Environmental Engineering Reference
In-Depth Information
an increase in the hydrogen spillover effect and the binding energy between
metal nanoparticles and supporting material facilitated by nitrogen doping.[18]
Several recent theoretical or computational studies have also found that
chemical doping or nanocomposite can improve the hydrogen storage per-
formance of graphene or GO [19-22]. For example, detailed first-principles
calculations based on density functional theory have been carried out on
graphene with Li atoms with the objective to determine how the Li coverage
pattern affects the hydrogen storage capacity [22]. Results indicate that
hydrogen storage capacity can be increased to 16 wt% by adjusting the cov-
erage of Li atoms on graphene to the (root 3 × root 3) pattern at both sides.
This study demonstrates the importance of the details of the surface coverage
of the metal atoms as well as the potential of metal-modified graphene for
hydrogen storage.
Carbon nanofbers (CNFs) are cylindric carbon-based nanostructures with
graphene layers arranged as stacked cones, cups, or plates. Carbon nanotubes
discussed earlier are nanofibers with graphene layers wrapped into perfect
cylinders. Similar to other carbon nanostructures, CNFs have been studied
for hydrogen storage with encouraging results. For example, CNFs synthe-
sized by a catalytic pyrolysis method can store 4 wt% or higher of hydrogen,
similar to CNTs [23]. Similarly, turbostratic CNFs with a rough surface, open
pore walls, and a defect structure, produced by the thermal decomposition
of alcohol in the presence of an iron catalyst and a sulfur promoter at 1100°C
under a nitrogen atmosphere in a vertical chemical vapor deposition reactor,
showed hydrogen storage capacities 1.5 and 5 wt% for the as-produced and
exfoliated forms, respectively [24]. The defects on the surface and expand-
able graphitic structure are considered important to increasing the hydrogen
uptake. In a more recent study that compares the hydrogen adsorption capac-
ity of different types of carbon nanofibers (platelet, fishbone, and ribbon)
and amorphous carbon measured as a function of pressure and temperature,
more graphitic/ordered carbon materials have been found to adsorb less
hydrogen than the more amorphous ones, and functionalization (oxygen
surface group incorporation) and Ni-modification considerably improve the
hydrogen adsorption capacity [25]. Figure 7.4 shows representative TEM
images of parent carbon materials: (a) amorphous carbon, (b) ribbon CNFs,
(c) platelet CNFs, and (d) fishbone CNFs. The functionalization helps the
development of pores and accessibility of internal surface while Ni-modification
enhances the spillover effect, which involves the initial H
2
adsorption and
dissociation (metal catalyzed process) followed by the dissociated H migra-
tion through the metal and its anchorage to the carbonaceous structure [26].
Activated carbon (AC) is a highly porous, modified synthetic carbon that
contains crystalized graphite and amorphous carbon with a high specific
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