EGAS41 - Swansea University

41 st EGAS CP 63 Gdańsk 2009
Nuclear quantum effects in Hydrogen storage materials
G. Ludueña ∗ , D. Sebastiani
Max-Planck-Institut für Polymerforschung, Mainz, Germany
∗ Corresponding author: luduena@mpip-mainz.mpg.de,
We present the structural properties of Lithium Imide (Li 2 NH) and Amide (LiNH 2 ),
investigated by means of Path Integral Molecular Dynamics (PIMD [1, 2]). This kind of
complex hydrates have higher gravimetric hydrogen density than conventional hydrogen
storage materials, due to its composition from light atoms (a capacity of 11.5wt% is
reported [3]). These materials are, however, still not well understood [4]. Experiments
suggest different H adsorption sites [5, 6], but they mostly cannot give a definitive answer
to the storage mechanism. The potential energies for the protons is relatively flat which
can lead to quantum delocalization, as Hydrogen can behave non-classically. Specifically
for Li 2 NH, the symmetry of the material and the proximity of several possible sites within
a flat potential energy surface can lead to quantum delocalization of the protons over six
octahedral equipotential sites around each Nitrogen. We aim at a deeper understanding
of this phenomenon, by means of ab-initio PIMD simulations.
In this study all atoms in the material are treated as quantum particles, while in
standard ab-initio calculations only electrons are treated to that accuracy. The results
for different initial structures and levels of quantum description are compared. NMR
spectra are calculated and compared with experimental solid state NMR results, showing
good agreement. The results show that protons tend to be bond to Nitrogen atoms while
showing a degree of delocalization (fig. 1).
Figure 1: Structure of crystalline Li 2 NH. Proton clouds (light gray) represent their quantum
delocalization.
References
[1] D. Ceperley, Rev. Mod. Phys. 67, 279 (1995)
[2] M. Tuckerman et al., J. Chem. Phys. 99, 2796 (1993)
[3] P. Chen et al., Nature 420, 302 (2002)
[4] W. David et al., JACS 129, 1594-1601 (2007)
[5] K. Ohoyama et al., Phys. Soc. Jpn. 74, 1765 (2005)
[6] Ch. Zhang, M. Dyer, Ali Alavi, J. Phys. Chem. B 109, 22089 (2005)
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