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

Abstract

• The dehydrogenation induction period is completely eliminated. • 9.4 wt% H 2 can be reversibly hydrogenated at 200 °C. • The catalytic mechanism of K 2 TiF 6 is investigated in detail. • TiB 2 nano-particles can stably exist during 10 cycles. • F substitution can destabilize LiBH 4 and LiF to de/rehydrogenate. 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 2LiBH 4 -MgH 2 with K 2 TiF 6 is synthesized, in which K 2 TiF 6 can react with LiBH 4 to form TiB 2 , LiF, KBH 4 . 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 K 2 TiF 6 and LiBH 4 can reduce the practical capacity, excess LiBH 4 was added into the composite to offset the reduced dehydrogenation capacity. The cycling performances of the composite with excess LiBH 4 are greatly improved. The composite with excess LiBH 4 can completely absorb 9.4 wt% H 2 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 2LiBH 4 -MgH 2 . Characterization analyses and theoretical calculations indicate the in situ formed TiB 2, LiF, and KBH 4 can perform stable synergetic catalytic effect on the hydrogen storage performances of 2LiBH 4 -MgH 2 from thermodynamic and kinetic aspects. The TiB 2 nanoparticles can improve dehydrogenation kinetics by acting as heterogeneous nucleation agents and modifying kinetic model. KBH 4 can form eutectic composites with LiBH 4 to start dehydrogenation at low temperatures. LiF can transform into LiH 1-x F x during dehydrogenation and LiBH 4-x F x during rehydrogenation. Such fluoride substitution can thermodynamically destabilize LiH and LiBH 4 to improve hydrogen de/absorption properties. The novel synergetic mechanism provides a new and comprehensive inspiration for improving the reversible hydrogen storage properties of 2LiBH 4 -MgH 2 .