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Materials 2017, 10, 1399 2 of 10 been extensively studied [11–15], but only a few studies have considered its hydrogen storage properties. Borophene and graphene [16] have a similar 2D planar structure with a large specific surface area. Moreover, the relative atomic mass of B atom is smaller than the relative atomic mass of C atom. Therefore, we suspect that borophene has better hydrogen storage properties than graphene (it exhibits a triangular lattice with different periodic arrangements and is flat without obvious vertical undulation). Feng et al. [10] reported that β12-borophene is more stable than the other two types of borophene. Chen et al. [17] used the first-principles method to study the hydrogen storage properties of Ca-β12-borophene and found that it has a larger adsorption energy compared to other types of borophene. Therefore, we selected β12-borophene as the research focus. In this work, we performed theoretical calculations for the hydrogen storage properties of pure β12-borophene and Li-β12-borophene based on the first-principle study. We found that H2 molecules were completely dissociated into two H atoms that were adsorbed on the B–B bridge sites to form H–B covalent bonds, thus making it difficult to dissociate. Comparison of the improvement in hydrogen storage properties of graphene found that the graphene surface was modified by alkali metal (Li, Na, K) [18], alkali-earth metal (Ca) [19], light metal (Al) [20] and transition metals (Cu, Pd, Y) [21–24], which can change the chemical activity of the graphene surface and could effectively change the hydrogen storagecapability. The quality of alkali metal (Li atoms) is very light, which helps to enhance the hydrogen storage density [25]. The transition metal atom-modified nanostructures are highly reactive and can easily cause the dissociation of H2 molecules, which is detrimental to the reversible storage of hydrogen [26]. Therefore, we selected the lightest Li atom to modify the β12-borophenen. H2 adsorbed on Li-β12-borophene by physical adsorption, which improved the reversible hydrogen storage performance and significantly increased the amount of hydrogen storage. It is expected that this work can provide theoretical support for β12-borophene being used as hydrogen materials. 2. Computational Methods All density functional theory (DFT) calculations are carried out using the Cambridge Sequential Total Energy Package (CASTEP) [27], and the DFT evaluation is based on the plane-wave expansion. We use the Generalized Gradient Approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [28] to describe exchange and correlation effects. The van der waals forces of H2 adsorption on Li-β12-borophenen is modified by DFT-D methods. While the DFT-D perform poorly for energetics in layered materials [29], it is important to deal with the molecules adsorption system. We select the Ultrasoft Pseudopotential [30] to describe the interaction of electron-ion, and the electron wave functions are expanded by plane wave. The convergence tolerance energy, the force on each atoms and displacement convergence criterions are set to 5.0 × 10−6 eV/atom, 0.01 eV/Å and 0.001 Å, respectively. All atoms are relaxed in our calculations. In order to eliminate the interaction of the interlayer we select the vacuum thickness 20 Å. Considering the calculation accuracy and computational efficiency, all calculations are using a cutoff energy of 600 eV and 9 × 16 × 5 k-point mesh in the Brillouin zone. The adsorption energy (Eads) and average adsorption energy (Eads) of H2 adsorption on Li-β12-borophene are calculated by the following formulas [31]: Eads = EiH2+nLi+β12−borophene − E(i−1)H2+nLi+β12−borophene − EH2 (1) E =E −E −iE /i (2) ads iH2+nLi+β12−borophene nLi+β12−borophene H2 The average adsorption energy of Li atom on β12-borophenec [32] is defined as: E =E −E −nE /n (3) b nLi+β12−borophene β12−borophene Li where EiH2+nLi+β12−borophene, E(i−1)H2+nLi+β12−borophene and EnLi+β12−borophene are the total energy of the n Li-β12-borophene with i, i − 1 H2 molecules and β12-borophene with n Li atoms, respectively.PDF Image | Li-Decorated Borophene as Potentia for Hydrogen Storage
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