(102c) Dehydrogenation Thermodynamics of Hydrogen Storage Material Manganese Borohydride: First-Principles Study

Light weight, low cost, highly reversible hydrogen
storage systems are essential for low temperature PEM fuel cell powered
vehicles¹. 
Complex hydrides of alkali and alkaline earth elements are promising candidates
for hydrogen storage, but typically have heats of reaction that are too high to
be of use for fuel cell vehicles.  So, transition metal borohydride complexes
as hydrogen storage materials have recently attracted great interest.  Mn(BH₄)₂,
among all other transition complex borohydrides, is stable at room tempearature², and
is considered to be a potential candidate for on-board applications as it has
high theoretical hydrogen storage capacity of 10 wt% and also a reasonably low
decomposition temperature compared to complex alkali and alkaline earth
borohydrides.  To be a viable candidate, a hydride must have a reaction free
energy that lies in a range of values to allow reversible H₂ storage
at practical temperatures and pressures.  Thermodynamic data such as standard
enthalpy of formation and Gibbs energy of dehydrogenation reaction required to
assess the usability of the candidate material for practical applications are
not available.  We have used density functional theory and lattice dynamics
calculations to predict the crystal structure of the candidate material³,
the reaction enthalpies and identify suitable dehydrogenation reaction
thermodynamics.  The electronic density of states shows that manganese
borohydride is a half-metallic hydrogen storage material.  The electronic
structure analysis also implies polar covalent bonds both between Mn and H and
between B and H, with the lowest degree of polarity between Mn and H.  It was also
found that the reaction Mn(BH₄)₂ = Mn + 2B + 4H₂(g)
is the most favorable reaction during dehydrogenation of manganese borohydride.

Reference:

¹      Schlapbach, L. and
Zuttel, A., Nature 414, 353 (2001).

²      Grochala, W. and Edwards, P. P., Chem. Rev. 104,
1283 (2004).

³      Choudhury, P., Bhethanabotla, V. R., and Stefanakos,
E., Phys. Rev. B 77, 134302 (2008).