Combined X-ray and Raman Studies on the Effect of Cobalt Additives on the Decomposition of Magnesium Borohydride

Magnesium borohydride (Mg(BH 4 ) 2 ) is one of the most promising hydrogen storage materials. Its kinetics of hydrogen desorption, reversibility, and complex reaction pathways during decomposition and rehydrogenation, however, present a challenge, which has been often addressed by using transition metal compounds as additives. In this work the decomposition of Mg(BH 4 ) 2 ball-milled with CoCl 2 and CoF 2 additives, was studied by means of a combination of several in-situ techniques. Synchrotron X-ray diffraction and Raman spectroscopy were used to follow the phase transitions and decomposition of Mg(BH 4 ) 2 . By comparison with pure milled Mg(BH 4 ) 2 , the temperature for the γ → ε phase transition in the samples with CoF 2 or CoCl 2 additives was reduced by 10–45 °C. In-situ Raman measurements showed the formation of a decomposition phase with vibrations at 2513, 2411 and 766 cm −1 in the sample with CoF 2 . Simultaneous X-ray absorption measurements at the Co K-edge revealed that the additives chemically transformed to other species. CoF 2 slowly reacted upon heating till ~290 °C, whereas CoCl 2 transformed drastically at ~180 °C.