Mechano-chemical synthesis of manganese borohydride (Mn(BH4)2) and inverse cubic spinel (Li2MnCl4) in the (nLiBH4 + MnCl2) (n = 1, 2, 3, 5, 9 and 23) mixtures and their dehydrogenation behavior

Abstract

Manganese borohydride (Mn(BH4)2) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH4) and manganese chloride (MnCl2) by applying high energy ball milling for 30 min. For the first time a wide range of molar ratios n = 1, 2, 3, 5, 9 and 23 in the (nLiBH4 + MnCl2) mixture was investigated. During ball milling for 30 min the mixtures release only a very small quantity of H2 that increases with the molar ratio n but does not exceed ∼0.2 wt.% for n = 23. However, longer milling duration leads to more H2 released. For the equimolar ratio n = 1 the principal phases synthesized are Li2MnCl4, an inverse cubic spinel phase, and the Mn(BH4)2 borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH4)2. With the n increasing from 3 to 23 LiBH4 is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH4)2. Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 °C shows the release of hydrogen as a principal gas with a maximum intensity around 130–150 °C accompanied by a miniscule quantity of borane B2H6. The intensity of the B2H6 peak is 200–600 times smaller than the intensity of the corresponding H2 peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH4)2 up to about 270–280 °C. At 100 °C under 1 bar H2 pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly ∼4 wt.% H2 which is a very large amount of H2 considering that the mixture also contains 2 mol of LiCl salt. The H2 quantities experimentally desorbed at 100 and 200 °C do not exceed the maximum theoretical quantities of H2 expected to be desorbed from Mn(BH4)2 for various molar ratios n. It clearly confirms that the contribution from B2H6 evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100–200 °C. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant k and the lowest apparent activation energy for dehydrogenation, EA ∼ 102 kJ/mol. Decreasing or increasing the molar ratio n below or above 3 increases the apparent activation energy. Ball milled mixtures with the molar ratio n = 2 and 3 discharge slowly H2 during storage at room temperature and 40 °C. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m2/g substantially enhances the rate of discharge at 40 °C.