86
8 Chemisorption
and electronic characteristics are still ambiguous. HongJiang et al. have used four
isomers of Na 6 C 60 and Li 6 C 60 and analyzed them for their hydrogen storage capacity
with DFT model. The effect of pressure and temperature was determined in all the
isomers by employing the statistical thermodynamic methods. For the determination
of hydrogen storage capacity, the doped fullerenes were allowed to interact with 36
atoms of hydrogen at 300
0 C and 100 bar pressure. The hydrogen binding on the
surface of fullerenes followed two mechanisms with different binding energies. In
the first mechanism, the hydrogen atoms are adsorbed directly onto the surface of
C 60 carbon atoms resulting in the formation of chemical bond. The other mechanism involves the hydrogen molecules which gets physically adsorbed onto lithium
or sodium atoms. Hydrogen binds in the molecular form via charge polarization
mechanism as the Li and Na are positively charged because of their metallic nature.
The study concluded that Li-doped fullerene has better hydrogen storage capacity
compared to the Na-doped fullerene because of greater stability of the Li-doped
fullerene. The highest gravimetric density shown by one of the Li-doped fullerene
is 4.5 wt.% where it is 4 wt. % for sodium-doped material [14].
8.3 Chemisorption of Hydrogen and Carbon Nanotubes
Carbon nanotubes are involved actively in the storage of hydrogen. Several studies
have reported the use of carbon nanotubes in chemisorption of hydrogen. Nikitin et al.
have investigated chemisorption of hydrogen in single-walled carbon nanotubes. In
order to determine whether these nanotubes can be used for hydrogen chemisorption, atomic hydrogen was used for the hydrogenation of the nanotubes. The study
concluded that the maximum hydrogen adsorption of the nanotubes is dependent
on diameter of nanotube. The study claimed that the nanotubes with the diameter
of ~2.0 nm can have up to 100% hydrogenation, with complete stability at 25 °C.
This implies that the particular carbon nanotubes can have 7 wt. % hydrogen storage
capacity via the establishment of reversible C-H bonds [15].
A study has reported the dissociative chemisorption of hydrogen on carbon
nanotubes depending upon the first-principle calculations. The chemisorption of
hydrogen is the result of C-H bond formation after the breakage of the H–H
bond, when high pressure is applied on the two neighboring carbon nanotubes. The
chemisorbed hydrogen is released when the applied pressure is removed from the
nanotubes, hence making the process reversible. Fullerenes also depicts similar type
of behavior in vicinity of hydrogen when pressure is applied [16].
There are three possible types of geometries shown by carbon nanotubes after
the adsorption of hydrogen. The first is arch-type geometry which is shown by the
nanotubes when the ions of hydrogen get adsorbed to the top spots of carbon atoms.
This chemisorption shown by hydrogen ions is exothermic process. The hydrogen
gets adsorbed on the exterior of the carbon nanotubes where each hydrogen atom
gets attached to the carbon atom with sp
3 hybridization. This arrangement of the
atoms increases the diameter of the carbon nanotubes to 6.88 nm–7.78 nm. Another
8 Chemisorption
and electronic characteristics are still ambiguous. HongJiang et al. have used four
isomers of Na 6 C 60 and Li 6 C 60 and analyzed them for their hydrogen storage capacity
with DFT model. The effect of pressure and temperature was determined in all the
isomers by employing the statistical thermodynamic methods. For the determination
of hydrogen storage capacity, the doped fullerenes were allowed to interact with 36
atoms of hydrogen at 300
0 C and 100 bar pressure. The hydrogen binding on the
surface of fullerenes followed two mechanisms with different binding energies. In
the first mechanism, the hydrogen atoms are adsorbed directly onto the surface of
C 60 carbon atoms resulting in the formation of chemical bond. The other mechanism involves the hydrogen molecules which gets physically adsorbed onto lithium
or sodium atoms. Hydrogen binds in the molecular form via charge polarization
mechanism as the Li and Na are positively charged because of their metallic nature.
The study concluded that Li-doped fullerene has better hydrogen storage capacity
compared to the Na-doped fullerene because of greater stability of the Li-doped
fullerene. The highest gravimetric density shown by one of the Li-doped fullerene
is 4.5 wt.% where it is 4 wt. % for sodium-doped material [14].
8.3 Chemisorption of Hydrogen and Carbon Nanotubes
Carbon nanotubes are involved actively in the storage of hydrogen. Several studies
have reported the use of carbon nanotubes in chemisorption of hydrogen. Nikitin et al.
have investigated chemisorption of hydrogen in single-walled carbon nanotubes. In
order to determine whether these nanotubes can be used for hydrogen chemisorption, atomic hydrogen was used for the hydrogenation of the nanotubes. The study
concluded that the maximum hydrogen adsorption of the nanotubes is dependent
on diameter of nanotube. The study claimed that the nanotubes with the diameter
of ~2.0 nm can have up to 100% hydrogenation, with complete stability at 25 °C.
This implies that the particular carbon nanotubes can have 7 wt. % hydrogen storage
capacity via the establishment of reversible C-H bonds [15].
A study has reported the dissociative chemisorption of hydrogen on carbon
nanotubes depending upon the first-principle calculations. The chemisorption of
hydrogen is the result of C-H bond formation after the breakage of the H–H
bond, when high pressure is applied on the two neighboring carbon nanotubes. The
chemisorbed hydrogen is released when the applied pressure is removed from the
nanotubes, hence making the process reversible. Fullerenes also depicts similar type
of behavior in vicinity of hydrogen when pressure is applied [16].
There are three possible types of geometries shown by carbon nanotubes after
the adsorption of hydrogen. The first is arch-type geometry which is shown by the
nanotubes when the ions of hydrogen get adsorbed to the top spots of carbon atoms.
This chemisorption shown by hydrogen ions is exothermic process. The hydrogen
gets adsorbed on the exterior of the carbon nanotubes where each hydrogen atom
gets attached to the carbon atom with sp
3 hybridization. This arrangement of the
atoms increases the diameter of the carbon nanotubes to 6.88 nm–7.78 nm. Another