Mn-based Borohydride Synthesized by Ball-milling KBH4 and MnCl2 for Hydrogen Storage

In this work, a mixed-cation borohydride (K2Mn(BH4)4) with P21/n structure was successfully synthesized by mechanochemical milling of the 2KBH4–MnCl2 sample under argon. The structural and thermal decomposition properties of the borohydride compounds were investigated using XRD, Raman spectroscopy, FTIR, TGA-MS and DSC. Apart from K2Mn(BH4)4, the KMnCl3 and unreacted KBH4 compounds were present in the milled 2KBH4–MnCl2. The two mass loss regions were observed for the milled sample: one was from 100 to 160 °C with a 1.6 ± 0.1 wt% loss (a release of majority hydrogen and trace diborane), which was associated with the decomposition of K2Mn(BH4)4 to form KBH4, boron, and finely dispersed manganese; the other was from 165 to 260 °C with a 1.9 ± 0.1 wt% loss (only hydrogen release), which was due to the reaction of KBH4 with KMnCl3 to give KCl, boron, finely dispersed manganese. Simultaneously, the formed KCl could dissolve in KBH4 to yield a K(BH4)xCl1−x solid solution, and also react with KMnCl3 to form a new compound K4MnCl6.