Tailoring of Metal Borohydrides for Hydrogen Storage Applications

Abstract

ABSTRACTRecent investigations on thermodynamical stabilities of metal borohydrides were reviewed. The first-principles calculations indicated that the heat of formation normalized by the number of BH4 complexs, ΔHboro, show a good correlation with the Pauling electronegativitiese of M, χp, which is represented by the liner relation, ΔHboro = 252.8χp − 396.4 in the unit of kJ/mol BH4. In order to clarify the correlation between the stability of borohydrides and the electronegativity χp of M, M(BH4)n (M = Mg, Ca, Sc, Ti, V, Cr, Mn, Zn, Zr and Al; n = 2-4) were systematically synthesized by mechanical milling. The thermal desorption analyses indicated that Td correlate with χ p of M; Td decrease with increasing the values of χp, in M(BH4)n. Furthermore, the correlation can be reasonably extended to double cation ones, (ZrLin-4)(BH4)n. For single cation, M(BH4)n (M = Mn, Zn and Al; χp ≥ 1.5) desorb borane besides hydrogen, and M(BH4)n (M = Ti, V and Cr; χp ≥ 1.5) desorbe small amount of hydrogen provably due to desorption reaction during milling. Therefore χp is an indicator to approximately estimate the stability of M(BH4)n, and appropriateχp in M(BH4)n is expected to be smaller than 1.5. The enthalpy change for the desorption reaction, ΔHdes, is estimated using our predicted ΔHboro and the reported data for decomposition product, ΔHhyd/borideboro, which shows a good correlation with the observed Td. These results are useful for exploring M(BH4)n with appropriate stability for hydrogen storage applications