 350 °C and 400 °C under vacuum conditions. We observed the formation of nanocrystalline/amorphous CaB₆ mainly and found only small contributions from amorphous B for annealing times larger than 2 h. For short annealing times of 0.5 h at 400 °C we observed neither CaB₁₂H₁₂ nor CaB₆. The results indicate a reaction pathway in which Ca(BH₄)₂ decomposes to B and CaH₂ and finally reacts to form CaB₆. These findings confirm the potential of using Ca(BH₄)₂ as a hydrogen storage medium and imply the desired cycling capabilities for achieving high-density hydrogen storage materials.