Ultrahigh reversible hydrogen capacity and synergetic mechanism of 2LiBH4-MgH2 system catalyzed by dual-metal fluoride

Developing convenient and applicable strategies to synthesize hydrogen storage composites with high capacity and favorable reversibility is vital in the field of novel energy materials. Herein, a system of 2LiBH4-MgH2 with K2TiF6 is synthesized, in which K2TiF6 can react with LiBH4 to form TiB2, LiF, KBH4. Such composite possesses low onset dehydrogenation temperatures, completely eliminated dehydrogenation induction period and fast kinetics with low activation energies of 100.3 kJ/mol. Considering that the reaction between K2TiF6 and LiBH4 can reduce the practical capacity, excess LiBH4 was added into the composite to offset the reduced dehydrogenation capacity. The cycling performances of the composite with excess LiBH4 are greatly improved. The composite with excess LiBH4 can completely absorb 9.4 wt% H2 at 200 °C, close to the practical operating temperatures of the fuel cell system on vehicle. This is exceedingly outstanding among the reversible properties of 2LiBH4-MgH2. Characterization analyses and theoretical calculations indicate the in situ formed TiB2, LiF, and KBH4 can perform stable synergetic catalytic effect on the hydrogen storage performances of 2LiBH4-MgH2 from thermodynamic and kinetic aspects. The TiB2 nanoparticles can improve dehydrogenation kinetics by acting as heterogeneous nucleation agents and modifying kinetic model. KBH4 can form eutectic composites with LiBH4 to start dehydrogenation at low temperatures. LiF can transform into LiH1-xFx during dehydrogenation and LiBH4-xFx during rehydrogenation. Such fluoride substitution can thermodynamically destabilize LiH and LiBH4 to improve hydrogen de/absorption properties. The novel synergetic mechanism provides a new and comprehensive inspiration for improving the reversible hydrogen storage properties of 2LiBH4-MgH2.