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+nLiBH(4) -> M(BH4)(n)+nLiCl. The thermal desorption properties of M(BH4), were investigated by gas-chromatography and mass-spectroscopy combined with thermogravimetry. The results indicate that the hydrogen desorption temperature T-d of M(BH4)(n) correlates with the Pauling electronegativity chi(p) of M; that is, T-d decreases with increasing values of chi(p). The components of desorbed gas for M = Ca, Sc, Ti, V and Cr (chi(p) <= 1.5) are hydrogen only, while those for M = Mn, Zn and Al (chi(p) >= 1.5) contain borane and hydrogen. Therefore, the appropriate chi(p) in M(BH4)(n) for hydrogen storage is expected to be smaller than 1.5, and the resulting predicted relationship between the heat of forrilation of M(BH4)(n) and chi(p) is a useful indicator for estimating the enthalpy change for the desorption reaction for M(BH4)(n). (C) 2007 Elsevier B.V. All rights reserved.