(424d) Destabilized Libh4 / Mgh 2 for Reversible Hydrogen Storage

DOE and FreedomCAR technical targets of 6.0 and 9.0 wt.% are set forth capacities to realize a ?holy grail? hydrogen storage systems for 2010 and 2015 respectively [1]. Alkali metal complex hydrides with high theoretical hydrogen capacity, for example: LiBH₄ (18 wt.%), are being investigated for their properties to store large hydrogen quantities. The catalytic ad-mixing of SiO₂, enhances the performance of LiBH₄ [2]. However, the cyclic reversibility seems poor due to thermodynamic limitations. Recently, a destabilization mechanism was adopted in order to improve the cycling capacity of LiBH₄ by incorporating half a mole of MgH₂ [3]. We have successfully synthesized the complex hydride mixtures LiBH₄ + ½MgH₂ + Xmol% ZnCl₂ catalyst (X=2, 4, 6, 8 and 10) by an inexpensive mechano-chemical process. The structural characterization by X-ray powder diffraction profiles exhibits the presence of LiCl, MgH₂ and LiBH₄ as majority and minority phases. Besides, the LiCl peaks relative intensity increases with increase of ZnCl₂ concentration. The thermal decomposition (gravimetric weight loss) and heat flow measurements have been performed by simultaneous DSC and TGA techniques. An earlier decomposition temperature of 270° C has been observed for the 10mol% ZnCl₂ doped LiBH₄/MgH₂ in comparison to the pristine LiBH₄ compound. The pressure-composition-temperature isotherms of the destabilized LiBH₄ show an extended plateau pressure around 4-5 bars at 350° C with a good cyclic stability. Further experimental analysis using different catalysts and amounts of reactants is currently under way to tailor the hydrogenation properties of the complex borohydride systems.

References: [1] http://www.eere.energy.gov/hydrogenandfuelcells/storage/storage_challeng... [2] A. Züttel, S. Rentsch, P. Fischer, P. Wenger, P. Sudan, Ph. Mauron, Ch. Emmenegger, J Alloys Compd. 356-357, 515 (2003). [3] J. Vajo, S. Skeith, F. Mertens, J. Phys. Chem. B, 109, 2005, 3719-3722.