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Materials 2017, 10, 1399 5 of 10 There are three stable adsorption structures of two Li atoms after adsorption on the β12-borophene as shown in Figure 3d–f, respectively. One of the most stable adsorption sites involves the two Li atoms being located on both sides of the same B ring. The distance between Li and the nearest B is 2.298 Å. Materials 2017, 10, 1399 5 of 10 The average adsorption energy is −3.106 eV, which is larger than the cohesive energy of Li atoms. After optimization, the relaxation of β -borophene is very small. Each Li atom in the Li-β -borophene Li atoms. After optimization, the rela1x2ation of β12-borophene is very small. Each Li atom1i2n the Li-β12- systemisanactiveadsorptionsite,allowingalargenumberofH moleculestobeadsorbedaround borophene system is an active adsorption site, allowing a larg2e number of H2 molecules to be the Li atom in order to significantly increase the hydrogen storage capacity. adsorbed around the Li atom in order to significantly increase the hydrogen storage capacity. The charge transfer between atoms can be analyzed by Mulliken analysis [40], which shows The charge transfer between atoms can be analyzed by Mulliken analysis [40], which shows that that the charge was transferred from Li to B. From the Partial Densities of States (PDOS) of the the charge was transferred from Li to B. From the Partial Densities of States (PDOS) of the Li-β -borophene structure in Figure 4, we found the peak of B atom’s 2p orbital overlaps with Li-β122-borophene structure in Figure 4, we found the peak of B atom’s 2p orbital overlaps with the the peak of the Li atom’s 1s orbital. This suggests a strong hybridization between B and Li atoms. peak of the Li atom’s 1s orbital. This suggests a strong hybridization between B and Li atoms. A similar binding mechanism has also been confirmed in other metal-modified nanostructures [41]. A similar binding mechanism has also been confirmed in other metal-modified nanostructures [41]. In addition, it can be seen from the PDOS that the metal properties of β -borophene did not change In addition, it can be seen from the PDOS that the metal properties of β12-borophene did not change after modification of Li atom. after modification of Li atom. Figure 4. Partial density of states (PDOS) of Li-decorated β12-borophene system. Figure 4. Partial density of states (PDOS) of Li-decorated β12-borophene system. 3.2.2. Adsorption of H2 Molecules on Li-β12-Borophene 3.2.2. Adsorption of H2 Molecules on Li-β12-Borophene We investigated the adsorption properties of H2 molecules on Li-β12-borophene. Figure 5 shows We investigated the adsorption properties of H2 molecules on Li-β12-borophene. Figure 5 shows the optimized geometries of 1–7 H2 molecules adsorbed on the Li-modified β12-borophene. Table 1 the optimized geometries of 1–7 H2 molecules adsorbed on the Li-modified β12-borophene. Table 1 lists lists the adsorption energy and average adsorption energy calculated by the GGA PBE functional and the adsorption energy and average adsorption energy calculated by the GGA PBE functional and DFT-D methods. First, we investigate the adsorption sites of H2 molecules on Li-β12-borophene. For DFT-D methods. First, we investigate the adsorption sites of H2 molecules on Li-β12-borophene. For the the first adsorbed H2 molecules, many adsorption sites were considered in order to find the most first adsorbed H2 molecules, many adsorption sites were considered in order to find the most stable site. stable site. The most stable site involves H2 being parallel to the β12-borophene plane, which is The most stable site involves H2 being parallel to the β12-borophene plane, which is opposite to the H2 opposite to the H2 vertical adsorption on Ca-β12-borophene [17]. After adsorption, the corresponding vertical adsorption on Ca-β12-borophene [17]. After adsorption, the corresponding rH-H of the adsorbed rH-H of the adsorbed H2 is 0.756 Å, which is larger than the distance of free H2 (0.753 Å). To investigate H2 is 0.756 Å, which is larger than the distance of free H2 (0.753 Å). To investigate the maximum the maximum storage capacity of single Li atom-modified β12-borophene, more H2 was added around storage capacity of single Li atom-modified β12-borophene, more H2 was added around Li gradually. Li gradually. The minimum distance between the H and Li atom are range of 2.164 to 6.368 Å. The The minimum distance between the H and Li atom are range of 2.164 to 6.368 Å. The first four H2 first four H2 molecules were parallel to the β12-borophene and were around the Li atom at the same molecules were parallel to the β12-borophene and were around the Li atom at the same level. When the level. When the fifth H2 molecule was added to the system, two H2 molecules moved to an upper fifth H2 molecule was added to the system, two H2 molecules moved to an upper layer after relaxation. layer after relaxation. This may be due to the limited space around the Li atom and the repulsive This may be due to the limited space around the Li atom and the repulsive interactions between the interactions between the adsorbed H2. The average adsorption energy slowly reduced from −0.385 to adsorbed H2. The average adsorption energy slowly reduced from −0.385 to −0.210 eV/H2 due to the −0.210 eV/H2 due to the strong steric interactions between the adsorbed H2. Interestingly, the strong steric interactions between the adsorbed H2. Interestingly, the adsorption energy suddenly rose adsorption energy suddenly rose to –0.388 eV after the second H2 molecule was added to the system. to –0.388 eV after the second H2 molecule was added to the system. With an increase in the number of With an increase in the number of H2 molecules, the H2 molecules becomes further away from the Li H2 molecules, the H2 molecules becomes further away from the Li atom and the adsorption weakens. atom and the adsorption weakens. The average adsorption energy was at its minimum (−0.210 eV/H2) The average adsorption energy was at its minimum (−0.210 eV/H2) when the seventh H2 molecule when the seventh H2 molecule was adsorbed. At this time, the hydrogen storage capacity reached was adsorbed. At this time, the hydrogen storage capacity reached 5.90 wt %, which exceeded the ideal 5.90 wt %, which exceeded the ideal hydrogen storage capacity (over 5.5 wt %). In order to further increase the hydrogen storage capacity, we added two Li atoms to decorate the β12-borophene to adsorb H2 molecules. 2Li-β12-borophene can adsorb up to 14 H2 molecules and the minimum average adsorption energy is −0.220 eV. The hydrogen storage capacity can reach up to 10.85 wt %, which is larger than the hydrogen storage capacity with a gravimetric hydrogen density of 9.5 wt % of the Ca-β12-borophene/H2 system [17]. The optimized structure is shown in Figure 5h–n. The averagePDF Image | Li-Decorated Borophene as Potentia for Hydrogen Storage
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