Environmental Engineering Reference
In-Depth Information
caps of CNTs, resulting in an increase in the active adsorption site for physi-
cal sorption of H
2
while the Pt nanoparticles promoted chemical sorption of
hydrogen via spillover mechanism that involves additives (e.g., Ru, Pt, and
Pd) that act as catalysts for dissociation of hydrogen molecules to hydrogen
atoms, which move from the catalytic sites to the surface of CNTs and finally
become adsorbed.
Fullerenes are molecules composed entirely of carbon, in the form of
hollow spheres, ellipsoids, or tubes. CNTs discussed earlier belong to the
family of fullerenes with cylindrical shapes, also called buckytubes. A special
spherical fullerene structure with 60 carbon atoms is called buckminster-
fullerene or buckyball, as it resembles the ball used in football (soccer) [8].
Buckminsterfullerene or C
60
has a cage-like fused-ring structure (truncated
icosahedron), made of 20 hexagons and 12 pentagons, with a carbon atom
at each vertex of each polygon and a bond along each polygon edge. It has
been considered as a potential hydrogen storage material because of its
ability to react with hydrogen via the C=C double bonds. Theoretically,
C
60
H
60
can be formed that would correspond to a hydrogen content of
∼7.7 wt% [1]. Experimentally, this is challenging to realize due to the
requirement of very high temperature, about 823-873 K [9]. Strategies have
been developed to overcome the challenge, including doping of C
60
using
metal atoms such as Li [10-13]. One example is the recent work on Li-doped
fullerene (Li
x
-C
60
-H
y
) that is capable of reversibly storing hydrogen through
chemisorption at elevated temperatures and pressures [13]. This system is
unique in that hydrogen is closely associated with lithium and carbon upon
rehydrogenation of the material and that the weight percent of H
2
stored in
the material is closely linked to the stoichiometric ratio of Li : C
60
in the
material. Under optimized conditions, a Li-doped fullerene with a Li : C
60
mole ratio of 6 : 1 can reversibly desorb up to 5 wt% H
2
with an onset
temperature of ∼270°C, which is significantly lower than the desorption
temperature of hydrogenated fullerenes (C
60
H
x
) and pure lithium hydride
(decomposition temperature 500−600 and 670°C, respectively). However,
the Li
x
-C
60
-H
y
system still suffers from the same drawbacks as typical
hydrogenated fullerenes (high desorption temperature and release of hydro-
carbons) because the fullerene cage remains mostly intact and is only slightly
modified during multiple hydrogen desorption/absorption cycles.
Graphene, a substance made of pure carbon with atoms arranged in a
regular hexagonal pattern similar to graphite but in a one-atom thick sheet,
has been considered for hydrogen storage as well, even though the studies
have been limited so far. It potentially can have a very high hydrogen storage
density. Graphene has been converted into graphane reversibly using a stream
of hydrogen atoms, and graphane can release the stored hydrogen by heating
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