Thermal Conversion of Unsolvated Mg(BH) to BH in the Presence of MgH.

In the search for energy storage materials, metal octahydrotriborates, M(BH) , = 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(BH) to BH as-synthesized and in the presence of MgH. The conversion of our unsolvated Mg(BH) starts at ∼100 °C and yields ∼22 wt % of BH 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 MgH to unsolvated Mg(BH) drastically increases the thermal conversion to 85-88 wt % of BH while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH substantially decreases the formation of (closo-hydro)borates and provides the necessary H for the BH-to-BH conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH under moderate conditions of temperature (200 °C) in less than 1 h, making the MgBH-MgH system very promising for energy storage applications.