Decomposition behaviour of Mn(BH4)2 formed by ball-milling LiBH4 and MnCl2

Manganese borohydride-based materials are of interest for hydrogen storage, as the Mn(BH4)2 compound has a theoretical hydrogen capacity of 9.53 wt.%. However, more needs to be known about the decomposition behaviour, in order to try to introduce reversibility. In this work, samples were prepared by ball-milling xLiBH4 and MnCl2 (where x = 2 or 3). The Mn(BH4)2 phase had XRD reflections consistent with the P3112 structure proposed by Černý et al., and Raman spectroscopy indicted that the [BH4]− units were in an ionic tetrahedral configuration. The 3LiBH4 + MnCl2 sample contained excess LiBH4 which exhibited an orthorhombic to hexagonal phase change at 93 °C indicating partial substitution of Cl− for [BH4]−. Thermal decomposition of Mn(BH4)2 occurred between 130 and 181 °C for the 2LiBH4 + MnCl2 sample and 105–145 °C for 3LiBH4 + MnCl2, with the concurrent evolution of hydrogen and diborane. Analysis of the decomposition products shows the formation of amorphous boron, and the probable formation of manganese metal (deduced from the presence of manganese oxide in a sample subsequently exposed to air).

► In this study Mn(BH4)2 was prepared by ball milling LiBH4 + MnCl2.
► DSC and TGA-MS showed that thermal decomposition started at 130 °C and finished at 180 °C with a total mass loss of a 8.5 wt.%.
► Concurrent desorption of hydrogen and diborane (approximately 9:1) was observed.
► The presence of excess LiBH4 lowered the decomposition by about 25 °C, to 105–145 °C.
► Ex situ FTIR on decomposed samples deliberately exposed to air show the formation of manganese oxide, indicating that finely dispersed manganese may be formed as a decomposition product. The presence of boron bonding from an allotrope of boron was also observed.