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

[Image: see text] 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.