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).