(42a) Theoretical Analysis of the Hydrogen Storage Capacity Limitations of Single-Walled Carbon Nanotubes | AIChE

(42a) Theoretical Analysis of the Hydrogen Storage Capacity Limitations of Single-Walled Carbon Nanotubes

Authors 

Muniz, A. R. - Presenter, University of Massachusetts at Amherst
Maroudas, D. - Presenter, University of Massachusetts

When
single-walled carbon nanotubes (SWCNTs) are exposed to a hydrogen plasma,
atomic hydrogen is chemisorbed onto their graphene walls. One potential
application of this process is the use of SWCNTs as media for hydrogen storage.
Experimental studies have reported storage capacities obtained by this process
as high as 7 wt %; this is equivalent to a nanotube wall coverage by H atoms of
almost 100%, which is the theoretical maximum value. However, in many
experiments, lower H storage capacities have been reported consistently and the
reasons for this limitation are not well understood. For practical applications
of this process, a fundamental understanding is required of the structural
changes undergone by SWCNTs upon hydrogenation.

In
this presentation, we report results of a computational atomic-scale analysis of the
effects of atomic hydrogen chemisorption on the structure and deformation of SWCNTs.
The analysis is based on classical molecular-dynamics (MD) and Monte Carlo (MC)
simulations of structural and compositional relaxation, as well as targeted
first-principles density functional theory (DFT) calculations that complement
and validate the classical simulation results. In the MD and MC simulations,
the interatomic interactions are described according to the Adaptive Interatomic
Reactive Empirical Bond Order (AIREBO) potential. The DFT calculations are
performed within the generalized gradient approximation and employ plane-wave
basis sets, ultrasoft pseudopotentials, and supercell models.

                We
find that H chemisorption induces structural changes in SWCNTs associated with sp2-to-sp3
bonding transitions, which cause deformation and amorphization of the SWCNT
wall. A particularly important effect is the ?swelling? of the nanotube,
consistently with experimental observations. The corresponding computed radial
and axial strains depend on the H coverage and on the SWCNT diameter and chirality.
Most importantly, we find that there is a critical H coverage (typically ³ 30%), beyond which the
radial and axial SWCNT strains start increasing linearly with H coverage and sp3-hybridized
C atoms prevail; at sub-critical coverages, the strain levels are negligible
and sp2-hybridized C atoms dominate. When SWCNTs arranged in bundles are exposed to atomic
hydrogen, this swelling effect upon hydrogenation may limit the total amount of
hydrogen that can be chemisorbed on the SWCNT walls. The swelling of the SWCNTs
results in a decrease in the intertube spacing within the bundle, which may
hinder the diffusion of atomic hydrogen through the interstitial space of the
bundle. This mass-transfer limitation may cause the non-uniform hydrogenation
of the SWCNTs in the bundle and, consequently, a decrease in the total amount
of hydrogen that can be chemisorbed in the SWCNTs and stored in the bundles. We
present an analytical model for this phenomenon. The model is parameterized
according to the atomic-scale computations of SWCNT swelling and provides
estimates of the maximum tube wall coverage that can be obtained as a function
of the bundle density and the properties of the individual nanotubes in the
bundle. The model predictions are assessed by comparisons with large-scale
MD/MC simulations of hydrogenation of SWCNT bundles.