Compaction of LiBH4-MgH2 doped with MWCNTs-TiO2 for reversible hydrogen storage

According to catalytic effects of TiO₂ on kinetic properties of hydrides and thermal conductivity of multiwall carbon nanotubes (MWCNTs) favoring heat transfer during de/rehydrogenation, improvement of dehydrogenation kinetics of compacted 2LiBH₄-MgH₂ by doping with MWCNTs decorated with TiO₂ (MWCNTs-TiO₂) is proposed. Via solution impregnation of Ti-isopropoxide on MWCNTs and hydrothermal reaction to produce TiO₂, high surface area and good dispersion of TiO₂ on MWCNTs surface are obtained. Composite of 2LiBH₄-MgH₂ is doped with 5–15 wt. % MWCNTs-TiO₂ and compacted into the pellet shape (diameter and thickness of 8 and 1.00–1.22 mm, respectively). By doping with 15 wt. % MWCNTs-TiO₂, not only fast dehydrogenation kinetics is obtained, but also reduction of onset dehydrogenation temperature (ΔT = 25 °C). Besides, gravimetric and volumetric hydrogen storage capacities of compacted 2LiBH₄-MgH₂ increase to 6.8 wt. % and 68 gH₂/L, respectively, by doping with 15 wt. % MWCNTs-TiO₂ (～twice as high as undoped sample). The more the MWCNTs-TiO₂ contents, the higher the apparent density (up to ～1.0 g/cm³ by doping with 15 wt. % MWCNTs-TiO₂). The latter implies good compaction, resulting in the development of volumetric hydrogen capacity. In the case of mechanical stability during cycling, compacted 2LiBH₄-MgH₂ doped with at least 10 wt. % MWCNTs-TiO₂ maintains the pellet shape after rehydrogenation. Although increase of porosity (up to 30%), leading to the reduction of thermal conductivity, is detected after rehydrogenation of compacted 2LiBH₄-MgH₂ doped with 15 wt. % MWCNTs-TiO₂, comparable kinetics during cycling is obtained. This benefit can be achieved from thermal conductivity of MWCNTs.