Theoretical Analysis of the Hydrogen Storage Capacity Limitations of Single-Walled Carbon Nanotubes

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.