Mechano-chemical activation of the (3LiBH4 + TiF3) system, its dehydrogenation behavior and the effects of ultrafine filamentary Ni and graphene additives

Abstract

The influence of milling energy input, Q_TR (kJ g⁻¹), during ball milling and additives such as ultrafine filamentary Ni and graphene (reduced graphene oxide), on the occurrence of the solid-state mechano-chemical reaction and resulting microstructure, were investigated for the (3LiBH₄ + TiF₃) system. The new phases LiF and Ti are observed after injecting the energy input Q_TR = 72.8 kJ g⁻¹ (1 h ball milling). A mechanical dehydrogenation phenomenon occurs during mechano-chemical reaction. The ultrafine filamentary Ni additive does not measurably accelerate the rate of mechanical dehydrogenation while the rate of mechanical dehydrogenation with graphene is initially slow and then dramatically increases up to 5 h ball milling (Q_TR = 364 kJ g⁻¹). Thermal desorption of ball milled samples occurs at a very low temperature of 60 °C. The addition of 5 wt% filamentary Ni mildly reduces the apparent average activation energy for desorption. The highest average apparent activation energy of 95.2 ± 1.9 kJ mol⁻¹ is exhibited by a sample with 5 wt% graphene milled for 1 h which dramatically decreases after 5 h ball milling. The X-ray diffraction intensity of the LiF and Ti peaks greatly increases after thermal dehydrogenation. The principal gas released during thermal dehydrogenation is hydrogen although the 1 h ball milled (Q_TR = 72.8 kJ g⁻¹) sample shows a very small quantity of diborane gas, B₂H₆, which ceased to be released after 5 h ball milling. It clearly shows that the release of B₂H₆ during thermal dehydrogenation depends on the quantity of milling (mechanical) energy injected into the powder mixture. Differential scanning calorimetry measurements show exothermic peaks for all samples regardless of the milling energy input. The ball milled samples release H₂ during long term storage at room temperature.