Thermodynamical stabilities of metal-borohydrides

Metal-borohydrides M(BH4)n (M = Ca, Sc, Ti, V, Cr, Mn, Zn (fourth period in periodic table) and Al; n = 2–4) were synthesized by a mechanical milling process according to the following reaction: MCln + nLiBH4 → M(BH4)n + nLiCl. The thermal desorption properties of M(BH4)n were investigated by gas-chromatography and mass-spectroscopy combined with thermogravimetry. The results indicate that the hydrogen desorption temperature Td of M(BH4)n correlates with the Pauling electronegativity χP of M; that is, Td decreases with increasing values of χP. The components of desorbed gas for M = Ca, Sc, Ti, V and Cr (χP ≤ 1.5) are hydrogen only, while those for M = Mn, Zn and Al (χP ≥ 1.5) contain borane and hydrogen. Therefore, the appropriate χP in M(BH4)n for hydrogen storage is expected to be smaller than 1.5, and the resulting predicted relationship between the heat of formation of M(BH4)n and χP is a useful indicator for estimating the enthalpy change for the desorption reaction for M(BH4)n.