I.       Motivation/Background

In hydrogen storage materials, one recent challenge is to find light weight complex solid hydrides which have high gravimetric theoretical capacity exceeding the 2010 DOE target of 6.5 wt % for on-board vehicular application. The technical challenge is to find materials that can exhibit favorable thermodynamics and kinetics for hydrogen de-sorption and absorption, and have ability to store a sufficient amount of hydrogen by weight as well as by volume percent measures. Boro-hydride complexes as hydrogen storage materials have recently raised great interest. Most of the commonly known boro-hydrides are found to be unsuitable for on-board application because of either high stability (resulting from a very high decomposition temperature) or completely irreversible nature.¹ Zn(BH₄)₂ is considered to be a potential candidate for on-board application as it has high theoretical hydrogen storage capacity of 8.4 wt% and also a reasonably low  decomposition temperature (85 ^oC)² compared to complex alkali borohydrides (which decompose above 300 ^oC).

A combined theoretical and experimental approach can better lead us to design a suitable complex material for hydrogen storage.  Here, we explore the complex hydrides using density functional theory (DFT) to provide information at the electronic level that is hard to obtain by experimental methods. Such calculations can also unravel the transition states of the molecules undergoing chemical transformation at complex solid surfaces. Phonon spectrum helps us to understand the finite temperature crystal stability and such calculations are also performed in this work.

II.    Computational details and methodology

First-principle calculations were performed in this work on Zn(BH₄)₂ using DFT within the local density approximation (LDA) and projector augmented wave (PAW) method and a plane wave basis set to calculate the total energies, as implemented in the Vienna Ab initio Simulation Package (VASP).^3-5 We started our calculations from known similar complex formula units X(YZ₄)₂ (X=Be, Ca, Mg and Cu; Y=Al and B; Z=H, Cl, Br and I) for Zn(BH₄)₂ to find the most stable crystal structure. We have optimized the atomic positions and cell parameters for each model and finally optimized the cell volume against the total energy of the system. We have calculated the enthalpy for the formation equation of Zn(BH₄)₂ from its elemental components as given below:

Zn + 2B + 4H₂(g) = Zn(BH₄)₂

The calculated results indicated that those for space group Pmc2₁(i.e. Mg(BH₄)₂ model) give the highest negative enthalpy of formation for Zn(BH₄)₂ which can be considered the most stable structure. We have also calculated the electronic total density of states (DOS) and Electron Localization Function (ELF) of Zn(BH₄)₂ to gain insights into the nature of the material and bonding between the elements.

Based on the optimized crystal structure from DFT, lattice dynamics based on the harmonic approximation using the direct force constant method in the PHONON⁴ code was used to calculate the phonon density of states (DOS) to confirm the finite temperature crystal stability.

III. Results/Discussion

The above methods were used to establish the stable crystal structure with appropriate lattice parameters & atomic positions and the electronic structure of the high theoretical hydrogen storage capacity material Zn(BH₄)₂. These results as well as the 0 K enthalpy of formation including zero point energy corrections, 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.       G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).

4.       G. Kresse and J. Furthmüller, Comput. Mater. Sci. 6, 15 (1996).

5.       G. Kresse and J. Hafner, Phys. Rev. B 47, R558 (1993).

6.       K. Parlinski, Software PHONON (2005)