Atomistic insights into the H2 adsorption and desorption behavior of novel Li-functionalized polycrystalline CNTs

Abstract

At present, hydrogen storage by physisorption is being investigated on novel polycrystalline carbon nanotubes (PCNTs) in order to utilize them for large-scale applications. Large-sized nanotubes are susceptible to structural flaws known as polycrystallinity, which evolve owing to the inherent constraints of growth kinetics and purification during the synthesis process. In this study, molecular dynamics simulations (MDS) were used to analyze the adsorption and desorption behavior of PCNTs employing hybrid grand canonical Monte Carlo (GCMC) simulations coupled with microcanonical ensembles. Moreover, a distinctive potential energy distribution technique was used to compute the adsorption metrics. We further investigated the size dependency effects, including curvature, length, and grain boundaries (GBs), on hydrogen adsorption functionalities. The MDS revealed that PCNTs with a moderate grain size of 4 nm are energetically stable and adsorption compatible with maximum gravimetric densities of 8.22 and 1.03 wt% at 77 and 300 K, respectively. Furthermore, the optimized PCNTs were functionalized with Li adatoms of varied concentrations and simulated at various temperatures and pressures. Afterward, adsorption/desorption kinetics and isosteric enthalpies of Li-functionalized PCNTs (Li-PCNTs) were computed to assess the synergistic impacts of Li adatoms and GBs. Li-PCNTs have shown remarkable performance in terms of hydrogen storage and deliverability. Consequently, at 77 and 300 K, Li-PCNTs doped with a 25% adatom concentration exhibited the maximum hydrogen uptake of 10.36 wt% and 2.13 wt%, respectively. Additionally, at 300 and 77 K, the averaged isosteric enthalpy of adsorption, estimated from the adsorption isotherms, was about 7.07 kJ/mol and 4.03 kJ/mol, respectively. Therefore, despite their ability to prevent metal clustering, PCNTs have a modest impact on adsorption metrics. Our findings open up new avenues for the utilization of PCNTs in large-scale hydrogen storage applications with suitable functionalization strategies. © 2023 Elsevier Ltd