7.3 Hydrogen Physisorption and Carbon Nanotubes
77
nanotubes and sheets. These energies are less than the energies exhibited by carbon
nanotubes and graphene. The study found the 1 eV/molecule energy barrier from
molecular physisorption to dissociative chemisorption of H 2 on these materials [16].
Different studies have reported different concentrations of hydrogen adsorption
on carbon nanotubes. In an investigation, purified and modified samples of carbon
nanotubes have been reported for up to 8 wt.% storage of hydrogen. Temperatureprogrammed desorption (TPD) spectroscopy was used for the analysis of adsorption
of H 2 in carbon nanotubes. The characterization depicted that the hydrogen gets
adsorbed in the empty cavities and canals of the carbon nanotubes. The same study
has also envisioned that the carbon nanotubes with length and diameter of 2 and
1.63 nm, respectively, may achieve the target set by the USA Department of Energy
[17].
Ye et al. have reported the physisorption of hydrogen on crystalline ropes on
single-walled carbon nanotubes, with the hydrogen-storing capacity of more than 8
wt. %. The study elaborated that the H 2 first gets adsorbed at the exterior surfaces of
the carbon nanotubes that are exposed. However, when the conditions of pressure and
temperature are altered to 40 bar and −193 °C, a phase transition takes place and all
the nanotubes get separated from each other and all their surfaces become exposed,
allowing hydrogen to physiosorbed on them. A nanotube–nanotube cohesive energy
is obtained due to the pressure of phase transition for most of the material of 5 meV
per carbon atom. The crystalline order in the nanotubes has strong effect on this
small cohesive energy. It was reported in the investigation that the H to C atoms ratio
of about 1.0 was achieved for carbon nanotubes at −193 °C and > 12 MPa pressure
[18].
Different types of carbon nanofibers were analyzed by Chambers et al. for the
adsorption of hydrogen. They reported that the tubular nanofibers can store 11 wt.%,
platelet form can adsorb 45 wt.%, and the herringbone can store 67 wt. % of hydrogen
at 25 °C and 12 MPa pressure. The release of the hydrogen from these materials
can be achieved by successive lowering of the pressure to approximate atmospheric
conditions at 25 °C [19].
In a study, molecular dynamics simulation was used for physisorption of molecular
hydrogen on single-walled vacant defect carbon nanotubes. The study focused on the
effects of different parameters like size of vacant defect, pressure, and temperature on
hydrogen physisorption. It was found that hydrogen can be stored inside the carbon
nanotube via the vacant defect when the size of the defect is more than the threshold.
The vacant defect size control allows the researchers to extract molecular hydrogen
from a mixture of gas and stock it up inside the carbon nanotubes. The physisorption
was found to be favored by high pressure and low temperature. Moreover, the storage
proficiency of the materials was also enhanced by the introduction of more defects,
which in practice is the reduction in carbon number of single-walled carbon nanotubes
[5].
In an investigation, a combination study with semi-empirical research, forcematching and theoretical method was made for the determination of the optimal
hydrogen adsorption ability of an open-ended single-walled carbon nanotube, as a
function of diameter. Semi-empirical analysis was done to establish the adsorption
77
nanotubes and sheets. These energies are less than the energies exhibited by carbon
nanotubes and graphene. The study found the 1 eV/molecule energy barrier from
molecular physisorption to dissociative chemisorption of H 2 on these materials [16].
Different studies have reported different concentrations of hydrogen adsorption
on carbon nanotubes. In an investigation, purified and modified samples of carbon
nanotubes have been reported for up to 8 wt.% storage of hydrogen. Temperatureprogrammed desorption (TPD) spectroscopy was used for the analysis of adsorption
of H 2 in carbon nanotubes. The characterization depicted that the hydrogen gets
adsorbed in the empty cavities and canals of the carbon nanotubes. The same study
has also envisioned that the carbon nanotubes with length and diameter of 2 and
1.63 nm, respectively, may achieve the target set by the USA Department of Energy
[17].
Ye et al. have reported the physisorption of hydrogen on crystalline ropes on
single-walled carbon nanotubes, with the hydrogen-storing capacity of more than 8
wt. %. The study elaborated that the H 2 first gets adsorbed at the exterior surfaces of
the carbon nanotubes that are exposed. However, when the conditions of pressure and
temperature are altered to 40 bar and −193 °C, a phase transition takes place and all
the nanotubes get separated from each other and all their surfaces become exposed,
allowing hydrogen to physiosorbed on them. A nanotube–nanotube cohesive energy
is obtained due to the pressure of phase transition for most of the material of 5 meV
per carbon atom. The crystalline order in the nanotubes has strong effect on this
small cohesive energy. It was reported in the investigation that the H to C atoms ratio
of about 1.0 was achieved for carbon nanotubes at −193 °C and > 12 MPa pressure
[18].
Different types of carbon nanofibers were analyzed by Chambers et al. for the
adsorption of hydrogen. They reported that the tubular nanofibers can store 11 wt.%,
platelet form can adsorb 45 wt.%, and the herringbone can store 67 wt. % of hydrogen
at 25 °C and 12 MPa pressure. The release of the hydrogen from these materials
can be achieved by successive lowering of the pressure to approximate atmospheric
conditions at 25 °C [19].
In a study, molecular dynamics simulation was used for physisorption of molecular
hydrogen on single-walled vacant defect carbon nanotubes. The study focused on the
effects of different parameters like size of vacant defect, pressure, and temperature on
hydrogen physisorption. It was found that hydrogen can be stored inside the carbon
nanotube via the vacant defect when the size of the defect is more than the threshold.
The vacant defect size control allows the researchers to extract molecular hydrogen
from a mixture of gas and stock it up inside the carbon nanotubes. The physisorption
was found to be favored by high pressure and low temperature. Moreover, the storage
proficiency of the materials was also enhanced by the introduction of more defects,
which in practice is the reduction in carbon number of single-walled carbon nanotubes
[5].
In an investigation, a combination study with semi-empirical research, forcematching and theoretical method was made for the determination of the optimal
hydrogen adsorption ability of an open-ended single-walled carbon nanotube, as a
function of diameter. Semi-empirical analysis was done to establish the adsorption