(600e) New Material Li-Mn-B-H System as Hydrogen Storage Candidate

A recent challenge in hydrogen
storage is to find light weight complex solid hydrides which have higher
gravimetric capacity larger than 6.0 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
phase transition and decomposition temperatures of Li-Mn-B-H correspond to
95-100 ^oC and 135-155 ^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.  Study
of thermogravimetric (TGA) and desorption kinetic profiles of the undoped and
doped Li-Mn-B-H system showed that 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              Zu¨ttel, 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 462, 294 (2008).