Hydrogen sorption kinetics and mobility on Li-functionalized polycrystalline carbon nanotube bundles: A molecular dynamics study

Abstract

Despite significant advancements in the synthesis of large-sized carbon nanotubes, there remain challenges associated with their polycrystalline nature and their tendency to agglomerate and form bundles. This study investigated the molecular interaction of hydrogen with polycrystalline carbon nanotube (PCNT) bundles and Li-functionalized PCNT (Li-PCNT) bundles using molecular dynamics simulations (MDS) based on the grand canonical Monte Carlo (GCMC) method. With MDS, a novel potential energy distribution technique was implemented to evaluate the hydrogen adsorption metrics. Here, five simulation protocols were employed: (i) the effect of intertube spacings

(ii) the energetic and accessible area assessment

(iii) the storage performance of PCNT bundles

(iv) the storage and energetic assessment of Li-PCNT bundles

and (v) the mobility of hydrogen. Our calculations revealed an optimal intertube spacing of 10 Å for PCNT bundles with maximum gravimetric densities of 8.15 wt% and 1.3 wt% at 77 K and 300 K, respectively. Moreover, the results suggest that hydrogen adsorption is not very sensitive to the hexagonal or heterogeneous arrangement of PCNTs. Subsequently, a Li-PCNT bundle engineered with 20 at.% Li-adatoms demonstrates remarkable hydrogen adsorption metrics, attributed to the charge polarization effect, which enhances physisorption at the grooves and surfaces of PCNTs. The adsorption isotherms revealed a maximum gravimetric density of 4.25 wt% and 2.18 wt% at 200 K and 300 K, respectively. Moreover, Li-PCNT bundles exhibited average adsorption energies of 0.088 eV/H2 at 200 K and 0.091 eV/H2 at 300 K. Meanwhile, Li-functionalization hindered the diffusivity of hydrogen molecules, showing the highest and lowest diffusion coefficients of 9×10−7 m2/s and 3.3×10−7 m2/s at 20 bar and 100 bar, respectively. Our study emphasizes that Li-PCNT bundles can meet the hydrogen storage standards set by the US Department of Energy, thereby paving the way for large-scale implementation. © 2024 Elsevier Ltd