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J. Phys. Chem. C 2007, 111, 12124-12128
Role of Lithium Vacancies in Accelerating the Dehydrogenation Kinetics on a LiBH4(010) Surface: An Ab Initio Study A. J. Du,†,‡ Sean C. Smith,*,†,‡ X. D. Yao,‡ and G. Q. Lu‡ Centre for Computational Molecular Science and ARC Centre for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The UniVersity of Queensland, Brisbane Queensland 4072, Australia ReceiVed: May 27, 2007
In this work, ab initio density functional calculations were performed to explore the effect of surface lithium vacancies on the initial dehydrogenation kinetics of lithium borohydride. We found that some B-H bonds in neighboring BH4-1 complexes around the vacancy became elongated (weakened). The activation barriers for the recombination of H atoms to form H2 were decreased from 3.64 eV for the stoichiometrically complete LiBH4(010) surface to 1.53 and 0.23 eV in the presence of mono- and di-vacancies, respectively. Our results indicate that the creation of Li vacancies may play a critical role in accelerating the dehydrogenation kinetics of LiBH4.
Introduction To develop a viable hydrogen storage system is becoming increasingly important for promoting the hydrogen economy. Among various hydrogen storage materials currently under study,1-9 complex hydrides8-9 have attracted considerable interest since the discovery by Bogdanovic and Schwickardi that a small amount of TiCl3 dopant into NaAlH4 could facilitate accelerated and reversible hydrogen release under moderate conditions.9 The release of hydrogen could in principle reach 5.6 wt %, occurring via two steps.9-10 The U.S. Department of Energy has targeted 6.5 wt % for reversibly adsorbed hydrogen for many applications including the automotive industry. Apparently, the storage capacity in sodium alanate is not high enough for the practical application in transportation at atmospheric temperature and pressure conditions. More recent experimental efforts have been directed toward LiBH4,11 which has a higher volumetric hydrogen density (18.5 wt %).12 However, BH4-1 is highly stable, thus necessitating high decomposition temperatures.13 Consequently, there has been considerable interest in finding possible destabilization strategies that would facilitate the release of molecular hydrogen at lower temperatures. Experimentally, the addition of SiO2 to LiBH4 was found to liberate 9 wt % hydrogen starting from 523 K through the reaction LiBH4 f LiH + B + H2.14 More recently, Au et al. reported the destabilization of LiBH4 by using metals, metal hydrides, and metal chlorides. The most effective composite material in their study was LiBH4 + 0.2MgCl2 + 0.1TiCl3, which starts desorbing 5 wt % of hydrogen at 60 °C and can be rehydrogenated to 4.5 wt % at 600 °C.17-18 Theoretically, Miwa et al. have proposed that partial substitution of Li+ cations with more electronegative elements, such as Mg or Cu, may be an effective way of lowering the dehydrogenation temperature of LiBH4.15-16 Our recent calculations have shown that the neutral BH4 complex is highly unstable and will result in the formation of a hydrogen molecule.19 * Corresponding author. Fax: 617-33654623; e-mail:
[email protected]. † Centre for Computational Molecular Science. ‡ ARC Centre for Functional Nanomaterials.
Since there is no experimental evidence to show that such large scale mass transport occurs via diffusion of molecular hydrogen from the bulk, a computational exploration of recombinative desorption of H2 from the surface is a logical next step. The bond between Li+ and (BH4)-1 in LiBH4 is well-known to be highly ionic. To suppress the charge transfer between them could significantly weaken the ionic bond interactions within the BH4 complex, effectively making the hydrogen more labile. Creating a neutral Li vacancy should be the most effective way since the anionic charge on the surrounding BH4 complexes is thereby reduced. The energy for the formation of a single Li vacancy is calculated to be as high as 3.77 eV. However, it is not unreasonable to suggest that lithium vacancies could be produced during the high-energy ball milling process. Additionally, surface catalysts such as TiCl3, etc. may also help to form a vacancy through chemical reactions as experimentally observed in sodium alanate.20-21 A very important objective of this work is therefore to understand the intrinsic role of surface Li vacancies in the desorption phase of the hydrogen recycling process on the LiBH4 surface. Improved understanding of these underlying mechanisms will assist in the rational design of improved materials for practical automotive applications. Computational Methods Our calculations were performed using the plane-wave basis VASP code22-23 implementing the generalized gradient approximation (GGA) of the PBE exchange correlation functional24 and the projector augmented wave method.25-26 The LiBH4(010) surface was modeled by using a (3 × 3) surface unit cell with 108 LiBH4 units containing 648 atoms. We note that the present model system is much larger than other theoretical studies to date. A gamma point was used for the Brillouin zone sampling, and the cutoff energy for plane waves was 320 eV. The vacuum space was up to 16 Å, which is large enough to guarantee a sufficient separation between periodic images. We should also note here that all distances between periodical images of vacancies exceeded 15 Å, and hence, longrange vacancy-vacancy interactions are assumed negligible. To determine the activation barriers and minimum energy paths
10.1021/jp074096w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
Dehydrogenation Kinetics on a LiBH4(010) Surface
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Figure 1. Optimized geometries for (a) clean, (c) mono-vacancy incorporated, and (e) di-vacancy incorporated LiBH4(010) surfaces. Panels b, d, and f are the corresponding B-H bond length distributions within BH4-1 anion complexes around the vacancy. Red, green, small white, and yellow balls represent Li atoms, B atoms, H atoms, and H atoms to be removed to form one hydrogen molecule at a far distance, respectively.
(MEPs) for the recombination of molecular hydrogen from the LiBH4(010) surface, the nudged elastic band (NEB) method was used.27-28 This method involves optimizing a chain of images that connect the reactant and product state. Each image is only allowed to move into the direction perpendicular to the hypertangent. Hence, the energy is minimized in all directions except for the direction of the reaction path. A damped molecular dynamics was used to relax ions until the force in each image was less than 0.02 eV/Å. Results and Discussion The LiBH4(010) surface was chosen for this work since it has a lower surface energy than the (001) and (110) surfaces.29 Figure 1a presents the optimized geometry for a clean LiBH4(010) surface, and the yellow balls represent the two H atoms that are found subsequently to recombine into molecular hydrogen from the surface. Figure 1b presents the bond length distribution
of the BH4-1 complexes shown in Figure 1a. The B-H bond length in bulk LiBH4 is shown to be constant.13,30 However, some B-H bonds within the top layer BH4-1 complexes on the LiBH4(010) surface become elongated, and others are shortened. This can be attributed to the reduction of anion charge in the surface BH4-1 complexes. To explore the effect of a surface Li vacancy, one and two Li atoms were removed on the LiBH4(010) surface, respectively. Geometry optimizations were then performed by using a conjugated gradient method. Figure 1c,e presents the final configurations for the LiBH4(010) surface in the presence of mono- and di-vacancies, respectively. Clearly, the BH4-1 complexes adjacent to the vacancy(s) are strongly distorted. This effect is much more pronounced in the case of Li di-vacancies. Additionally, one notes that two of the H atoms approach each other to separations of 1.03 and 0.87 Å, respectively, for the mono- and di-vacancy systems, respectively (see the yellow balls shown in Figure 1c,e). The B-H
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Figure 3. Minimum energy profiles for the recombination of two H atoms from clean, Li mono-vacancy, and Li di-vacancy LiBH4(010) surfaces to form one hydrogen molecule at a large separation from the surface.
we began to optimize the geometries with two H atoms either within the same or at different BH4-1 complexes removed. The removal energy was obtained by calculating the total energy difference as shown in the following equation:
Eremoval ) Eslab + EH2 - Eslab-2H
Figure 2. Optimized structures for (a) clean, (b) Li mono-vacancy, and (c) Li di-vacancy LiBH4(010) surfaces after removing two H atoms to form one hydrogen molecule at a large separation from the surface. Red, green, small white, and yellow balls represent Li atoms, B atoms, H atoms, and H atoms within B2H6-like structures, respectively.
bond length distributions around the mono-vacancy and divacancies were also analyzed and are plotted in Figure 1d,f. It can be seen clearly that some B-H bond lengths are elongated (weakened). The activation barrier for the recombinative dissociation of two H atoms from the surface to form molecular hydrogen in the gas phase thus should be decreased. Proceeding from this point, we studied the structure and energetics for the removal of two H atoms on the clean LiBH4(010) surface to form one hydrogen molecule at a large separation from the surface. On the clean LiBH4(010) surface,
where Eslab, EH2, and Eslab-2H represent the total energies for the optimized slab geometry, one hydrogen molecule, and optimized slab with two hydrogen atoms removed, respectively. The removal energies of hydrogen were calculated to be 1.35 eV for two H atoms within the same BH4- complex and 3.52 eV for two H atoms from adjacent BH4-1 complexes. Figure 2a presents the final configuration after two H atoms at the same BH4-1 complexes were removed from the clean LiBH4(010) surface. In a similar procedure as for the clean LiBH4(010) surface, two H atoms were then removed from the LiBH4(010) slab in the presence of mono- and di-vacancies, and the remaining structures were optimized as shown in Figure 2b,c. The final geometry structures after removing two H atoms are quite similar. BH2 will move to one neighboring BH4-1 complex, and one H atom in BH4-1 then transfers to BH2 and forms two BH3-like structures. A vertical B2H6-like structure31 is finally formed after the loss of H2 from the clean LiBH4(010) surface, while a B2H6-like structure parallel to the surface is formed at the surface after the loss of H2 in the presence of vacancies. Importantly, the removal energies became much smaller (0.08 and -0.453 eV for the mono- and di-vacancy systems, respectively). On the basis of the substantial effect of Li vacancies on the H2 removal energies obtained previously, the MEPs for the desorption of H2 molecules from the clean and defect LiBH4(010) surfaces were computed. In our NEB calculation, the ISs were chosen from the optimized geometry as shown in Figure 1a,c,e (two yellow balls indicate two H atoms to be recombined into a hydrogen molecule). The FSs were set as the geometries shown in Figure 2a,b,c plus one hydrogen molecule at a distance 4.5 Å from the LiBH4(010) surface. A chain of images was then built lying between ISs and FSs and minimizing the appropriate effective force acting on each image. The energy profiles, as revealed by our NEB calculations, are reported in Figure 3. The effective activation barriers were calculated
Dehydrogenation Kinetics on a LiBH4(010) Surface
J. Phys. Chem. C, Vol. 111, No. 32, 2007 12127 (see Figure 4b), respectively. Hence, relative to a clean LiBH4(010) surface, the substitution of Ti does appear to result in the surface hydrogen becoming more labile with respect to recombinative desorption. The impact is, however, not as great as demonstrated previously for Li vacancies (cf. removal energies of 0.08 and -0.453 eV for single- and di-vacancies, respectively). As our present purpose is to explore the role of vacancies on the desorption kinetics of H2 from a LiBH4 surface, further computational exploration of substitution effects is reserved for a subsequent study. Conclusion In summary, ab initio density functional calculations have been performed to explore the impact of neutral lithium vacancies in a LiBH4(010) surface on the initial dehydrogenation kinetics. Our results reveal clearly that certain B-H bonds within BH4-1 complexes adjacent to the vacancy become elongated and weakened, causing the corresponding hydrogen atoms to be more labile with respect to recombinative desorption. As compared with the clean surface, the activation barriers for the recombination of two H atoms from the LiBH4(010) surface to form H2 in the presence of mono- and di-vacancies are decreased from 3.64 to 1.53 and 0.23 eV, respectively. These results suggest that surface Li vacancies may potentially play an important mechanistic role in accelerating the kinetics of dehydrogenation from LiBH4. Acknowledgment. We acknowledge generous grants of high-performance computer time from both the Computational Molecular Science Cluster Computing Facility at The University of Queensland and the Australian Partnership for Advanced Computing (APAC) National Facility. The authors also greatly appreciate the financial support of the Australian Research Council and The University of Queensland through the ARC Centre for Functional Nanomaterials. References and Notes
Figure 4. (a) Optimized geometry computed after substitution of a Ti atom for a surface Li atom and (b) cut-out local structure around the Ti atom and the corresponding bond lengths. Red, green, small white, and gray balls represent Li atoms, B atoms, H atoms, and Ti atoms, respectively.
to be 3.64, 1.53, and 0.23 eV for the recombination of a hydrogen molecule from the clean, mono-vacancy, and divacancy surfaces, respectively. Clearly, the activation barrier becomes much smaller in the presence of Li vacancies at the LiBH4(010) surface. This indicates that the desorption kinetics should be improved greatly in the presence of vacancy defects at the LiBH4(010) surface. As reported in ref 20, the substitution of a Na atom with a Ti atom on the sodium alanate surface can be effective in promoting dehydrogenation. To explore whether a similar effect may be possible in the present system, a Li atom was substituted with a Ti atom on the LiBH4(010) surface, and the relaxed structural properties were computed. Figure 4a presents the final geometry optimized by a conjugated gradient method. The local structure around the Ti atom was very similar to Ti(BH4)3- as reported in ref 32. Clearly, the neighboring BH4-1 units around the Ti atom are distorted. The detailed B-H bond lengths within the BH4-1 units are given in the cut-out local structure as shown in Figure 4b, with some B-H bonds clearly elongated (weakened). The energies for removing two H atoms on the elongated B-H bond pairs from the surface were calculated to be 0.98, 1.38, and 1.16 eV for H3 and H4, H5 and H6, and H9 and H11
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