Enhanced hydrogen storage of a functional material: Hf₂CF₂ MXene with Li decoration

Abstract

In this paper, the hydrogen storage properties of the Li-decorated stable Hf2CF2 MXene layer, obtained by the exfoliation of Al from Hf2AlC and F-termination, are considered by using first-principles calculations based on Density Functional Theory. First, the stability characteristics of the host structure (Hf2CF2 layer) are examined by investigating bulk Hf2AlC. To enhance the adsorbed number of H-2 molecules, the well-defined initial H-2 coordinates are constructed by CLICH (Cap-Like Initial Conditions for Hydrogens) and Monte Carlo-based algorithms. After the geometry optimizations of the designed H-2 systems on the Li/Hf2CF2 layer, the adsorption energies of nH(2)/Li/Hf2CF2 n = 1-10, 15, 20 and 25 systems are calculated, and the suitable values (0.2-0.6 eV/H-2) are obtained up to 15H(2). For n = 20 and 25 systems, which have adsorption energies of 0.15 eV/H-2 and 0.16 eV/H-2, respectively. The structural properties and adsorption geometries of these molecules are analyzed. Additionally, the partial density of the states, electron density difference maps, and Mulliken atomic charges are presented to identify the actual binding mechanism of the systems. The results reveal that the Li-decorated Hf2CF2 MXene layer can be preferred for the hydrogen storage applications due to its stable nature and the convenient adsorption characteristics.