Studies of the effects of TiCl3 in LiBH4/CaH2/TiCl3 reversible hydrogen storage system

In the present study, the effects of TiCl3 on desorption kinetics, absorption/desorption reversibility, and related phase transformation processes in LiBH4/CaH2/TiCl3 hydrogen storage system was studied systematically by varying its concentration (x = 0, 0.05, 0.15 and 0.25). The results show that LiCl forms during ball milling of 6LiBH4/CaH2/xTiCl3 and that as temperature increases, o-LiBH4 transforms into h-LiBH4, into which LiCl incorporates, forming solid solution of LiBH4·LiCl, which melts above 280 °C. Molten LiBH4·LiCl is more viscous than molten LiBH4, preventing the clustering of LiBH4 and the accompanied agglomeration of CaH2, and thus preserving the nano-sized phase arrangement formed during ball milling. Above 350 °C, the molten solution LiBH4·LiCl further reacts with CaH2, precipitating LiCl. The main hydrogen desorption reaction is between molten LiBH4·LiCl and CaH2 and not between molten LiBH4 and CaH2. This alters the hydrogen reaction thermodynamics and lowers the hydrogen desorption temperature. In addition, the solid–liquid nano-sized phase arrangement in the nano-composites improves the hydrogen reaction kinetics. The reversible incorporation/precipitation of LiCl at the hydrogen reaction temperature and during temperature cycling makes the 6LiBH4/CaH2/0.25TiCl3 nano-composite a fully reversible hydrogen storage material. These four states of LiCl in LiBH4/CaH2/TiCl3 system, i.e. “formed-solid solution-molten solution-precipitation”, are reported for the first time and the detailed study of this system is beneficial to further improve hydrogen storage property of complex hydrides.

► We systematically studied the effects of TiCl3 in LiBH4/CaH2/TiCl3 hydrogen storage system.
► It is found that adding 0.25 TiCl3 produces fully reversible hydrogen absorption and desorption and a lower desorption temperature.
► LiCl experiences four different states, i.e. “formed-solid solution-molten solution-precipitation”, in the whole desorption process of the system.
► The incorporation of LiCl into LiBH4 forms more viscous molten LiBH4·LiCl, leading to fast kinetics.
► The precipitation and re-incorporation of LiCl into LiBH4 lead to a fully reversible complex hydrogen storage system.