Thermal Conversion of Unsolvated Mg(B 3 H 8 ) 2 to BH 4 - in the Presence of MgH 2 .

In the search for energy storage materials, metal octahydrotriborates, M(B 3 H 8 ) 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(B 3 H 8 ) 2 to BH 4 - as-synthesized and in the presence of MgH 2 . The conversion of our unsolvated Mg(B 3 H 8 ) 2 starts at ∼100 °C and yields ∼22 wt % of BH 4 - 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 2 to unsolvated Mg(B 3 H 8 ) 2 drastically increases the thermal conversion to 85-88 wt % of BH 4 - while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH 2 substantially decreases the formation of (closo-hydro)borates and provides the necessary H 2 for the B 3 H 8 -to-BH 4 conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH 4 - under moderate conditions of temperature (200 °C) in less than 1 h, making the MgB 3 H 8 -MgH 2 system very promising for energy storage applications.