Abstract:                                                                                                                                            

Solid hydrogen storage materials are considered to be one of the better options for on board vehicular applications in light of their overall storage capacity, reversibility and also operational temperature (stability). Some of the complex hydrides have very high capacity but highly stable to decompose at lower temperature. For example, LiBH₄ has a storage capacity of 19.6 wt% but the decomposition temperature is more than 400 ^oC. Some of them are highly reversible but the capacity is low.  As an example, for NaAlH₄, the maximum reversibility is achieved at 5.4 wt% which is lower than the 2010 DOE target of 6.5 wt%. A new transition metal assisted complex borohydride, Zn(BH₄)_2, was recently reported with a high theoretical hydrogen storage capacity of 8.4 wt.% at the thermal decomposition temperature of 85 °C [Grochala and Edwards (2004)].  

In this work, an inexpensive mechanochemical approach of ball milling technique reported by our group (Srinivasan et. al (2006); Srinivasan et. al. (2006)) and others (Jeon and Cho (2006) was used to prepare the solvent free solid state Zn(BH₄)₂ by stoicheometrically mixing of ZnCl₂ and NaBH₄  according to the following reaction:  

ZnCl₂ + 2 NaBH₄ = 2NaCl + Zn(BH₄)₂

This work focuses on the extraction of pure Zn(BH₄)₂ from NaCl after the ball milling using solvent extraction methods, and the characterization by liquid FTIR analysis to confirm the B-H stretching and NMR analysis for hydrogen dynamics. Results of a density functional theory study of geometric as well as thermodynamic properties of pure crystalline Zn(BH₄)₂ will also be presented.  

In the present work the purification process was performed to extract the impurity NaCl under nitrogen/vacuum Schlenk manifold technique, using dry non-polar solvents in the solvent purification system. The mixture was then filtered to separate the NaCl from the Zn(BH₄)₂ dissolved in liquid phase. After that we performed a vacuum evaporation technique in the Schlenk manifold to get the solvent free pure Zn(BH₄)₂ from the liquid solvent. Preliminary analysis of this phase indicates that we are able to purify and isolate crystalline Zn(BH₄)₂.  Results of the complete study will be presented.                                                                                                            

Reference:                                                                                                                                           

1.      W. Grochala and P. P. Edwards, Chem. Rev. 104, 1283 (2004).

2.      E. Jeon and Y. Cho, Journal of Alloys and Compounds  (2006).

3.      Srinivasan, S., Dumbris, S., McElwee-White, L., Stefanakos, E., Goswami, Y. Thermal and Volumetric Studies of Complex Chemical Hydrides. Mater. Res. Soc. Symp. Proc. 2006; 885E-A07-03.1

4.      Srinivasan, S., Rivera, L., Stefanakos E., Goswami Y. Mechanochemical synthesis and characterization of new complex hydrides for hydrogen storage. Mater. Res. Soc. Symp. Proc. 2006; 0927-EE02-06 (2005)