Nanoscale PAPER. Carbon-tuned bonding method significantly enhanced the hydrogen storage of BN Li complexes. Dynamic Article Links C <

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1 Nanoscale Dynamic Article Links C < Cite this: Nanoscale, 2011, 3, Carbon-tuned bonding method significantly enhanced the hydrogen storage of BN Li complexes Qing-ming Deng, a Lina Zhao,* a You-hua Luo, c Meng Zhang, c Li-xia Zhao c and Yuliang Zhao* ab Received 6th July 2011, Accepted 10th September 2011 DOI: /c1nr10741k PAPER Through first-principles calculations, we found doping carbon atoms onto BN monolayers (BNC) could significantly strengthen the Li bond on this material. Unlike the weak bond strength between Li atoms and the pristine BN layer, it is observed that Li atoms are strongly hybridized and donate their electrons to the doped substrate, which is responsible for the enhanced binding energy. Li adsorbed on the BNC layer can serve as a high-capacity hydrogen storage medium, without forming clusters, which can be recycled at room temperature. Eight polarized H 2 molecules are attached to two Li atoms with an optimal binding energy of ev/h 2, which results from the electrostatic interaction of the polarized charge of hydrogen molecules with the electric field induced by positive Li atoms. This practical carbon-tuned BN Li complex can work as a very high-capacity hydrogen storage medium with a gravimetric density of hydrogen of 12.2 wt%, which is much higher than the gravimetric goal of 5.5 wt % hydrogen set by the U.S. Department of Energy for Introduction Hydrogen is very attractive as a clean energy source because of its efficiency, abundance and its use without generating any air pollution and greenhouse-gas emissions. 1 4 However, one of the most difficult challenges in realizing a hydrogen economy is synthesizing materials that can store hydrogen with high gravimetric and volumetric density at near ambient thermodynamic conditions. In the past, considerable attention has been focused on studying storage of hydrogen in nano-materials, because of the possibility of good reversibility, fast kinetics, and high capacity (large surface area) Recent studies showed that noncarbon nano-systems composed of light elements such as B and N offer many advantages to store hydrogen For instance, the BN layer is somewhat more resistant to oxidation than graphene, and thus more suitable for applications at room temperature, where graphene would be oxidized. 16 Furthermore, similar to carbon nanotubes, BN nanotubes are also regarded as possible hydrogen storage media. It has been found experimentally that the BN nanotubes can store as much as 2.6 wt% of hydrogen at 10 MPa. 17 Collapsed BN nanotubes exhibit a higher hydrogen storage capacity with 4.2 wt% of hydrogen. 18 However, the previous theoretical results indicated that perfect BN nanotubes could not adsorb hydrogen molecules effectively by a CAS Key Lab for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics and National Center for Nanoscience and Technology, Chinese Academy of Sciences (CAS), Beijing, , China. zhaoyuliang@ihep.ac.cn; linazhao@ihep.ac.cn b National Center for Nanoscience and Technology, Beijing, , China c Department of Physics, East China University of Science and technology, Shanghai, , China physisorption at room temperature and ambient pressure. 19 Rossato et al. suggested that BC 2 N nanotubes could serve as an effective hydrogen storage medium, and the binding energies for the H 2 molecules were slightly dependent on the adsorption site. 20 Previous experimental and theoretical studies have shown that doping alkali or transition metal (TM) atoms on BN and carbon systems could appreciably increase hydrogen uptake largely due to the enhanced adsorption energy of H 2 due to the metal However, TM atoms basically prefer being clustered to being individually dispersed on nano-materials because of the large cohesive energy of bulk TM (4.0 ev). 26,27 Moreover, strong interaction of TM and H 2 makes it difficult for the H 2 molecules to dissociate completely. For instance, the first hydrogen molecule introduced, which would form to H Ti bond, cannot be dissociated from the Ti atoms at the room temperature. 28 Ni and Rh atoms adsorb three and four molecules of hydrogen by chemisorption, respectively. 23 In this regard, alkali metal (AM) atoms have been proposed as possible alternatives to TM atoms, since the cohesive energy of AM is much smaller than that of TM. Among the AMs, Li seems to be promising since not only is it the lightest metal in the periodic table but also a positively charged Li ion in free space has been shown to store six hydrogen molecules with desirable adsorption energy. 29,30 However, the binding energies of Li on BN fullerene (0.725 ev) and BN tubes (0.09 ev) are much smaller than the cohesive energy of bulk Li (1.63 ev), which means Li atoms are easily aggregated. 24,31 33 Therefore, it is necessary to enhance the Li bonding on these BN materials to achieve a higher gravimetric density of hydrogen. Some novel BC and BNC mixed nanostructures have been widely researched. 20,30,34,35 Wu et al. investigated the interaction between 4824 Nanoscale, 2011, 3, This journal is ª The Royal Society of Chemistry 2011

2 Li atoms and boron carbide nanotubes. 36 They showed that the boron substitution greatly enhanced the binding energy of Li atoms to the nanotube. Thus, moderate substitutional doping in BN materials could also enhance the binding energy of Li to BN layer for hydrogen storage. Herein, we present systematic theoretical studies on hydrogen storage media that use doping carbon atoms on BN monolayer to enhance the bonding of Li atoms. We focused on the following questions: (i) Can the strength of the bonding between Li atoms and BN monolayers be enhanced by doping with C atoms, and where do Li atoms bind and distribute on the BNC layer? (ii) How strong is the binding energy of Li on the BNC layer, and what is the mechanism of the bonding? (iii) How strong is the adsorption of hydrogen with Li atoms and what is the hydrogen gravimetric density of the BNC Li complex? We showed that Li atoms could be bound strongly on both sides of a BNC monolayer at high adsorbing concentrations and high temperature due to donating their electrons to the substrate and s p and p p hybridization. Clustering of Li atoms is suppressed on BNC layers, and the Li atom adsorbs multiple H 2 molecules with a desirable binding energy of ev. This BNC Li complex has the following advantages over existing hydrogen storage materials: (i) BNC Li complex can reach the gravimetric capacity of 12.2 wt % hydrogen, which is much higher than the gravimetric goal of 5.5 wt % hydrogen set by the U.S. Department of Energy for (ii) The BN complex is more resistant to oxidation than graphene, and thus more suitable for applications at room temperature. (iii) Because BNC systems have been successfully synthesized, 38,39 and the B or N atoms on the BN layer can be displaced by C atoms through controlled radiation damage, 40,41 so BNC Li complexes are very promising to be synthesized experimentally. Methods The first-principles calculations were performed using spinpolarized density functional theory (DFT) implemented in DMOL3 code. The exchange correlation terms were considered by the Perdew Berke Ernzerhof functional of the generalized gradient approximation (GGA). 42,43 Under the periodic boundary condition, the interlayer distance was set at 25 A, which was large enough to minimize the artificial interlayer interactions. Fully relaxed geometries were obtained by optimizing all atomic positions until the energy, maximum force, maximum displacement were less than Ha, Ha A 1 and A, respectively. A double numerical-polarized basis set was employed, which included all occupied atomic orbitals with a second set of valence atomic orbitals plus polarized d-valence orbitals. The k-points samplings were 3 3 2in the Brillouin zone. The accuracy of the PBE/DND combination was evaluated through the following data. 44 The bond length and the binding energy of the H 2 molecule parallel to its axis were calculated to be A and 4.47 ev, respectively, which agreed well with corresponding experimental values of A and 4.45 ev. The lattice constant of the BN layer has been optimized and the calculated value of A is also in good agreement with the experimental value of A. Results and discussion In order to search for hydrogen storage nano-materials consisting of BN monolayers and Li atoms, we first considered three states of carbon doping with BN layers. Fig. 1 shows the top views of optimized 4 4 cells of BNC N, BNC B, and BNC BN models. For BNC N and BNC B, a single C atom substitutes a N or B atom randomly in a supercell. For BNC BN, we used two Fig. 1 (a), (b) and (c) show the three different states of BNC layers and five possible adsorption sites for Li atoms. (a1), (b1) and (c1) show the total energy of the five optimized structures of Li atoms on BNC N, BNC B and BNC BN, respectively, where the energy of the lowest energy configuration is set to zero. The insets of (a1), (b1) and (c1) show the lowest energy configurations. d1 is the distance between Li atoms and BNC layers. This journal is ª The Royal Society of Chemistry 2011 Nanoscale, 2011, 3,

3 carbon atoms to replace one boron and one nitrogen atom randomly. After optimizing all possible structures of the 4 4 BNC BN layer, we found that the carbon atoms prefer to stay together rather than to disperse on the BNC BN layer. Firstly, we focused on the site preference of Li on one side of the BNC layers with three states as shown in Fig. 1. Structural 4 4 BNC N optimization shows that the hollow site 2 is the most favorable adsorption site for Li atoms among the five sites, which is A above the layer with a binding energy (E e ) of 5.22eV. Li atoms prefer to stay on the site above 2 of the C atoms for both of BNC B and BNC BN, which are A and A between Li and substrates with E e values of 2.80 and 2.28eV Fig. 2 (a) (d) Red and green spots represent the most favorable adsorption sites on the two sides of the 4 4 pristine BN, BNC N, BNC B and BNC BN layers. (a1) (d1) show the difference charge densities of two Li atoms on pristine BN, BNC N, BNC B and BNC BN layers. Red and green isosurfaces ( electrons/ A 3 ) correspond to accumulation and charge depletion regions. d1 and d2 are the distances between two Li atoms and the substrates. (a2) (d2) show projected density of states (PDOS) of BN and BNC Li complex. The blue and red lines represent the Li 2s and 2p states, respectively Nanoscale, 2011, 3, This journal is ª The Royal Society of Chemistry 2011

4 respectively. A denser Li coverage, which is energetically more favorable, is attained if one Li is adsorbed on each 2 2 cell of BNC layers with a Li Li distance of A. In this dense 2 2 coverage, the adsorption sites are the same as the dense 4 4 coverage, but the binding energies decrease, which are 4.55, 1.97 and 1.43eV for BNC N, BNC B and BNC BN, respectively. We also calculated the binding energy of a pristine BN layer coated with Li atoms. The result shows that the E e values are 1.97 and 0.85 ev for the coverage 4 4 and 2 2, respectively, which indicate that doping carbon atoms on the BN layer can significantly enhance the Li bond on this material. Secondly, we considered the double-sided adsorption of Li atoms. We introduced the second Li atom to the other side of the pristine BN and BNC layers in order to investigate how successive Li atoms will be distributed. In Fig. 2(a) (d), structural optimizations show that the second Li atom on BNC N tends to stay on the site above the C atom, while Li prefers to reside on the site above the N atoms on pristine BN and BNC B layers. For BNC BN, the Li atom stands on the second carbon atom. We noted the distances between two Li atoms and the substrates as d1 and d2, respectively. The d1 and d2 of the BN layer are and A, which are much longer than for BNC Li systems, indicating their weak binding energy for Li above the layer. The average binding energies ( E) of4 4(2 2) BN, BNC N, BNC B and BNC BN with Li atoms are 0.96 ev (0.67 ev), 2.85 ev (2.35 ev), 1.82eV (1.35 ev) and 1.93 ev (1.14 ev), respectively. We noticed that the pristine BN layer s E with Li atoms was much smaller than the cohesive energy of bulk Li (1.63 ev) which meant that Li would aggregate together instead of dispersing on the layer with high concentration of Li. By contrast, Li atoms can be seized strongly by the carbon doped BN layer and distribute much more densely than on the pristine BN layer on both sides, which is due to the larger E. The average binding energy of BNC N and Li atoms is the largest one in the BNC Li systems. By increasing Li coverage from 4 4to22, repulsion between positively charged Li atoms becomes stronger, thus their E decreases by about ev. Our results showed that stable and uniform coverage of Li atoms on the BNC layers (reaching Q ¼ 25%) could be attained for double-sided adsorption forming 2 2 pattern. It is found in this dense 2 2 coverage, the E of BNC B and BNC BN are smaller than the cohesive energy of bulk Li, which can make the Li atoms form clusters. To verify the possibility of clustering on the Li coated BNC B and BNC BN layers, we have tested the systems by doping the third and fourth Li atoms near the metal sites and optimized the system without any geometrical restrictions. The optimized structures are shown in Fig. 3(a) and (b). The optimized geometry shows that Li atoms undergo clustering to form dimmers. The Li Li distance of BNC B in optimized structures is A, which is much longer than for the Li dimers (2.673 A). 45 Therefore, the Li atoms would not form clusters in 2 2 pattern. However, the bond lengths of Li dimers (2.735 A) on the BNC BN monolayer are very close to those of the Li dimers. Hence, when a denser Li coverage is attained on a 2 2 cell of BNC BN, the Li dimers are energetically more favorable. To elucidate the numerical results presented above, we turned to the electronic structure analyses of the cases either without or with C doping. Hirshfeld charge analysis is shown in Table 1. The result indicates that the Li dopant becomes a cation of BN Li and BNC Li complex, whereas all C atoms carry negative charges. When some of the B and N atoms are substituted by C atoms, the Li atoms donate the largest amount of electrons to C and carry and e with the 4 4 coverage respectively. However, for the BNC N Li complex, less electrons will be transferred from the Li atoms to the BNC N substrate. For the pure BN monolayer, the charges carried by Li atoms are only e. By increasing Li coverage from 4 4to2 2, adsorbed Li atoms become less positively charged for the BNC B and BNC BN, but the charges of Li for BN and BNC N are nearly the same as for the 4 4 coverage. We have also performed charge transfer analysis for the Li atoms on BN and BNC layers. Fig. 2 (a1), (b1), (c1) and (d1) show their charge differences, defined as Dr ¼ r Li/substrate r substrate r Li. Here r Li/substrate, r substrate and r Li represent the charge densities of the Li covering on BN and BNC layers, the BN or BNC layers and Li freestanding state, respectively. The red isosurface corresponds to an electron accumulation zone, while the green one is an electron depletion zone. Clearly, Li atoms donate more their electrons to the BNC substrates rather than to pristine BN ones. It is observed that charges near Li atoms are depleted and are increased near C, and B atoms such that the sides near Li atoms attain partial positive charge and the substrates attain partial negative charge. In contrast, there is a slight charge accumulation between the adsorbed Li and pristine BN layer. Fig. 2 (a2), (b2), (c2) and (d2) show the projected density of states (PDOS) of BN Li, BNC N Li, BNC B Li, BNC BN Li, respectively. It shows that an electron depletion of the Li s orbital occurs in Fig. 2 (a2), indicating that the Li atom on pristine BN layer is totally caused by the ionic interactions. Fig. 2 (b2), (c2) and (d2) clearly show that the increased states of Li 2p as compared with (a2) are due to the 2p orbitals receiving more electrons from the highest occupied molecular orbital of BNC complex. S p and p p hybridization of Li atoms appear when carbon atoms are doped on the BN layer. Based on the above analysis, it can be concluded that the type of bond between Li and BNC layers is predominantly ionic and partially covalent, which is responsible for the enhanced binding energy of the BNC Li complex. Finally, we discuss hydrogen adsorption on BNC Li complex. The first H 2 introduced to the 2 2 BNC layers remains in the form of a hydrogen molecule. The H H bond length expands from to A due to the polarization interaction between the Li ion and the H 2. We noticed that the positive Li ion could produce the stronger local electric field to significantly improve the adsorption of H 2 (more than the neutral Li atoms do). Therefore, we further calculated the average binding energy (E b ) of multiple H 2 molecules on the Li atom adsorbed on BNC Li Fig. 3 The optimized geometric structures for the Li dimmers on (a) the BNC B monolayer and (b) BNC BN. The bond lengths of dimmer Li are named as d. This journal is ª The Royal Society of Chemistry 2011 Nanoscale, 2011, 3,

5 Table 1 The Hirshfeld charge analysis for the BNC-Li complex of the 4 4(2 2) cell Complex BN Li 2 BNC N Li 2 BNC B Li 2 BNC BN Li 2 Charge on the first Li 0.056(0.054) 0.233(0.228) 0.338(0.173) 0.408(0.304) Charge on the second Li 0.056(0.062) 0.161(0.165) 0.207(0.112) 0.317(0.256) Charge on the Carbon NA 0.290( 0.296) 0.066( 0.052) 0.268/ ( 0.269/ 0.099) Fig. 4 The optimized hydrogen binding configurations at the maximum number of H 2 adsorbed on the 2 2 BNC N, BNC B and BNC BN, respectively. The red and green bars represent the calculated H 2 average binding energies (E b ) of the n (1 4) H 2 absorbed by each Li atom on the two sides of the substrate with the 4 4 and 2 2 coverage respectively, where g d is the gravimetric density of hydrogen storage with the 2 2 coverage Nanoscale, 2011, 3, This journal is ª The Royal Society of Chemistry 2011

6 complex. Fig. 4 shows the 2 2 optimized atomic structures for the configuration with the maximum number of adsorbed H 2 molecules on the each Li atom attached to the BNC N, BNC B and BNC BN. It appears that a Li dimer can hold four hydrogen molecules in the 2 2 BNC BN layer. For the other complex systems, the calculated E b of the H 2 molecules on the Li atom attached to the substrate is around ev/h 2 with the GGA method as shown in Fig. 4 (a), (b) and (c). The GGA exchange correlation usually underestimates the binding energy by 0.08 ev/h 2, because it does not consider van der Waals interactions. 46 Thus, our predicted binding energy should be around ev/h 2. By using the Langmuir equation, Bhatia and Myers investigated the optimum thermodynamic conditions for hydrogen adsorption and found the average optimal adsorption enthalpy should be 15.1 kj mol 1, if operated between 1.5 and 30 bar at 298 K. When the pressure is increased to 100 bar, the optimal value becomes 13.6 kj mol 1. The adsorption energy per H 2 within the ideal energy window of ev should give a satisfactory release temperature. 47 Therefore, the BNC Li complex in our study should give an acceptable hydrogen release temperature with the E b within the ideal energy window of ev. Furthermore, the gravimetric capacity of hydrogen of BNC N, BNC B and BNC BN can reach 11.6, 12.2 and 10.5wt %, respectively. Conclusions In conclusion, using density functional theory, we found doping carbon atoms on the BN monolayer could significantly strengthen the Li bond on the BN layer. There are two reasons for the enhanced binding energy. Firstly, when doping some carbon atoms on the BN layer, they will break the p-bonding network to provide available frontier orbitals to receive more electrons from Li. Secondly, s p and p p hybridization of Li atoms appears above the BNC layer when carbon atoms are doped. We also found that each Li atom could absorb four hydrogen molecules, so at full coverage the BNC Li complex can yield a H 2 storage capacity of 12.2 wt%. This BNC Li complex has some advantages over the existing ones. The BN complex is more suitable for applications at room temperature. More importantly, the BNC Li complex is very promising to be synthesized experimentally. At the same time, Li adsorbed on the BNC layer can serve as a high-capacity hydrogen storage medium, without forming clusters, which can be recycled at room temperature. Therefore, we hope that the theoretical prediction here will encourage experimental studies for searching for reversible hydrogen storage materials. Acknowledgements This work was supported by the National Basic Research Program of China (973 program: 2011CB933400, 2010CB and 2010CB933600), and CAS Knowledge Innovation Program. References 1 J. Alper, Science, 2003, 299, R. D. Cortright, R. R. Davda and J. A. Dumesic, Nature, 2002, 418, L. 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