The effect of milling energy input during mechano-chemical activation synthesis (MCAS) of the nanocrystalline manganese borohydride (Mn(BH4)2) on its thermal dehydrogenation properties

•A nanocrystalline Mn(BH4)2 hydride was synthesized by MCAS.•Its crystallite (grain) size decreases with increasing milling energy input.•Mn(BH4)2 decomposes in endothermic reaction releasing H2 and forming amorphous Mn and B.•Mn(BH4)2 (with LiCl) is capable of desorbing up to ∼ 4.5 wt.% at 100 °C.•Activation energy closely follows the average powder particle size variations.

A manganese borohydride, Mn(BH4)2, co-existing with a nanocrystalline LiCl salt, which is a reaction “dead-weight” byproduct, was successfully synthesized by the mechano-chemical activation synthesis (MCAS) during ball milling the (nLiBH4 + MnCl2) mixtures having the molar ratios n = 2 and 3, using the total milling energy input, QTR, from 36.4 to 364 kJ/g. The crystallite (grain) size of the synthesized nanocrystalline Mn(BH4)2 hydride attains 21 ± 5.0 nm for the energy input QTR = 36.4 kJ/g and then it is further reduced to 18 ± 1.0 nm for QTR = 145.6 kJ/g and finally to 14 ± 0.5 nm for QTR = 364 kJ/g. The crystallite (grain) size of LiCl is very close to 30 nm regardless of the milling energy input, QTR. During continuous heating in a Differential Scanning Calorimeter (DSC), Mn(BH4)2 decomposes in endothermic reaction releasing H2 and forming amorphous Mn and B in the process. The synthesized nanocrystalline Mn(BH4)2 hydride, co-existing with a nanocrystalline LiCl salt, is capable of desorbing up to ∼ 4.5 wt.% at 100 °C. The values of the apparent activation energy for dehydrogenation obtained in the present work are very low. The apparent activation energy for the n = 3 nanocomposite decreases monotonically from ∼70 to ∼59 kJ/mol with increasing milling energy input whereas the apparent activation energy for the n = 2 nanocomposite decreases from about 65 kJ/mol for QTR = 36.4 kJ/g to about 53 kJ/mol for QTR = 145.6 kJ/g and then again increases to ∼59 kJ/mol for the QTR = 364 kJ/g. These changes closely follow the variations in the average powder particle size obtained with the varying milling energy input. For the milling energy input QTR = 36.4 and 145.6 kJ/g the average powder particle size decreases to 14.9 ± 6.6 and 7.5 ± 2.6 μm, respectively, and subsequently increases reaching the average size of 16.1 ± 6.3 μm for the milling energy input QTR = 364 kJ/g. On the other hand, the apparent activation energy for dehydrogenation doesn't depend on the average crystallite (grain) size. The amorphous Mn and B elements are also formed after isothermal dehydrogenation. The synthesized Mn(BH4)2 hydride is very stable and doesn't excessively release H2 during a long-term storage at room temperature for over 120 days under a slight overpressure of argon.