Destabilizing LiBH4 with a Metal (M = Mg, Al, Ti, V, Cr, or Sc) or Metal Hydride (MH2 = MgH2, TiH2, or CaH2)

We experimentally investigate several hydrogen storage reactions based on thermodynamic destabilization of LiBH4. The destabilized mixtures include nine M(H2)−LiBH4 compositions, where M(H2) = Al, Mg, Ti, Sc, V, Cr, MgH2, CaH2, or TiH2, which were selected on the basis of favorable thermodynamics predicted by recent first-principles computational study (Siegel, D. J.; Wolverton, C.; Ozoliņš, V. Phys. Rev. B: Condens. Matter, Mater. Phys. 2007, 76, 134102). For all compositions, our measurements reveal significant kinetic barriers for hydrogen release, evidenced by high desorption temperatures (>300 °C) and exceedingly slow hydrogen release rates. Characterization of the desorbed reaction phases indicate that less than half of the mixtures examined (M(H2) = MgH2, Mg, Al, and CaH2) follow the thermodynamically expected reaction pathway, resulting in the formation of metal boride products (MgB2, AlB2, and CaB6, respectively). Hydrogen release/uptake data for these compositions indicate that the MgH2−(LiBH4)2 and Al−(LiBH4)2 reactions are reversible (10.2 and 6.7 wt %, respectively) at increased desorption pressures of 5 and 3 bar H2, respectively, while CaH2−(LiBH4)6 is irreversible under the conditions tested. For the remaining compositions, M(H2) = Sc, V, Cr, Ti, and TiH2, we surmise that substantial limitations in kinetics inhibit the expected formation of metal boride products. Finally, we discuss how the relative physical properties, in particular the melting point of the reactants and products, correlate with desorption route and reversibility.