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^-1H₂. 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[BH₄] 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 ΔH = -39.3 kJ mol^-1H₂ 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] (ΔH = -74.9 kJ mol^-1H₂). However, only the second reaction step (MgH₂ → Mg) is reversible at the condition up to 623 K at 10 MPa of hydrogen.