Reactive Vapor-Phase Additives toward Destabilizing y-Mg(BH4)2 for Improved Hydrogen Release

Magnesium borohydride (Mg(BH₄)₂) is a promising candidate for material-based hydrogen storage due to its high hydrogen gravimetric/volumetric capacities and potential for dehydrogenation reversibility. Currently, slow dehydrogenation kinetics and the formation of intermediate polyboranes deter its application in clean energy technologies. In this study, a novel approach for modifying the physicochemical properties of Mg(BH₄)₂ is described, which involves the addition of reactive molecules in the vapor phase. This process enables the investigation of a new class of additive molecules for material-based hydrogen storage. The effects of four molecules (BBr₃, Al₂(CH₃)₆, TiCl₄, and N₂H₄) with varying degrees of electrophilicity are examined to infer how the chemical reactivity can be used to tune the additive-Mg(BH₄)₂ interaction and optimize the release of hydrogen at lower temperatures. Control over the amounts of additive exposure to Mg(BH₄)₂ is shown to prevent degradation of the bulk γ-Mg(BH₄)₂ crystal structure and loss of hydrogen capacity. Trimethylaluminum provides the most encouraging results on Mg(BH₄)₂, maintaining 97% of the starting theoretical Mg(BH₄)₂ hydrogen content and demonstrating hydrogen release at 115 °C. These results firmly establish the efficacy of this approach toward controlling the properties of Mg(BH₄)₂ and provide a new path forward for additive-based modification of hydrogen storage materials.