Transformation Kinetics of LiBH 4 –MgH 2 for Hydrogen Storage
The reactive hydride composite (RHC) LiBH 4 –MgH 2 is regarded as one of the most promising materials for hydrogen storage. Its extensive application is so far limited by its poor dehydrogenation kinetics, due to the hampered nucleation and growth process of MgB 2 . Nevertheless, the poor kinetics can be improved by additives. This work studied the growth process of MgB 2 with varying contents of 3TiCl 3 ·AlCl 3 as an additive, and combined kinetic measurements, X-ray diffraction (XRD), and advanced transmission electron microscopy (TEM) to develop a structural understanding. It was found that the formation of MgB 2 preferentially occurs on TiB 2 nanoparticles. The major reason for this is that the elastic strain energy density can be reduced to ~4.7 × 10 7 J/m 3 by creating an interface between MgB 2 and TiB 2 , as opposed to ~2.9 × 10 8 J/m 3 at the original interface between MgB 2 and Mg. The kinetics of the MgB 2 growth was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, describing the kinetics better than other kinetic models. It is suggested that the MgB 2 growth rate-controlling step is changed from interface- to diffusion-controlled when the nucleation center changes from Mg to TiB 2 . This transition is also reflected in the change of the MgB 2 morphology from bar- to platelet-like. Based on our observations, we suggest that an additive content between 2.5 and 5 mol% 3TiCl 3 ·AlCl 3 results in the best enhancement of the dehydrogenation kinetics.