Molecular modeling of hydrogen adsorption and mobility in transition metal-functionalized polycrystalline CNTs using an energy-centered approach

Abstract

Despite notable progress in synthesizing large-sized carbon nanotubes (CNTs), persisting challenges arise from their inherent polycrystallinity and tendency to agglomerate. The investigation focuses on the interactions of molecular hydrogen with polycrystalline CNTs (PCNTs) functionalized with various concentrations of transition metals (Fe, Ni, and Ti) through molecular dynamics simulations (MDS). A novel potential energy distribution (PED) technique, integrated with grand canonical Monte Carlo (GCMC) simulations, was employed to identify and calculate the adsorbed hydrogen. Comparative analysis revealed that Ti-functionalized PCNTs (Ti-PCNTs) exhibited superior hydrogen adsorption capacity, characterized by deep local minima in PED with broad distribution, outperforming Fe and Ni-PCNTs. At 100 bar, 10 at% Ti-PCNT exhibits the highest hydrogen storage capacity, with a maximum of 7.14 wt% at 200 K and 5.2 wt% at 300 K. In comparison, 10 at% Ni-PCNT and Fe-PCNT attain maximum gravimetric densities of 3.31 wt% (1.75 wt%) and 3.19 wt% (1.7 wt%) at 200 K (300 K), respectively. Notably, bundled Ti-PCNTs (intertube spacing: 10 Å) achieved a substantial hydrogen gravimetric density of 6.6 wt% at 300 K and 100 bar, exceeding the US Department of Energy's targets. Subsequently, the average adsorption energies were calculated as 0.115 eV/H2 and 0.110 eV/H2 for isolated and bundled Ti-PCNTs at 200 K, increasing to 0.128 eV/H2 and 0.123 eV/H2 at 300 K, indicating stable and stronger adsorption states relative to undoped PCNTs. Furthermore, functionalization reduced hydrogen mobility, as Ti-PCNTs exhibited lower diffusion coefficients compared to Ni and Fe-PCNTs. Specifically, the diffusion coefficients for a 10 at% Ti-PCNT bundle were calculated as 9.47 × 10-8 m2/s and 1.85 × 10-7 m2/s at 200 K and 300 K, respectively. Ultimately, the integrated PED-MDS approach demonstrated robustness in evaluating adsorption metrics, endorsing Ti-PCNT as a promising candidate for efficient hydrogen storage. The proposed computational framework will facilitate the development and optimization of novel materials that could further improve hydrogen storage performance. © 2024 Elsevier B.V.