Synthesis, characterisation and decomposition properties of manganese-based borohydrides for hydrogen storage

Abstract

In this work, a series of Mn-based borohydrides were synthesized by mechanochemical milling of ABH$_4$-MnCl$_2$ (where A=Li, Na, K) under argon. Samples prepared by milling xLiBH$_4$-MnCl$_2$ (where x = 2 or 3) resulted in the formation of Mn(BH$_4$)$_2$ and LiCl in a stoichiometric ratio. Thermal decomposition of Mn(BH$_4$)$_2$ was observed between 105 and 180°C with the concurrent evolution of hydrogen and diborane. Analysis of the decomposition products suggested the presence of amorphous B, Mn and LiCl. When NaBH$_4$ was used as a reagent, Mn(BH$_4$)$_2$ was also observed, however instead of NaCl, a solid solution of NaCl$_x$(BH$_4$)$_1$$_-$$_x$ and Na$_6$MnCl$_8$ were formed. Mn(BH$_4$)$_2$ decomposed between 110 and 170°C for xNaBH$_4$-MnCl$_2$ with the concurrent evolution of hydrogen and diborane. Analysis of the decomposed samples showed that amorphous B and Mn formed (110 to 170°C) and NaCl$_x$(BH$_4$)$_1$$_-$$_x$ reacted with Na$6$MnCl$_8$ to produce NaCl (250 to 475°C). When KBH$_4$ was used as a reagent, Mn(BH$_4$)$_2$ was not observed. Rather, K$_2$Mn(BH$_4$)$_4$, KMnCl$_3$ and unreacted KBH$_4$ were observed. The mass losses for the milled sample were observed between 100 to 160°C with a release of hydrogen and trace diborane, forming KBH$_4$, B, and Mn, and between 165 to 260°C with only hydrogen release, giving KCl, B and Mn