Transformation Kinetics of LiBH4-MgH2 for Hydrogen Storage

The reactive hydride composite (RHC) LiBH4-MgH2 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 MgB2. Nevertheless, the poor kinetics can be improved by additives. This work studied the growth process of MgB2 with varying contents of 3TiCl(3)center dot AlCl3 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 MgB2 preferentially occurs on TiB2 nanoparticles. The major reason for this is that the elastic strain energy density can be reduced to similar to 4.7 x 10(7) J/m(3) by creating an interface between MgB2 and TiB2, as opposed to similar to 2.9 x 10(8) J/m(3) at the original interface between MgB2 and Mg. The kinetics of the MgB2 growth was modeled by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, describing the kinetics better than other kinetic models. It is suggested that the MgB2 growth rate-controlling step is changed from interface- to diffusion-controlled when the nucleation center changes from Mg to TiB2. This transition is also reflected in the change of the MgB2 morphology from bar- to platelet-like. Based on our observations, we suggest that an additive content between 2.5 and 5 mol% 3TiCl(3)center dot AlCl3 results in the best enhancement of the dehydrogenation kinetics.

Automated summary

The addition of 3TiCl(3)center dot AlCl3 is found to improve the dehydrogenation kinetics of the reactive hydride composite LiBH4-MgH2. The growth process of MgB2 is studied, and it is observed that MgB2 prefers to form on TiB2 nanoparticles due to the creation of an interface that reduces the elastic strain energy density. The kinetics of MgB2 growth is described by the JMAK equation, and it is suggested that the nucleation center change from Mg to TiB2 leads to a transition from interface- to diffusion-controlled growth, resulting in a change in MgB2 morphology.