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.