Hydrogen Sorption Behavior of the ScH_2−LiBH_4 System: Experimental Assesment of Chemical Destabilization Effects

Abstract

The hydrogen storage reaction ScH_2 + 2LiBH_4 → ScB_2 + 2LiH + 4H_2 (8.91 wt %), based on the thermodynamic destabilization of LiBH_4, is predicted to have a reaction enthalpy of ΔH_(300K) = 34.1 kJ/mol H_2. The isothermal kinetic desorption behavior in this system was measured. At temperatures up to 450 °C, less than 5 wt % H_2 is released, which is only half of the theoretical capacity. Powder X-ray diffraction data indicate that LiBH_4 has decomposed into LiH in the final desorption product, but the data provide no evidence that ScH_2 has participated in the reaction. Magic angle spinning NMR (MAS NMR) results do not show that the expected ScB_2 equilibrium product phase has formed during desorption. While the addition of 2 mol % TiCl_3 catalyst does improve desorption kinetics at 280 °C, it does not otherwise assist the destabilization reaction. The calculated reaction enthalpy suggests that this system should be of interest at moderate temperatures, but the large heats of formation of the reactant phases in this system appear to play a critical role in determining overall kinetics. Furthermore, the formation of a Li_2B_(12)H_(12) intermediate phase was determined by MAS NMR, which is an undesirable stable product if reaction reversibility is to be accomplished.