New insights into the solid-state hydrogen storage of nanostructured LiBH4-MgH2 system

In this study, the nano-mixture of LiBH4 + MgH2 is prepared by ball milling (BM) of 1 mol MgH2 with in-situ aerosol-spraying (AS) of 1 mol of LiBH4 (called BMAS). It is shown, for the first time, that Mg(BH4)2 can be formed via the reaction between MgH2 and LiBH4 through the BMAS process and it contributes to H2 release at temperature ≤265 °C. Three parallel H2 release mechanisms have been identified from the BMAS powder. These include (i) H2 release from the decomposition of nano-LiBH4 and then Li2B12H12 decomposition product reacts with nano-MgH2 to release H2, (ii) H2 release from the decomposition of nano-Mg(BH4)2, and (iii) H2 release from the decomposition of nano-MgH2. Together, these three mechanisms result in 4.11 wt% H2 release in the solid-state at temperature ≤265 °C, which is among the highest quantities ever reported for LiBH4 + MgH2 mixtures to date. Furthermore, the H2 release temperature for each mechanism described above is lower than the corresponding temperature reported using other synthesis methods. In addition, the predicted property of a small amount of the Fe3B phase in the BMAS powder in absorbing more H2 than releasing is confirmed experimentally for the first time in this study. All these enhancements are achieved in the solid-state without any catalyst, which highlights the efficacy of mechanical activation and nanoengineering as well as the future opportunity to further improve the reversible hydrogen storage properties of LiBH4 + MgH2 in solid-state.