Superior catalytic effect of AlF3: Changing the dehydrogenation pathway and improving the hydrogen storage performance of 2LiBH4-MgH2 system

The 2LiBH 4 -MgH 2 composite has garnered extensive attention for industrial-scale hydrogen storage due to its excellent hydrogen capacity which has potential in addressing the increasing requirements for elevated energy density and extended cycle life. However, its practical application is still restricted by sluggish dehydrogenation kinetics and unsatisfactory thermodynamic stability. In this study, these limitations are addressed by introducing aluminum fluoride (AlF 3 ) as a catalyst, and the catalytic effect and mechanism are further investigated. The 2LiBH 4 -MgH 2 doped with 10% AlF 3 presents an extremely low onset dehydrogenation temperature of 181.8 °C. Specially, the nucleation period of MgB 2 is further decreased to 137 min, which is 49.91% shorter than that of the unmodified sample. In the case of dehydrogenation, it can release 6.33 wt% H 2 in 15 min at 400 °C. Kinetic analysis shows that the dehydrogenation activation energy is reduced to 174.93 kJ/mol, which is 65.65 kJ/mol lower than that of the ball-milled 2LiBH 4 -MgH 2 . Notably, in terms of hydrogen absorption, 9.38 wt% H 2 is absorbed within 34.2 min at 400 °C. When temperature reaching up to 350 °C, the composite can absorb 4.57 wt% H 2 within 1 h. Mechanistic analysis reveals that AlF 3 reacts with LiH and MgB 2 to generate LiF, MgF 2 , Li 3 AlF 6 and MgAlB 4 , and these products could alter the reaction pathway and further reduce the reaction energy barrier. Additionally, the reversible phase transformations of Al/Mg/Mg 17 Al 12 offer sufficient active sites during the re/dehydrogenation. Finally, MgF 2 and LiF serve as electron-shuttling media, thereby enhancing the hydrogen diffusion kinetics. This study not only proposes a viable method for improving the hydrogen storage behavior of 2LiBH 4 -MgH 2 , but also broadens the understanding of fluoride-assisted catalytic mechanisms.