Abstract
The hampered kinetics of reactive hydride composites (RHCs) in hydrogen storage and release, which limits their use for extensive applications in hydrogen storage S1and energy conversion, can be improved using additives. However, the mechanism of the kinetic restriction and the additive effect on promoting the kinetics have remained unclear. These uncertainties are addressed by utilizing versatile transmission electron microscopy (TEM) on the LiBH4-MgH2 composite under the influence of the 3TiCl3·AlCl3 additives. The formation of the MgB2 phase, as the rate-limiting step, is emphatically studied. According to the observations, the heterogeneous nucleation of MgB2 relies on different nucleation centers (Mg or TiB2 and AlB2). The varied nucleation and growth of MgB2 are related to the in-plane strain energy density at the interface, resulting from the atomic misfit between MgB2 and its nucleation centers. This leads to distinct MgB2 morphologies (bars and platelets) and different performances in the dehydrogenation kinetics of LiBH4-MgH2. It was found that the formation of numerous MgB2 platelets is regarded as the origin of the kinetic improvement. Therefore, to promote dehydrogenation kinetics in comparable RHC systems for hydrogen storage, it is suggested to select additives delivering a small atomic misfit.