Hydrogen is a promising clean energy technology, however, its onboard application relies on high capacity storage. Chemical storage based on complex magnesium borohydride, Mg(BH4)2, offers one of the highest theoretical gravimetric (14.8 wt.%) and volumetric (112 g/L) hydrogen storage capacity. Yet practical challenges remain due to kinetic limitations to realize full reversibility for de-/re-hydrogenations. Early theoretical work based on density functional theory predicts hydrogen desorption temperature between 20 and 75°C based on Mg(BH4)2 –&gt; MgB2 + 4H2, however, in experiments, hydrogen release have only been observed above 200°C. It is speculated that this discrepancy is originated from the formation of various BxHy intermediates that act as a thermodynamic sink or a kinetic barrier during hydrogen desorption. In this work, we use first-principles methods to explore initial dehydrogenation, from BH4 to B3Hx, in bulk γ-Mg(BH4)2 and to identify the preferred reaction pathways for the formation of key B2Hx and B3Hx intermediates that limit the overall kinetics of BH4 to B3Hx transformation. Our results not only provide valuable thermodynamic and kinetic information regarding each of the elemental reaction steps during BH4 to B3Hx conversion, but also shed light on the apparent contradiction between previous theoretical prediction and experimental observation regarding the existence of B3H8 and MgH2 intermediate phases.

First-principles study of initial dehydrogenation pathways in Mg(BH4)2

ACS Spring 2022

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