Effect of Transition Metal Fluorides on the Sorption Properties and Reversible Formation of Ca(BH4)2

Light metal borohydrides are considered as promising materials for solid state hydrogen storage. Because of the high hydrogen content of 11.5 wt % and the rather low dehydrogenation enthalpy of 32 kJ mol −1 H 2, Ca(BH 4 ) 2 is considered to be one of the most interesting compounds in this class of materials. In the present work, the effect of selected TM-fluoride (TM = transition metal) additives on the reversible formation of Ca(BH 4 ) 2 was investigated by means of thermovolumetric, calorimetric, Fourier transform infrared spectroscopy, and ex situ, and in situ synchrotron radiation powder X-ray diffraction (SR-PXD) measurements. Furthermore, selected desorbed samples were analyzed by 11 B{ 1 H} solid state magic angle spinning nuclear magnetic resonance (MAS NMR). Under the conditions used in this study (145 bar H 2 pressure and 350 °C), TiF 4 and NbF 5 were the only additives causing partial reversibility. In these two cases, 11 B{ 1 H} MAS NMR analyses detected CaB 6 and likely CaB 12 H 12 in the dehydrogenation products. Elemental boron was found in the decomposition products of Ca(BH 4 ) 2 samples with VF 4, TiF 3, and VF 3 . The results indicate an important role of CaB 6 for the reversible formation of Ca(BH 4 ) 2 .