Abstract
Magnesium borohydride (Mg(BH4)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(BH4)2 ball-milled with CoCl2 and CoF2 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(BH4)2. By comparison with pure milled Mg(BH4)2, the temperature for the γ → ε phase transition in the samples with CoF2 or CoCl2 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 CoF2. Simultaneous X-ray absorption measurements at the Co K-edge revealed that the additives chemically transformed to other species. CoF2 slowly reacted upon heating till ~290 °C, whereas CoCl2 transformed drastically at ~180 °C.
Highlights
IntroductionMetal borohydrides (or tetrahydroborates) are complex hydrides containing BH4− anions counterbalanced by metal cations
Metal borohydrides are complex hydrides containing BH4− anions counterbalanced by metal cations
In our previous works [26,42] we have shown that Ni- and Co-based additives have an effect on the kinetics of hydrogen desorption and absorption in γ-Mg(BH4)2 [13]
Summary
Metal borohydrides (or tetrahydroborates) are complex hydrides containing BH4− anions counterbalanced by metal cations. Group I and II borohydrides (except Be) have pure ionic interactions between the cations and BH4− and strong B-H bonding, which renders the compounds stable, decomposing only above 200 °C with typically pure H2 release [1,2]. In transition metal (TM) or mixed cation borohydrides, on the contrary, the interaction between the cation(s) and H− of the BH4−. TM borohydrides can, in many cases, be obtained by reaction of the corresponding chloride with an alkali borohydride [5,8]. Due to their exceptionally high gravimetric hydrogen content LiBH4), metal borohydrides, both stable and unstable, have been extensively studied for hydrogen storage applications [2,9,10].
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