Hydrogen storage properties of Mg[BH4]2

Among the large variety of possible complex hydrides only few exhibit a large gravimetric hydrogen density and stability around 40 kJ mol(-1)H(2). Mg[BH4](2) is based on theoretical approaches a complex hydride with an equilibrium hydrogen pressure of approximately 1 bar at room temperature and a hydrogen content of 14.9 mass%. The reaction of Li[BH4] with MgCl2 at elevated temperatures was investigated as a possible route to synthesize Mg[BH4](2). Li[BR4] reacts with MgCl2 at a temperature >523 K at a pressure of 10 MPa of hydrogen, where the product contains LiCl and Mg[BH4](2) The desorption pc-isotherm of the product obtained at 623 K shows two flat plateaus, which indicates that the decomposition of the product consists of a two-step reaction. The products of the first and the second decomposition reaction were analyzed by means of X-ray diffraction and found to be MgH2 and Mg, respectively. The enthalpy for the first decomposition reaction was determined to be Delta H = -39.3 kJ mol(-1)H(2) by the Van't Hoff plot of the equilibrium measurements between 563 K and 623 K, which is significantly lower than that for pure Li[BH4] (Delta H = -74.9 kJ mol(-1)H(2)). However, only the second reaction step (MgH2 -> Mg) is reversible at the condition up to 623 K at 10 MPa of hydrogen. (c) 2007 Elsevier B.V. All rights reserved.