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-LiBH4 + nano-MgH2 mixture with a 0.5:1 molar ratio, which has been shown to have the ability to reversibly release and absorb ∼5.7 wt% H2 at 265 °C, have been investigated in detail. The study reveals that the solid-state dehydrogenation kinetics of the MgH2 + 0.5 LiBH4 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 MgB2. The interfacial area between MgH2 and LiBH4 plays a critical role in the nucleation and growth of LiH and MgB2, and thus influence the dehydrogenation kinetics and H2 storage capacity of the MgH2 + 0.5 LiBH4 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 MgH2 + LiBH4 mixture, the nucleation and growth rates of LiH and/or MgB2 should be enhanced in the future, while the interfacial area between MgH2 and LiBH4 should be increased and maintained to be as large as possible during hydrogen uptake/release cycles.