  Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a
single ethylene molecule can form stable complexes with light transition metals
(TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and
14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to
include a large number of other metals and different isomeric structures. We
obtained interesting results for light metals such as Li. The ethylene molecule
is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which
then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2
and absorption capacity of 16 wt %, a record high value reported so far. The
stability of the proposed metal-ethylene complexes was tested by extensive
calculations such as normal-mode analysis, finite temperature first-principles
molecular dynamics (MD) simulations, and reaction path calculations. The phonon
and MD simulations indicate that the proposed structures are stable up to 500
K. The reaction path calculations indicate about 1 eV activation barrier for
the TM2-ethylene complex to transform into a possible lower energy
configuration where the ethylene molecule is dissociated. Importantly, no
matter which isometric configuration the TM2-ethylene complex possesses, the TM
atoms are able to bind multiple hydrogen molecules with suitable binding energy
for room temperature storage. These results suggest that co-deposition of
ethylene with a suitable precursor of TM or Li into nanopores of light-weight
host materials may be a very promising route to discovering new materials with
high-capacity hydrogen absorption properties.
