Thermodynamics of Dehydrogenation of the 2LiBH4-Mg2FeH6 Composite

Joint decomposition of hydrides may be energetically favored, if stable mixed compounds are formed. This "hydride destabilization" improves the energetics of H-2 release from hydrogen storage materials. The sequence of dehydrogenation reactions of the 2LiBH(4)-Mg2FeH6 composite was studied by PCI (pressure-composition-isotherm) and TPD (temperature-programmed-desorption) techniques in a Sievert apparatus. Produced phases were identified by ex-situ X-ray diffraction and FTIR spectroscopy. Three distinct plateaus are detected on each isotherm: A, B, and C on decreasing pressure. The A reaction, involving formation of FeB, MgH2, and LiH, occurs at higher pressure/lower temperature than dehydrogenation of either pure hydrides; these are then effectively destabilized thermodynamically. The B process is plain decomposition of MgH2, and in C, the magnesium produced reacts with left LiBH4, forming MgB2 and LiH. The B + C sequence is fully reversible, and it corresponds to two-step dehydrogenation of the LiBH4/MgH2 system. Reaction enthalpies and entropies were obtained through van't Hoff plots of all processes, thus providing a full thermodynamic characterization of the system. Interestingly, it ensues that destabilization of pure hydrides with reaction A is due primarily to an entropic rather than enthalpic effect.