B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH 4 , Mg(BH 4 ) 2 , LiSc(BH 4 ) 4 , and the mixed Ca(AlH 4 ) 2 -LiBH 4 system. The 11 B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K 2 B 12 H 12 that contains the [B 12 H 12 ] 2anion. The presence of [B 12 H 12 ] 2in the partially decomposed borohydrides was further confirmed by high-resolution solution 11 B and 1 H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.

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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