In the search for energy storage materials, metal octahydrotriborates, M(B₃H₈)_n, n = 1 and 2, are promising candidates for applications such as stationary hydrogen storage and all-solid-state batteries. Therefore, we studied the thermal conversion of unsolvated Mg(B₃H₈)₂ to BH₄^– as-synthesized and in the presence of MgH₂. The conversion of our unsolvated Mg(B₃H₈)₂ starts at ∼100 °C and yields ∼22 wt % of BH₄^– along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of activated MgH₂ to unsolvated Mg(B₃H₈)₂ drastically increases the thermal conversion to 85–88 wt % of BH₄^– while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH₂ substantially decreases the formation of (closo-hydro)borates and provides the necessary H₂ for the B₃H₈-to-BH₄ conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH₄^– under moderate conditions of temperature (200 °C) in less than 1 h, making the MgB₃H₈-MgH₂ system very promising for energy storage applications.

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.