Solid-state hydrogen desorption of 2 MgH2 + LiBH4 nano-mixture: A kinetics mechanism study.

The dehydrogenation reaction pathway and rate-limiting step of a nano-LiBH 4 + nano-MgH 2 mixture with a 0.5:1 molar ratio, which has been shown to have the ability to reversibly release and absorb ∼5.7 wt% H 2 at 265 °C, have been investigated in detail. The study reveals that the solid-state dehydrogenation kinetics of the MgH 2 + 0.5 LiBH 4 mixture at 265 °C is nucleation-and-growth controlled. The rate-limiting step for dehydrogenation via the two parallel reaction pathways has been identified through examination of the elementary reactions as the nucleation and growth of reaction products LiH and MgB 2. The interfacial area between MgH 2 and LiBH 4 plays a critical role in the nucleation and growth of LiH and MgB 2 , and thus influence the dehydrogenation kinetics and H 2 storage capacity of the MgH 2 + 0.5 LiBH 4 mixture. X-ray diffraction, SEM analysis and specific surface area measurements reveal that the evolution of the powder characteristics before and after isothermal hydrogen uptake/release cycles is consistent with the kinetics observation and analysis. This study indicates that to further improve the dehydrogenation kinetics of the MgH 2 + LiBH 4 mixture, the nucleation and growth rates of LiH and/or MgB 2 should be enhanced in the future, while the interfacial area between MgH 2 and LiBH 4 should be increased and maintained to be as large as possible during hydrogen uptake/release cycles. Image 1 • The solid-state dehydrogenation kinetics of the MgH 2 + LiBH 4 mixture is nucleation-and-growth controlled. • The rate-limiting step for dehydrogenation is the nucleation and growth of reaction products LiH and MgB 2. • The interfacial area between MgH 2 and LiBH 4 plays a critical role in the nucleation and growth of LiH and MgB 2. • Decay in the reaction kinetics is associated with particle growth and decrease in the interfacial area between reactants. [ABSTRACT FROM AUTHOR]