Nanotechnological APPROACHES IN THE
DEVELOPMENT OF Materials for Hydrogen Storage
Actual
developments in the field of hydrogen storage mainly deal with the development
of materials based on the principles of chemisorption (metal hydrides in
general) and physisorption. A nanotechnological approach has turned out to be
highly beneficial in this field.
Complex
aluminum hydrides, the so-called alanates, are chemisorption materials
with high gravimetric storage densities for hydrogen. It will be shown that
their dehydrogenation temperature depends on the grain size and that the kinetics
of decomposition and hydrogen uptake are governed by nucleation and growth of
the new phases [1,2]. Kinetic data suggest that diffusion processes in the
solid limit the rate of their rehydrogenation. Hence, shortening of diffusion
paths would be necessary to enhance the kinetics, e.g. by reduction of the
grain size of the dehydrogenated material.
Kinetic
barriers interfere with the hydrogen uptake and release
and it has been tried to reduce the barriers by using appropriate dopants. In
various studies Ti turned out to be the most active element for the process. It
will be shown that a nanocomposite consisting of sodium alanate (NaAlH4)
and a catalytic amount of small ligand stabilized Ti clusters (Ti13)
shows considerably increased exchange rates for H when compared to a
state-of-the-art catalyst.
Nanoscale
physisorption materials have regained importance after a new class of
nanomaterials with very high specific surface areas has been tested for
hydrogen storage. Microporous isoreticular metal-organic frameworks (IR-MOFs)
[3] seem to have the potential to store several weight% of hydrogen at room
temperature and moderate pressures. In order to optimize these structures,
theoretical investigations have been made [4] and results of a work will be shown
about the binding energy of molecular hydrogen interacting with various
(substituted) aromatic hydrocarbons.
[1] M. Fichtner, O. Fuhr,
O. Kircher, J. Rothe, Nanotechnology 14 (2003) 778-785.
[2] O. Kircher and M.
Fichtner, J. Appl. Phys.
(submitted)
[3] N.L. Rosi et al.,
Science 300 (2003) 1127
[4] O. Huebner, A. Gloess,
M. Fichtner, and W. Klopper, J. Phys. Chem. B (submitted).