Kinetics studies of the reversible partial decomposition reaction in Mg(BH4)2

• Mg(BH4)2 releases up to 6.8 wt% H2 in the intermediate decomposition reaction to amorphous MgxByHz compounds.
• Rehydrogenation at T < 285 °C and 120 bar H2 yields in 2.5 wt% uptake and crystalline Mg(BH4)2.
• Kinetics modeling suggests that the desorption reaction(s) follow a complex pathway with different limiting steps.
• Rehydrogenation reaction(s) are governed by nucleation at a constant rate and diffusion-controlled growth.

Magnesium borohydride Mg(BH4)2 is a promising candidate for hydrogen storage due to its high hydrogen content and theoretically predicted low decomposition temperature. Hydrogenation of the completely decomposed Mg(BH4)2 requires high temperature, high H2 pressure and long reaction time. However, the partially decomposed compound can be rehydrogenated in much milder conditions. In this work, we study the reversible intermediate decomposition reaction in Mg(BH4)2. Gravimetric and volumetric measurements have shown that Mg(BH4)2 released up to 6.8 wt% H2 below 285 °C with the formation of amorphous MgBxHy intermediate(s), as found by infrared spectroscopic analysis. No crystalline decomposition reaction products were detected by powder X-ray diffraction. Rehydrogenation at 260–280 °C yielded 2.5 wt% uptake and the formation of crystalline Mg(BH4)2. Kinetics modeling suggested that the decomposition is a complex process with possibly several reactions which are limited mostly by diffusion. The rehydrogenation reaction was governed by the Johns–Mehl–Avrami model with the nucleation at a constant rate and diffusion-controlled growth mechanism.