Molecular Dynamics Simulations on the Effects of Diameter and Chirality on Hydrogen Adsorption in Single Walled Carbon Nanotubes

We present systematic molecular dynamics simulation studies of hydrogen storage in single walled carbon
nanotubes of various diameters and chiralities using a recently developed curvature-dependent force field.
Our main objective is to address the following fundamental issues:  1. For a given H2 loading and nanotube
type, what is the H2 distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal
loading (H2 coverage)? 3. What is the diameter range and chirality for which H2 adsorption is most energetically
favorable? Our simulation results suggest strong dependence of H2 adsorption energies on the nanotube diameter
but less dependence on the chirality. Substantial lattice expansion upon H2 adsorption was found. The average
adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with
higher H2 loading. The calculated H2 vibrational power spectra and radial distribution functions indicate a
strong attractive interaction between H2 and nanotube walls. The calculated diffusion coefficients are much
higher than what has been reported for H2 in microporous materials such as zeolites, indicating that diffusivity
does not present a problem for hydrogen storage in carbon nanotubes.

本研究采用新近开发的曲率依赖性力场（curvature-dependent force field），针对不同管径与手性的单壁碳纳米管（single walled carbon nanotubes）开展了储氢行为的系统性分子动力学模拟研究。本研究的核心目标为厘清以下关键科学问题：1. 给定氢气装载量（H2 loading）与碳纳米管类型时，碳纳米管管束内的氢气分布状态如何？2. 针对特定类型的碳纳米管，其最大氢气装载量（即氢气覆盖度（coverage））为多少？3. 氢气吸附热力学优势最显著的管径范围与手性类型是什么？
模拟结果表明，氢气吸附能（adsorption energy）与碳纳米管管径存在显著相关性，但受手性类型的影响较小。研究发现氢气吸附过程会引发碳纳米管管束发生显著的晶格膨胀（lattice expansion）；平均吸附能随碳纳米管管径减小（曲率升高）而升高，随氢气装载量增加而降低。通过计算得到的氢气振动功率谱与径向分布函数（radial distribution functions）显示，氢气与碳纳米管壁之间存在较强的吸引相互作用。模拟得到的扩散系数（diffusion coefficients）远高于沸石（zeolites）等微孔材料中氢气的扩散系数，表明碳纳米管储氢体系不存在扩散性能瓶颈。