Mechano-chemical synthesis of manganese borohydride (Mn(BH_4)_2) and inverse cubic spinel (Li_2MnCl_4) in the (nLiBH_4 + MnCl_2) (n = 1, 2, 3, 5, 9 and 23) mixtures and their dehydrogenation behavior

摘要

Manganese borohydride (Mn(BH_4)_2) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH_4) and manganese chloride (MnCl_2 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 (nLiBH_4 + MnCl_2) mixture was investigated. During ball milling for 30 min the mixtures release only a very small quantity of H_2 that increases with the molar ratio n but does not exceed ～0.2 wt.% for n = 23. However, longer milling duration leads to more H_2 released. For the equimolar ratio n = 1 the principal phases synthesized are Li_2MnCl_4, an inverse cubic spinel phase, and the Mn(BH_4)_2 borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH_4)_2. With the n increasing from 3 to 23 LiBH_4 is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH_4)_2. Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 ℃ shows the release of hydrogen as a principal gas with a maximum intensity around 130-150 ℃ accompanied by a miniscule quantity of borane B_2H_6. The intensity of the B_2H_6 peak is 200-600 times smaller than the intensity of the corresponding H_2 peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH_4)_2 up to about 270-280 ℃. At 100 ℃ under 1 bar H_2 pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly ～4 wt.% H_2 which is a very large amount of H_2 considering that the mixture also contains 2 mol of LiCl salt. The H_2 quantities experimentally desorbed at 100 and 200 ℃ do not exceed the maximum theoretical quantities of H_2 expected to be desorbed from Mn(BH_4)_2 for various molar ratios n. It clearly confirms that the contribution from B_2H_6 evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100-200 ℃. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant κ and the lowest apparent activation energy for dehydrogenation, E_A ～ 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 H_2 during storage at room temperature and 40 ℃. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m~2/g substantially enhances the rate of discharge at 40 ℃.