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: LiBH
4 (18 wt.%), are being investigated for their properties to store large hydrogen quantities. The catalytic ad-mixing of SiO
2, enhances the performance of LiBH
4 [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 LiBH
4 by incorporating half a mole of MgH
2 [3]. We have successfully synthesized the complex hydride mixtures LiBH
4 + ˝MgH
2 + Xmol% ZnCl
2 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, MgH
2 and LiBH
4 as majority and minority phases. Besides, the LiCl peaks relative intensity increases with increase of ZnCl
2 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% ZnCl
2 doped LiBH
4/MgH
2 in comparison to the pristine LiBH
4 compound. The pressure-composition-temperature isotherms of the destabilized LiBH
4 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_challenges.html [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.