Calcium Borohydride for Hydrogen Storage:  Catalysis and Reversibility

We demonstrate a new solid-state synthesis route to prepare calcium borohydride, Ca(BH4)2, by reacting a ball-milled mixture of CaB6 and CaH2 in a molar ratio of 1:2 at 700 bar of H2 pressure and 400−440 °C. Moreover, doping with catalysts was found to be crucial to enhance reaction kinetics. Thermogravimetric analysis and differential scanning calorimetry revealed a reversible low-temperature to high-temperature endothermic phase transition at 140 °C and another endothermic phase transition at 350−390 °C associated with hydrogen release upon formation of CaB6 and CaH2, as was evident from X-ray diffraction analysis. Thus, since Ca(BH4)2 here is shown to be prepared from its anticipated decomposition products, the conclusion is that it has potential to be utilized as a reversible hydrogen storage material. The theoretical reversible capacity was 9.6 wt % hydrogen.