8.3 Chemisorption of Hydrogen and Carbon Nanotubes
87
type of geometry is zigzag, where adsorption of hydrogen takes place both inside and
outside of the carbon nanotubes giving the structure a zigzag appearance. Owing to
less strain on the C–C bonding this geometry is regarded as more stable than the arch
type. There is comparatively less increase in the average diameter to 7.13 nm. This
type of geometry allows large amounts of the hydrogen storage which can increase
with the increase in diameter [17]. However, the greater uptake of hydrogen can
produce repulsive energies resulting in the breakage of the nanotube walls. Lee et al.
reported that repulsive energies determine the stability and hydrogen uptake ability
of the carbon nanotubes [18].
Carbon nanotubes of varying purity were analyzed in a study, for their ability of
hydrogen adsorption at 25 MPa pressure and −196 °C temperature. It was found
that the carbon nanotubes with high purity can adsorb greater amount of hydrogen
as compared to the less pure carbon nanotubes. Additionally, in case of multiwalled
carbon nanotube, the enhanced adsorption of the hydrogen in highly pure multiwalled
carbon nanotubes is associated to high surface area and more available adsorption
spots [19].
In a computational investigation, first-principle calculations were made within
density functional theory (DFT) for the determination of hydrogen chemisorption on
carbon nanotubes.
The study elaborated chemisorption of one and two hydrogen atoms on the exterior
walls of varying armchair single-walled carbon nanotubes. The results of the study
showed that the two hydrogen atoms prefer bond formation at adjacent sites instead of
alternate carbon positions [21]. Different results on zigzag nanotubes are reported by
Dinadayalane et al. [22] Commonly, the exothermicity of chemisorption of hydrogen
decreases with the increase in the diameter of armchair nanotubes, whereas the
opposite is true for hydrogen chemisorption on zigzag nanostructures. The adsorption
of one and two hydrogen atoms expressively change the C-C bond distance of the
carbon nanotube. In the presence of hydrogen atoms, the carbon atom(s) adsorbs the
hydrogen atom(s) resulting in the alteration of hybridization of carbon from sp
2 to
sp
3 at the chemisorption spot(s) [21].
Different studies have reported modified carbon nanotubes for hydrogen storage
applications. For instance, the metal-doped carbon nanotubes have depicted
promising hydrogen adsorption. In some cases, the ability of hydrogen storage shown
40 times more hydrogen storage ability as compared to the pure carbon nanotubes.
The improved hydrogen adsorption can be associated to the fact that in modified
carbon nanotubes the initial hydrogen adsorption is done by the metallic nanoparticles, resulting in the subsequent dissociation of the hydrogen molecule. The atoms
thus formed are then and spilled to the nanotubes. A precise information of some of
metal-modified nanotubes involved in hydrogen storage is given in Table 8.1.
Hydrogen adsorption ability at ambient pressure and room temperature in varying
types of carbon nanotubes is very less than the target aimed by the USA Department of
Energy. However, it can be inferred form the data in Table 8.1 that at low temperature
such as −196 °C and high pressure like 4 MPa, some carbon nanotubes offer hydrogen
storage equal or above the target of energy department. Still that carbon nanotubes it
87
type of geometry is zigzag, where adsorption of hydrogen takes place both inside and
outside of the carbon nanotubes giving the structure a zigzag appearance. Owing to
less strain on the C–C bonding this geometry is regarded as more stable than the arch
type. There is comparatively less increase in the average diameter to 7.13 nm. This
type of geometry allows large amounts of the hydrogen storage which can increase
with the increase in diameter [17]. However, the greater uptake of hydrogen can
produce repulsive energies resulting in the breakage of the nanotube walls. Lee et al.
reported that repulsive energies determine the stability and hydrogen uptake ability
of the carbon nanotubes [18].
Carbon nanotubes of varying purity were analyzed in a study, for their ability of
hydrogen adsorption at 25 MPa pressure and −196 °C temperature. It was found
that the carbon nanotubes with high purity can adsorb greater amount of hydrogen
as compared to the less pure carbon nanotubes. Additionally, in case of multiwalled
carbon nanotube, the enhanced adsorption of the hydrogen in highly pure multiwalled
carbon nanotubes is associated to high surface area and more available adsorption
spots [19].
In a computational investigation, first-principle calculations were made within
density functional theory (DFT) for the determination of hydrogen chemisorption on
carbon nanotubes.
The study elaborated chemisorption of one and two hydrogen atoms on the exterior
walls of varying armchair single-walled carbon nanotubes. The results of the study
showed that the two hydrogen atoms prefer bond formation at adjacent sites instead of
alternate carbon positions [21]. Different results on zigzag nanotubes are reported by
Dinadayalane et al. [22] Commonly, the exothermicity of chemisorption of hydrogen
decreases with the increase in the diameter of armchair nanotubes, whereas the
opposite is true for hydrogen chemisorption on zigzag nanostructures. The adsorption
of one and two hydrogen atoms expressively change the C-C bond distance of the
carbon nanotube. In the presence of hydrogen atoms, the carbon atom(s) adsorbs the
hydrogen atom(s) resulting in the alteration of hybridization of carbon from sp
2 to
sp
3 at the chemisorption spot(s) [21].
Different studies have reported modified carbon nanotubes for hydrogen storage
applications. For instance, the metal-doped carbon nanotubes have depicted
promising hydrogen adsorption. In some cases, the ability of hydrogen storage shown
40 times more hydrogen storage ability as compared to the pure carbon nanotubes.
The improved hydrogen adsorption can be associated to the fact that in modified
carbon nanotubes the initial hydrogen adsorption is done by the metallic nanoparticles, resulting in the subsequent dissociation of the hydrogen molecule. The atoms
thus formed are then and spilled to the nanotubes. A precise information of some of
metal-modified nanotubes involved in hydrogen storage is given in Table 8.1.
Hydrogen adsorption ability at ambient pressure and room temperature in varying
types of carbon nanotubes is very less than the target aimed by the USA Department of
Energy. However, it can be inferred form the data in Table 8.1 that at low temperature
such as −196 °C and high pressure like 4 MPa, some carbon nanotubes offer hydrogen
storage equal or above the target of energy department. Still that carbon nanotubes it