Effects Of Rare Earth Fluorides On The Dehydrogenation Properties And Reversibility Of LiAlH₄

Complex hydrides are now popularly used as hydrogen storage materials due to their high hydrogen storage capacities ranging from 3.66wt% to 18.36wt%. To improve their poor hydrogen sorption properties, numerous approaches have been made by adding catalysts through ball milling in recent years. In the present work, effects of three rare earth metal fluorides REF3(RE=Ce, La, Y) on the dehydrogenation thermodynamic and kinetic properties of Li Al H4 are carefully investigated. In addition, due to the great catalytic effect on Li Al H4, Ce F3 is also added by two methods as a reactant in both 3Li Al H4/Ce F3 and 4Li Al H4/Ce F3 systems. The 3Li Al H4/Ce F3 prepared by mixing the reactants in THF solvent is also introduced for comparison. The hydrogen sorption behaviors especially the reversibility of Li Al H4 are deeply explored.In the study of adding REF3 as catalysts, the dehydrogenation behaviors were greatly altered by the addition of 5mol% REF3 and the destabilization ability of these catalysts on Li Al H4 has an order: Ce F3>La F3>YF3. For instance, the temperature programmed desorption(TPD) analyses show that the onset dehydrogenation temperature of Ce F3 doped Li Al H4 is sharply reduced by 90 oC compared to that of pristine Li Al H4. Based on differential scanning calorimetry(DSC) analyses, the dehydriding activation energies of the Ce F3 doped Li Al H4 sample are 40.9k J/mol H2 and 77.2k J/mol H2 for the first and second dehydrogenation stages, respectively, which decrease about 40.0k J/mol H2 and 60.3k J/mol H2 compared with those of pure Li Al H4. In addition, the sample doped with Ce F3 shows the fastest dehydrogenation rate among the REF3 doped Li Al H4 samples at both 125 oC and 150 oC during the isothermal desorption. The phase changes in REF3 doped Li Al H4 samples during ball milling and dehydrogenation are examined using X-ray diffraction and the mechanisms related to the catalytic effects of REF3 are proposed.In the systems where Ce F3 is introduced as a reactant, the three systems all dehydrogenate in two steps which are different from the three dehydrogenation steps of pure Li Al H4. Unlike many other known rare earth fluorides, Ce F3 reacts with Li Al H4 to form(Ce, Al) F3 phase during ball milling or mixing in the THF solvent and intermetallic phases such as Al3 Ce, Al4 Ce and Al11Ce3 during dehydrogenation. The 3Li Al H4/Ce F3 composite prepared by reactive ball milling boasts better hydrogen sorption properties. For instance, the onset and final dehydrogenation temperatures of Li Al H4 in the composite are respectively 120 oC and 110 oC lower than those of pure Li Al H4. At 150 oC under 4.5MPa hydrogen pressure, this composite shows reabsorption of 0.78wt% within 11 h. XRD results revealed that the Ce H2.51 and Al4 Ce formed during dehydrogenation may be the critical points for the significantly improved dehydrogenation properties. For the re-hydrogenation of the 3Li Al H4/Ce F3 composite, the Al-Ce-H composite formed by Al3 Ce may effectively lead to the further formation of Li3 Al H6 under a lower hydrogen pressure.