Destabilization of LiBH4 for reversible hydrogen storage has been studied using MgH2 as a destabilizing additive. Mechanically milled mixtures of LiBH4 + (1/2)MgH2 or LiH + (1/2)MgB2 including 2-3 mol % TiCl3 are shown to reversibly store 8-10 wt % hydrogen. Variation of the equilibrium pressure obtained from isotherms measured at 315-400 degrees C indicate that addition of MgH2 lowers the hydrogenation/dehydrogenation enthalpy by 25 kJ/(mol of H2) compared with pure LiBH4. Formation of MgB2 upon dehydrogenation stabilizes the dehydrogenated state and, thereby, destabilizes the LiBH4. Extrapolation of the isotherm data yields a predicted equilibrium pressure of 1 bar at approximately 225 degrees C. However, the kinetics were too slow for direct measurements at these temperatures.

Enhanced kinetics for hydrogen exchange in LiBH 4 incorporated within nanoporous carbon scaffolds are described. Dehydrogenation rates up to 50 times faster than those in the bulk material are measured at 300 °C in a nanostructured hydride formed by filling a porous carbon aerogel host with LiBH 4 . Furthermore, the activation energy for hydrogen desorption, measured using the approach developed by Ozawa, is reduced from 146 kJ/mol for bulk LiBH 4 to 103 kJ/mol for nanostructured LiBH 4 , and the faster kinetics result in desorption temperatures that are reduced by up to 75 °C. In addition, nanostructured hydrides exhibit increased cycling capacity over multiple sorption cycles. This work demonstrates that confinement within a porous scaffold host is a promising approach for enhancing hydrogen uptake and release in reversible light-metal complex hydrides.

Alloying with Si is shown to destabilize the strongly bound hydrides LiH and MgH 2 . For the LiH/Si system, a Li 2.35 Si alloy forms upon dehydrogenation, causing the equilibrium hydrogen pressure at 490°C to increase from approximately 5 × 10 -5 to 1 bar. For the MgH 2 /Si system, Mg 2 Si forms upon dehydrogenation, causing the equilibrium pressure at 300°C to increase from 1.8 to >7.5 bar. Thermodynamic calculations indicate equilibrium pressures of 1 bar at approximately 20°C and 100 bar at approximately 150°C. These conditions indicate that the MgH 2 /Si system, which has a hydrogen capacity of 5.0 wt %, could be practical for hydrogen storage at reduced temperatures. The LiH/Si system is reversible and can be cycled without degradation. Absorption/desorption isotherms, obtained at 400-500°C, exhibited two distinct flat plateaus with little hysteresis. The plateaus correspond to formation and decomposition of various Li silicides. The MgH 2 /Si system was not readily reversible. Hydrogenation of Mg 2 Si appears to be kinetically limited because of the relatively low temperature, <150°C, required for hydrogenation at 100 bar. These two alloy systems show how hydride destabilization through alloy formation upon dehydrogenation can be used to design and control equilibrium pressures of strongly bound hydrides.

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Sodalite zeolitic imidazolate frameworks containing Co (ZIF-67) and Zn (ZIF-8) were synthesized at room temperature under aqueous conditions in 10 min. A trialkylamine deprotonated the 2-methylimidazole ligand and nucleated the frameworks. Furthermore, the ligand acted as a structure directing agent in place of an organic solvent.