Researches On The Hydrogen Storage Performance Of LiBH₄-based Composites With Fluorides

4,which possesses high gravimetric and volumetric hydrogen densities (18.4 wt%and 121 kg-H2-m-3), has been attracting great interest as a promising hydrogen storage material. However, the stable thermodynamics and sluggish reaction kinetics have limited its practical use. In this paper, LiBH4-based composites with fluorides 6LiBH4+CaX2,2LiBH4+MgX2 (X=F, H) andχLiBH4+AlF3 (χ=l-5) composites were developed and studied in order to improve hydrogen storage properties of LiBHt-based composites.A new reversible hydrogen storage composite based on 6LiBH4+CaF2 was developed. It has a theoretical hydrogen capacity of 9.6 wt%(0.09 g-ml-1). The first dehydrogenation reached a capacity of 9.3 wt%, which is quite consistent with the theoretical value. The dehydrogenation of 6LiBH4+CaF2 yielded LiH2/3F1/3and CaB6-The composite revealed good reversibility of hydrogen storage regardless of hydrogen back pressure applied during the dehydrogenation. Catalysts like TiF3, CeF3, and NbCis were found to decrease the dehydrogenation temperature(around 50℃) and enhance the dehydriding kinetics.A comparative study was then performed on four LiBH4-based hydrogen storage composites 2LiBH4+MgX2 and 6LiBH4+CaX2 (X=H, F). The dehydrogenation of 2LiBH4+MgF2 demonstrated a strong dependence on the hydrogen back pressure, similar to that of 2LiBH4+MgH2. The reversible hydrogen storage of 2LiBH4+MgF2 was achieved under a back pressure of 0.5MPa at 450℃. Dehydrogenation under lower H2 pressures resulted in the production of Mg and thus a partial reversibility. A sufficiently high H2 back pressure was required to suppress the separate decomposition of LiBH4 and promote the formation of MgB2-In contrast, both 6LiBH4+CaH2 and 6LiBH4+CaF2 revealed reversible hydrogen storage properties regardless of the hydrogen back pressure. The formation conditions and structural difference between MgB2 and CaB6 may account for the observed differences in hydrogen storage properties of the Mg-and Ca-containing LiBH4 reactive composites.The study onχL1BH4+AIF3 (χ=l-5) demonstrated that the addition of AIF3 could decrease the dehydrogenation temperature effectively.χLiBH4+AIF3 began to desorb hydrogen at 80℃, which is 220℃lower than that of LiBH4. The XRD results indicated that the intermediate Li3ALF6 was formed during dehydrogenation.The formation of AIB2 was influenced mainly by dehydrogenation temperature. However, 3LiBH4+AlF3 had a poor reversibility in our experiment, maybe because only a small amount of AIB2 was produced in the reaction.