On the decomposition of the 0.6LiBH_4-0.4Mg(BH_4)_2 eutectic mixture for hydrogen storage

摘要

The dehydrogenation reaction of the 0.6UBH_4-0.4Mg(BH_4)_2 eutectic system was investigated by Temperature-Programmed-Desorption and Pressure-Composition-Isotherm methods, in the range of 25-540℃ and 0.1-150 bar of p(H_2). A sequence of four decomposition steps was found by TPD measurements; they occur at 235, 315, 365 and 460 °C for pfHJ = 3 bar, with a clear T decrease with respect to pure LiBH_4 and Mg(BH_4)_2 . In the PCI experiments, the first two steps could not be resolved but appeared merged in a single process. The amounts of H_2 release at each step and the △_Rh and △_Rs values derived from van't Hoff plots were analyzed and compared with known results for relevant possible reactions. A scheme of interpretation was then proposed for all four processes. In particular, a fraction of LiBH_4 and Mg(BH_4)_2 would react together in the range of 300-350 °C according to 2IABH_4 + Mg(BH_4)_2→2B + 2LiH + MgB_2 + 7H_2, thus explaining the quite large H_2 yield therein observed. The first and fourth steps correspond to decompositions of pure remaining Mg(BH_4)_2 and LiBH_4, respectively, and the third one to dehydrogenation of MgH_2 produced in the first step.