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.