Thermal Conversion of Unsolvated Mg(B₃H₈)₂ to BH₄^– in the Presence of MgH₂

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.