A recent challenge in hydrogen storage is to find light weight complex solid hydrides which have higher gravimetric capacity larger than 6.5 wt% and also can exhibit favorable thermodynamics and kinetics for hydrogen de-sorption and absorption for on-board vehicular applications.  The breakthrough discovery of Ti- catalyzed NaAlH₄^{1, 2} exhibiting reversible onboard hydrogen storage may not be the ideal system to attain the DOE 2010 and FreedomCAR technical targets.  This is due to the maximum achievable hydrogen storage capacity of 5.4 wt% for NaAlH₄, which is well below the DOE target of 2010.  So, borohydride complexes as hydrogen storage materials have recently attracted great interest.  The borohydride complexes NaBH₄ and LiBH₄ possess high hydrogen storage capacity of 13.0 wt% and 19.6 wt%, respectively.  However, the release of hydrogen from NaBH₄ is possible only by hydrolysis (reaction with H₂O) and this process is irreversible.  For the case of LiBH₄, the catalytic addition of SiO₂, significantly enhances its thermal desorption³ at 200 °C.  In general, thermal dehydrogenation and/or rehydrogenation of NaBH₄ or LiBH₄ are difficult to achieve because of the thermodynamic stability due to strong B-H interactions^{4, 5}.  It was found that thermal decomposition of Zn(BH₄)₂ comprises of not only the evolution of H₂, but also an appreciable amount of B–H (borane) compounds.  Lowering the decomposition temperature by Ni doping may lead to negligible release of boranes⁶.  

In this work, the inexpensive mechanochemical approach of ball milling technique was used to prepare a new class of solvent-free, solid-state complex borohydrides (Li-Mn-B-H) for on-board hydrogen storage by stoicheometrically mixing of MnCl₂ and LiBH₄.  It was found that the endothermic transition due to hydrogen or gaseous decomposition from the Li-Mn-B-H system precedes the low temperature phase transition of pure LiBH₄.  The dehydrogenation temperature of Li-Mn-B-H corresponds to 95-100 ^oC, whereas, this value for the low temperature phase transition of LiBH₄ is around 130 ^oC.  To reduce the decomposition temperature further, we attempted to dope the Li-Mn-B-H system with different molar concentrations of the nano-dopant.  Thermogravimetric (TGA) and desorption kinetic profiles of the undoped and doped Li-Mn-B-H system will also be presented.  The nanomaterial doped complex borohydride shows pronounced effects on the hydrogen release kinetics while lowering the decomposition temperature.

Reference:

¹              Bogdanovi, B. and Schwickardi, M., Journal of Alloys and Compounds 253-254, 1 (1997).

²              Jensen, C. M. and Zidan, R. A., U. S. Patent 6, 471935 (2002).

³              Zušttel, A., Rentsch, S., Fischer, P., et al., Journal of Alloys and Compounds 356-357, 515 (2003).

⁴              Lodziana, Z. and Vegge, T., phys. Rev. Lett. 93, 145501 (2004).

⁵              Frankcombe, T. J. and Kroes, G.-J., Phys. Rev. B 73, 174302 (2006).

⁶              Srinivasan, S., Escobar, D., Jurczyk, M., et al., Journal of Alloys and Compounds In Press, Corrected Proof.