Abstract
Density functional theory (DFT) is a valuable tool for calculating adsorption energies toward designing materials for hydrogen storage. However, dispersion forces being absent from the local/semi-local theory, it remains unclear as to how the consideration of van der Waals (vdW) interactions affects such calculations. For the first time, we applied diffusion Monte Carlo (DMC) to evaluate the adsorption characteristics of a hydrogen molecule on a (5,5) armchair silicon-carbide nanotube (H2-SiCNT). Within the DFT framework, we benchmarked various exchange-correlation functionals, including those recently developed for treating dispersion or vdW interactions. We found that the vdW-corrected DFT methods agree well with DMC, whereas the local (semilocal) functional significantly over (under)-binds. Furthermore, we fully optimized the H2-SiCNT geometry within the DFT framework and investigated the correlation between the structure and charge density. The vdW contribution to the adsorption was found to be non-negligible at ∼1 kcal/mol per hydrogen molecule, which amounts to 9–29% of the ideal adsorption energy required for hydrogen storage applications.
Highlights
Hydrogen energy is a promising energy resource for reducing greenhouse gas emissions.[1−3] To realize the industrial use of hydrogen energy, in the transportation sector, one of the most important developmental challenges is addressing the related storage issues safety and capacity.[2]
Silicon-carbide nanotubes (SiCNTs) are a typical nanostructure studied for the above purpose owing to their enhanced molecular interactions compared with those of the structurally related carbon nanotubes (CNTs)
We demonstrate the importance of incorporating van der Waals (vdW) interactions for a quantitative description of H2 adsorption on SiCNTs and related systems
Summary
Hydrogen energy is a promising energy resource for reducing greenhouse gas emissions.[1−3] To realize the industrial use of hydrogen energy, in the transportation sector, one of the most important developmental challenges is addressing the related storage issues safety and capacity.[2]. Silicon-carbide nanotubes (SiCNTs) are a typical nanostructure studied for the above purpose owing to their enhanced molecular interactions compared with those of the structurally related (and more common) carbon nanotubes (CNTs) This feature has been linked to the SiCNT polarized surface originating from the Si−C bonds.[6−9] Despite this, density functional studies based on local (LDA), semilocal (GGA), and hybrid (B3LYP) exchange-correlation (XC) functionals on pristine nanotubes estimated the adsorption energy of hydrogen between 0.7 and 1.98 kcal/mol,[10−13] which is still too small for storing hydrogen at the desired ambient temperature. In the Supporting Information, these potential biases in DMC are discussed and proved to be small enough for the purpose of this study
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