Thermodynamics of Dehydrogenation of the 2LiBH(4)-Mg2FeH6 Composite

Joint decomposition of hydrides may be energetically favored, if stable mixed compounds are formed. This "hydride destabilization" improves the energetics of H2 release from hydrogen storage materials. The sequence of dehydrogenation reactions of the 2LiBH4-Mg 2FeH6 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, MgH 2, 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 MgB 2 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. © 2012 American Chemical Society.