Study On The Hydrogen Storage Properties Of2LiBH₄-Li₃AlH₆System

LiBH4, which possesses high gravimetric and volumetric hydrogen densities (18.5wt%and121kg·H2/m3), has been attracted great interest as a promising hydrogen storage material. However, the stable thermodynamics and sluggish reaction kinetics have limited its practical use. In this paper, Al and three metal complex hydrides (LiAIH4, NaAlH4, Li3AIH6) were selected to compound with LiBH4. The influence of different transition metal fluoride catalysts (CeF3, NiF2and TiF3) and varying contents of TiF3were experimented to improving the de/rehydrogenation performance of LiBH4. The X-Ray Diffraction (XRD), synchronized Differential Scanning Calorimetry/Thermogravimetry (DSC/TG), Fourier Transform Infrared Spectrum (FTIR), Scanning electron microscopy (SEM) and Temperature Programmed Desorption (TPD), were employed to characterize the phase compositions, decomposition pathway, microstructures and de/rehydrogenation properties of the2LiBH4-Li3AlH6system and catalytic modification of fluoride catalysts.A comparative study was performed on four LiBH4-based hydrogen storage composites2LiBH4+M (M=Al, LiAIH4, NaAlH4, Li3AlH6). The dehydrogenation of2LiBH4-Li3AlH6system demonstrated a strong performance to reduce the dehydrogenation temperature of LiBH4to320℃. The decomposition process of2LiBH4-Li3AlH6involves two steps:firstly, Li3AIH6decomposes into LiH, Al and releases hydrogen; secondly, LiBH4and Al reaction to produce LiH, AlB2and H2, meanwhile. LiBH4decomposes into LiH, B and H2. The circulation reversibility of the2LiBH4-Li3AlH6system is poor, probably is due to the gradually reducing of the content of AlB2.Li3AlH6composite samples doped with5wt%transition metal fluorides (CeF3, NiF2and TiF3) were prepared by ball milling, and the effects of fluoride catalysts on the hydrogen storage performance of the samples were comparatively investigated. It is found that the three fluorides can enhance the dehydriding kinetics of the2LiBH4-Li3AlH6destabilized system to some extent. The TiF3doped composite exhibits the most prominent behavior in terms of the low dehydrogenation temperature and fast dehydriding rate. The activation energy for the decomposition of LiBH4of the TiF3doped2LiBH4-Li3AlH6composite was calculated to be118.3kJ/mol. which is much lower than that of the undoped composite (197.6kJ/mol). In addition, the experimental results show that the reversibility of the2LiBH4-Li3AlH6composite is improved by the doping of TiF3. which plays a catalytic role, strengthens the interaction between LiBH4and Li3AlH6and thus further improving the de/rehydrogenation performance.The study on varying contents of TiF3doped2LiBH4-Li3AlH6composite demonstrated that, with the increasing content of TiF3, the dehydrogenation temperature of LiBH4is decreased effectively and the rate of decomposition is faster, but the actual dehydrogenation capacity is decreased. By comprehensive consideration of the dehydrogenation temperature, rate and capacity, we consider that the10wt%of TiF3doped2LiBH4-Li3AlH6is the most effective composite.