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Materials 2017, 10, 1399 8 of 10 The bonding strength between atoms can be quantitatively analyzed based on the Mulliken charge population and bond population. Table 2 shows the Mulliken charge population before and after one H2 moleculebecomes absorbed on the Li-β12-borophene. H (1) and H (2) represent the two H atoms of the adsorbed H2 molecule; while B1, B5 and B6 are three B atoms that transfer the greatest amount of charge in the β12-borophene (as shown in Figure 1). The two H atoms have charges of 0.06 e and 0.05 e, respectively. In contrast, the Li atom loses 1.40 e, which occurs mainly in the H and Li atomic orbits. The Li atom transfers charge to the H2 molecules, resulting in the H2 molecules carrying more negative charge and Li atom showing positive charge. The interaction between the H2 molecules and the Li atom is consistent with the conclusion of the PDOS analysis. In addition, the B atoms obtains charge, with this charge transfer mainly occurring in the B-2p orbits and H-s orbits. This is in contrast with the Mulliken charge population of the β12-borophene/H2, in which the charge transfer mainly occurs in H and B atoms forming a covalent bond of H–B that is not favorable for the desorption of H2. Due to the β12-borophene being modified by Li atoms, H2 molecules and B atoms only have small interactions, resulting in the H2 molecules physically adsorbing on the Li-β12-borophene. This is conducive for H2 desorption and increases the hydrogen storage capacity. Table 2. Mulliken population analysis of the Li-β12-borophene before and after one H2 molecule adsorption. Atom Mulliken Before Adsorption/e After Adsorption/e s p Charge s p Charge −0.06 −0.05 −0.19 −0.15 −0.05 1.40 H (1) 1.0 1.06 H (2) 1.0 1.05 B1 0.82 2.18 0 0.83 2.36 B5 0.74 2.23 0.03 0.75 2.40 B6 0.65 2.40 −0.05 0.65 2.40 Li 3 0 0 1.60 4. Conclusions In summary, we performed a study on hydrogen storage properties of pure β12-borophene and Li-decorated β12-borophene through DFT calculations. It is found that H2 molecules are mainly adsorbed on pure β12-borophene as chemical adsorption with an adsorption energy of −0.536 eV. The H2 molecules are dissociated into two H atoms, which tend to the bridge of two B site and the H–B bond to form covalent bond. In order to improve the hydrogen storage performance of pure β12-borophene and increase the hydrogen storage capacity, we use the Li atom to modify the β12-borophene. It is found that a single Li atom adsorbed on the center of Boron ring with the adsorption energy −3.088 eV, the Li-β12-borophene can adsorb up to 7 H2 molecules with the average adsorption energy of −0.210 eV/H2. The charge transfer of the Li-β12-borophene/H2 is that H and B atoms lose electron, Li atom get electron. We use two Li atoms to modify β12-borophene to increase its hydrogen storage capacity. It is find that the two Li atoms are located at the same position on both sides of the same boron hole. 2Li-β12-borophene system can adsorb up to 14 H2 molecules and the hydrogen storage capacity up to 10.85 wt %. The average adsorption energy is range of −0.381 to −0.220 eV/H2, which is necessary for practical application [3,4]. Acknowledgments: This work was supported by the National Natural Science Foundation of China (grant number 51562022), the fund for the State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology (grant number SKLAB02014004), the Basic Scientific Research Foundation for Gansu Universities of China (grant number 05-0342), the Science and Technology Project of Lanzhou City (grant number 2011-1-10), the Natural Science Foundation of Gansu Province (Grant No. 17JR5RA123), and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (second phase).PDF Image | Li-Decorated Borophene as Potentia for Hydrogen Storage
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