Synergetic Effect of in Situ Formed Nano NbH and LiH1âxFx for

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Synergetic Effect of in Situ Formed Nano NbH and LiH1−xFx for Improving Reversible Hydrogen Storage Properties of the Li−Mg− B−H System Xuezhang Xiao, Liuting Zhang, Xiulin Fan, Leyuan Han, Jie Shao, Shouquan Li, Hongwei Ge, Qidong Wang, and Lixin Chen* Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A significant improvement in hydrogenation/dehydrogenation properties of 2LiH/MgB2 can be achieved by adding NbF5. The results show that the NbF5 additive is effective for enhancing the de/hydrogenation kinetics of the Li−Mg−B−H system and reducing the desorption temperatures of MgH2 and LiBH4. For the 2LiH− MgB2−0.03NbF5 sample, About 9.0 wt % hydrogen capacity is obtained rapidly under cyclic conditions of rehydrogenation within 20 min at 350 °C and dehydrogenation within 20 min at 400 °C; thus, catalytic improvement persists well in the subsequent reversible dehydrogenation cycles. Moreover, the sample could reversibly reabsorb and release more than 9.0 wt % hydrogen even at 250 and 375 °C, respectively. Microstructure analyses reveal that the NbF5 additive in improving the de/hydrogenation properties of Li−Mg−B−H system could be ascribed to the synergistic effect of in situ formed nano NbH particles acting as “active gateways” facilitating the diffusion of hydrogen, and the “favorable thermodynamic destabilization” from the reversible transition of LiH1−xFx caused by functionality of F-anion substitution. This fundamental understanding provides us with insights into the design and optimization of the catalytic method and species for the catalyzed Li−Mg−B−H system.

1. INTRODUCTION Hydrogen is considered to be one of the most important energy carriers for future mobile applications due to the advantages of high energy density, high elemental abundance, and environmentally benign product.1,2 Among hydrogen storage media, solid-state hydrides offer several advantages with respect to high volumetric hydrogen capacity, compactness, and safety despite its relatively low gravimetric hydrogen capacity.2 Currently, many research groups have paid more attention to light metal complex hydrides because of their high volumetric and gravimetric hydrogen capacities which are suitable for fuel cell vehicles. Lithium borohydride (LiBH4) is regarded as one of the promising candidates for safe and efficient hydrogen storage materials due to its high reversible hydrogen storage capacity (13.8 wt % H2). However, the strong and highly conjunct covalent and ionic bonds of LiBH4 extremely impose its problematic H-exchange thermodynamics and kinetics for reversible hydrogen storage.3,4 Up to now, a variety of novel approaches have been developed to address the thermodynamics and kinetics limitations of LiBH4, including nanoporous scaffolds,5−7 partial cation/anion substitution,8−11 thermodynamic destabilization of LiBH4 by transition metal oxide, fluoride, and hydride.12−17 The thermodynamic destabilization of LiBH4 displays great potential for adjusting © XXXX American Chemical Society

de/hydrogenation thermodynamics and enhancing its kinetics. Vajo et al.18 reported that the dehydrogenation enthalpy of 2LiBH4 + MgH2 is −46 kJ mol−1 H2, which is around 21 kJ mol−1 H2 lower than that of pure LiBH4. The theoretical hydrogen capacity of this Li−Mg−B−H system is still as high as 11.8 wt %, which undergoes the reversible hydrogen storage reactions under appropriate H2 back pressure by reaction 1.19 2LiBH4 + MgH2 ↔ 2LiBH4 + Mg + H 2 ↔ 2LiH + MgB2 + 4H 2

(1)

Although the reaction enthalpy of 2LiBH4/MgH2 is lowered, dehydrogenation process still occurs at high temperature with a relatively slow two-step kinetics, even doped with a few TiCl3 catalyst.18 Therefore, extensive efforts have been mainly focused on improving de/hydrogenation rate and reducing reaction temperature for the Li−Mg−B−H system by exploring high-performance catalysts. Sun et al. doped Nb2O5 into the 2LiBH4/MgH2 system and found Nb2O5 could significantly improve the de/hydrogenation kinetics due to the formation of Received: April 16, 2013 Revised: May 15, 2013

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dx.doi.org/10.1021/jp403766p | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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NbH2, which facilitates hydrogen diffusion.13 Recently, a series of Ti-based additives (such as Ti, TiH2, TiB2, TiCl3, TiF3, TiF4, TiO2, TiN, and TiC) were investigated for their effects on the improvements of de/hydrogenation properties of the LiH/ MgB2 mixture, and these additives were mostly responsible for decreasing hydrogen desorption temperature and increasing reaction kinetics of the Li−Mg−B−H system.20,21 Particularly, Wang et al. further demonstrated the advantage of TiF3 over its analogue TiCl3 on the decrease of desorption temperature of the Li−Mg−B−H system, resulting from F-substitution in the LiBH4 lattice.22 Quite recently, we investigated the effects of transition-metal fluoride additives (NbF5, TiF3, CeF3, LaF3, and FeF3) on dehydrogenation properties of the Li−Mg−B−H system.23 Among these transition-metal fluoride additives, NbF5 exhibited the most prominent performances in terms of low desorption temperature and fast kinetics for the Li−Mg− B−H system.23,24 Moreover, Mao et al. also found that NbF5 could improve the cycling de/rehydrogenation kinetics of 2LiBH4/MgH2, resulting from the suppression of forming Li2B12H12.25 Goudy et al. also confirmed that NbF5 is vastly superior to Nb2O5 and Mg2Ni for catalyzing the hydrogen desorption rates of the 2LiBH4/MgH2 system.26 However, the de/hydrogenation kinetics of the Li−Mg−B−H system is still relatively slow and the catalytic mechanism of NbF5 is not yet clear. A detailed understanding of the roles of NbF5 transitionmetal additive is indispensable for further improvements in reversible de/hydrogenation of the Li−Mg−B−H system. In addition, as far as we know, very few papers reported the direct catalytic enhancement of dehydrogenated produces (2LiH/ MgB2) in the Li−Mg−B−H system. Here we report a large degree of improvement in reversible de/hydrogenation properties of the Li−Mg−B−H system by doping a 2LiH/MgB2 mixture with NbF5 catalyst precursor, which provides a novel synergetic catalytic effect of in situ formed nano NbH and LiH1−xFx.

spectroscopy was performed to examine the microstructure and elemental distribution of the samples. A special sample holder filled with argon was used to prevent the air and moisture contamination during transfer and measurement. The differential scanning calorimetry and mass spectrometer (DSC-MS) measurements were conducted on a synchronous thermal analysis (Netzsch STA 449F3 analyzer/Netzsch Q403C mass spectrometer) with a heating rate of 3 °C/min from room temperature to 500 °C under flowing argon condition (high purity, 50 mL/min). Before DSC-MS measurements, all of the samples were previously hydrogenated at 350 °C with an initial hydrogen pressure of 8 MPa for 15 h. Hydrogen absorption/desorption properties of the samples were quantitatively examined with volumetric method by a carefully calibrated Sievert’s type apparatus. The as-synthesized samples were rehydrogenated at 250−350 °C and dehydrogenated at 355−400 °C with an initial hydrogen back pressure of 8 and 0.4 MPa, respectively. During the typical cyclic experiment, before dehydrogenation, all of the samples were previously hydrogenated at 350 °C with an initial hydrogen back pressure of 8 MPa for 15 h; before hydrogenation, all samples were subsequently dehydrogenated at 400 °C with an initial hydrogen back pressure of 0.4 MPa for 18 h. For comparison, the weight percent of hydrogen capacity was calculated on the basis of the total weight of the samples including the 2LiH−MgB2 and additives.

3. RESULTS AND DISCUSSION The DSC-MS characteristics of hydrogenated 2LiH−MgB2 with and without fluoride additives are shown in Figure 1. All

2. EXPERIMENTAL SECTION Commercial LiH (98%), MgB2 (99%), NbF5 (98%), NbCl5 (99%), LiF (99.99%), NbB2 (99%), and Nb (99.9%) powders were all purchased from Sigma-Aldrich Corp. and used without further purification. The NbH was synthesized by ball-milled Nb powder in the Planetary mill under a hydrogen pressure of 1 MPa for 3 h, which XRD pattern is shown in Figure S1. Approximately 1.5 g mixture of LiH/MgB2 with a mole ratio of 2:1 was ball-milled under Ar atmosphere for 6 h at 400 rpm in a stainless steel vial together with 60 g steel balls. For reference, the 2LiH−MgB2 doped with different additives (NbF5, NbCl5, LiF, NbH, and NbB2) were ball-milled under identical conditions. The ball-milling process was paused for 0.1 h every 0.2 h to avoid an increasing temperature of the sample. The samples of neat 2LiH−MgB2 and 2LiH−MgB2 doped with different catalytic additives (NbF5, NbCl5, LiF, NbH, and NbB2) were prepared through ball-milling method. The MgB2 and LiH main phases are detected in all XRD patterns of ballmilled samples (Figure S2). All experimental operations were performed in a high-pure Ar-filled glovebox, which was equipped with a recirculation system to keep the H2O and O2 levels below 1 ppm. The X-ray diffraction (XRD) experiments of the samples were performed on an X’Pert Pro (PANalytical, Netherlands) with Cu Kα radiation at 40 kV and 40 mA with the step size of 0.02° from 10° to 90° (2θ). Scanning electron microscopy (SEM, Hitachi SU-70) equipped with energy dispersive X-ray

Figure 1. DSC-MS results of (a) neat 2LiH−MgB2, (b) 2LiH−MgB2− 0.01NbF5, and (c) LiH−MgB2−0.03NbF5 samples after hydrogenated at 350 °C with an initial hydrogen pressure of 8 MPa for 15 h.

DSC curves exhibit four distinct endothermic peaks, which correspond to the structure transition of LiBH4 (113.6 °C), melting of LiBH4 (250−280 °C), decomposition of MgH2 (280−350 °C), and decomposition of LiBH4 (355−460 °C), respectively.19 MS results demonstrate that the gas released from all samples is pure H2 without B2H6, and the MS desorption peak temperatures are in good agreement with the DSC results. It can be found that the improved dehydrogenation of MgH2 and LiBH4 depends on the variations in concentrations of NbF5 additive. Compared with the neat 2LiH−MgB2, 2LiH−MgB2−0.01NbF5 shows slightly decreasB



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which rapidly releases 1 equiv. of hydrogen within 3 min, and further 3 equiv. of hydrogen within 30 min in the fifth cycle, as shown in the inset of left-hand side. Moreover, the promoting catalytic effect of NbF5 additive is also confirmed in the hydrogenation process. For example, the 2LiH−MgB2− 0.01NbF5 can absorb over 9.0 wt % hydrogen within 35 min, while the neat 2LiH−MgB2 absorbs 5.18 wt % hydrogen under same measurement condition. As shown in the inset of the right-hand side for Figure 2, the 2LiH−MgB2−0.03NbF5 rapidly absorbs more than 9.0 wt % hydrogen within 20 min at 350 °C. So far as we know, such enhanced de/hydrogenation properties of 2LiH−MgB2−0.03NbF5 have achieved great improvement in the Li−Mg−B−H system, compared with the previous reports.13,20−22,26 Figure 3 shows the isothermal hydrogenation/dehydrogenation curves of 2LiH−MgB2−0.03NbF5 sample at different

ing desorption temperatures of MgH2 and LiBH4. However, the dehydrogenations of MgH2 and LiBH4 dramatically shift toward lower peak temperatures at 333.2 and 422.2 °C for 2LiH−MgB2−0.03NbF5, which are 5 and 29.8 °C lower than the corresponding values of neat 2LiH−MgB2, respectively. The decreasing hydrogen desorption temperatures may be due to the F-anion substitution in Li−Mg−B−H system.22,27 Interestingly, excepted for the unchanged LiBH4 structure transition temperature at 113.6 °C, the sample doped with NbF5 additive exhibits a lower melting temperature than that of neat 2LiH−MgB2. The more NbF5 additive doped, the lower melting temperature appears. This phenomenon is likely due to the colligative effect in the NbF5 doped Li−Mg−B−H system. To further confirm and understand the pronounced effect of NbF5 additive in promoting the de/hydrogenation kinetics of the Li−Mg−B−H system, the isothermal de/hydrogenation curves of samples are presented in Figure 2. The samples were

Figure 3. Isothermal hydrogenation/dehydrogenation curves of 2LiH−MgB2−0.03NbF5 sample. (a) Hydrogenation at 250 and 300 °C under 8 MPa hydrogen back pressure; (b) dehydrogenation at 355 and 375 °C under 0.4 MPa hydrogen back pressure.

Figure 2. Isothermal dehydrogenation and hydrogenation curves of ball-milled (a) neat 2LiH−MgB2, (b) 2LiH−MgB2−0.01NbF5, and (c) LiH−MgB2−0.03NbF5 samples. The cycling isothermal dehydrogenation curves are presented in the inset of the left-hand side, and the magnified isothermal hydrogenation curves are presented in the inset of the right-hand side. All of the samples were dehydrogenated at 400 °C under 0.4 MPa hydrogen back pressure and hydrogenated at 350 °C under 8 MPa hydrogen back pressure.

temperatures. The hydrogenation curves exhibit one-step hydrogenation feature according to the reaction (2LiH +MgB2+4H2 ↔ 2LiBH4+MgH2) in Figure 3a. In contrast, two-step dehydrogenation feature displays in Figure 3(b), corresponding to the two step reaction described by Reaction (2LiBH4+MgH2 ↔ 2LiBH4+Mg+H2 ↔ 2LiH+MgB2+4H2). Thus one-step hydrogenation feature can be resulted from the LiBH4 and MgH2 formed simultaneously under fairly moderate conditions.19 It is believed that the relative unstable MgB2 with layer crystal structure, where each B atom is connected to a maximum of three other B atoms, is easy to react with LiH and H2 and simultaneously form LiBH4 and MgH2 under 8 MPa hydrogen back pressure. For the hydrogenation curves, the 2LiH−MgB2−0.03NbF5 sample can absorb 9.30 wt % H2 within 1200 min even with the hydrogenation temperature lowered to 250 °C. While as the hydrogenation temperature increases to 300 °C, the hydrogenation rate could be obviously enhanced and completely absorb 9.55 wt % H2 within 600 min. It takes only 65 min to absorb 90% of total hydrogenation capacity at 300 °C. As for the dehydrogenation curves, the sample displays obvious two-step reversible dehydrogenation reactions at 375 °C: releasing 2.43 wt % H2 in 7 min and further 6.85 wt % H2 in 600 min, respectively. As expected, the dehydrogenation rate is accelerated and the incubation period of MgB2 is dramatically shortened when the temperature is increased. Although the 2LiH−MgB2−0.03NbF5 sample only

dehydrogenated and rehydrogenated with an initial hydrogen back pressure of 0.4 and 8 MPa, respectively. We noticed that the de/hydrogenation kinetics of all samples could complete activation process and obtain stable cycling kinetics after the first hydrogenation (Figure S3 and S4). The de/hydrogenation kinetics of neat 2LiH−MgB2 is quite slow: it takes 10 h to absorb 10.56 wt % H2 at 350 °C and desorbs only 4.00 wt % H2 for 80 min at 400 °C, which is consistent with the previous report.21 In contrast, the 2LiH−MgB2 doped with NbF5 additive displays an improvement of reaction kinetics. Notably, the dehydrogenation rate becomes faster upon doping NbF5, especially for the second dehydrogenation step, indicating its beneficial effect on reducing incubation period of MgB2.28 In Figure 2, the hydrogenated 0.01NbF5 and 0.03NbF5 doped 2LiH−MgB2 can release 3.97 and 8.47 wt % hydrogen within 15 min at 400 °C, respectively. However, the neat 2LiH−MgB2 sample just releases 2.60 wt % hydrogen under identical measurement conditions. This result is also in good agreement with the MS analysis above. It can be found that the catalytic improvement of 2LiH−MgB2−0.03NbF5 persists well in its cycling capacity as the subsequent reversible dehydrogenation, C



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Figure 4. XRD patterns of the LiH−MgB2−0.03NbF5 sample: (a) before hydrogenation of the first cycle; (b) after hydrogenation of the first cycle at 350 °C; (c) after dehydrogenation of the first cycle at 400 °C.

desorbs ∼3.38 wt % H2 within 600 min at 355 °C, it can be concluded that the second dehydrogenation step (2LiBH4 + Mg ↔ 2LiH + MgB2 + 4H2) indeed occurs under the condition of dehydrogenation at 355 °C and 0.4 MPa H2, due to the capacity of the first dehydrogenation step is about 2.5 wt %. In a word, the 2LiH−MgB2−0.03NbF5 could reversibly absorb more than 9.0 wt % hydrogen even at 250 °C and mostly complete dehydrogenation below 375 °C. In order to clarify the pronounced effect of 2LiH−MgB2− 0.03NbF5 system, the XRD patterns in different hydrogenation and dehydrogenation stages are displayed in Figure 4. For the ball-milled 2LiH−MgB2−0.03NbF5, LiH and MgB2 main phases as well as Nb minor phase are detected. After hydrogenation, the main phases change to LiBH4 and MgH2 together with LiF minor phase. After dehydrogenation, the main phases turn back into LiH and MgB2. These findings confirm the completely reversible de/hydrogenation reaction of 2LiH−MgB2−0.03NbF5 system according to reaction 1. However, a few LiOH and MgO impurities are also observed in all samples, which maybe come from the raw powder itself or oxidation during measurement. Careful examination found that LiH peak at 2θ = 44.5° shifts slightly to the high-angle side due to the formation of LiH1−xFx phase by the solid-phase interaction between LiH and LiF, as discussed in Figure S2.10 It is important to note from Figure 4 (b) and (c) that, NbH phase could remain stable in the subsequent de/hydrogenation cycles. Even after 5 cycles, the NbH still maintains its phase structure (Figure S5). Based on the above results, it is postulated that the occurrences of reaction 2 in the ball-milling process and reaction 3 in the subsequent hydrogenation process. 5LiH + NbF5 → 5LiF + Nb + 2.5H 2

(2)

2Nb + H 2 → 2NbH

(3)

0.03NbF5 system, NbH can be formed after first hydrogenation and remain unchanged upon further de/hydrogenation cycles, thus improves the de/hydrogenation kinetics of Li−Mg−B−H system by facilitating hydrogen diffusion reported by previous study.13 Additionally, the asymmetric shoulder peak at 2θ around 44.8° could be further detected in the dehydrogenated sample even after several cycles (Figure S5). This result suggests that the reversible transition from LiF to LiH1‑xFx is taking place with the de/hydrogenation processes. Combined with the DSC-MS analysis above, the reversible transition of LiH1‑xFx is consistent with the hypothesis regarding functionality of F-anion substitution, which is responsible for thermodynamically destabilizing in the NbF5 catalyzed Li− Mg−B−H system.26,31 In an effort to understand whether or not NbH and LiH1−xFx mentioned above play the role in enhancement in dehydrogenation kinetics of hydrogenated 2LiH−MgB2−0.03NbF5 system, several additives such as NbF5, NbCl5, LiF, NbH, and NbB2 were used individually and/or together as additives in the Li− Mg−B−H system. Figure 5 illustrates the isothermal dehydrogenation curves of hydrogenated 2LiH−MgB2 doped with different additives. Before dehydrogenation, all of the samples were previously hydrogenated at 350 °C with an initial hydrogen back pressure of 8 MPa for 15 h. It can be found that the dehydrogenation kinetics of hydrogenated 2LiH−MgB2− 0.03NbCl5 is inferior to that of hydrogenated 2LiH−MgB2− 0.03NbF5, which further confirms the F-anion substitution resulted in favorable thermodynamics modification of Li−Mg− B−H system, as discussed in Figure 1. However, the individual LiF or NbH additive shows no remarkable effect in improving the dehydrogenation kinetics, while codoping LiF and NbH in 2LiH−MgB2 considerably improves the dehydrogenation kinetics of the Li−Mg−B−H system. So we conclude that, besides the functionality of F-anion substitution from LiF (LiH1−xFx), the NbH catalytic additive may be responsible for the improved dehydrogenation kinetics of hydrogenated LiH− MgB2−0.03NbF5. Additionally, it was ever reported that the transition-metal boride can be served as the effective catalyst in the improved dehydrogenation kinetics.20 So we further investigate the effect of NbB2 individual dopant and LiF− NbB2 codopants on the dehydrogenation of the Li−Mg−B−H

The formation enthalpies for LiH, NbF5, LiF, and NbH are −90.625, −1813.764, −616.931, and −4.953 kJ mol−1, respectively.29,30 Therefore, the enthalpy changes for the reaction 2 and 3 are calculated as −817.766 and −9.906 kJ mol−1, suggesting the formations of Nb and NbH are thermodynamically favorable. As for the 2LiH−MgB2− D



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Figure 5. Comparison of the isothermal dehydrogenation curves of undoped 2LiH−MgB2 and 2LiH−MgB2 doped with different additives (NbF5, NbCl5, LiF, and NbH). The isothermal dehydrogenation curves of undoped 2LiH−MgB2 and 2LiH−MgB2 doped with different additives (NbF5, LiF, and NbB2) are presented in the inset. All of the samples were dehydrogenated at 400 °C under 0.4 MPa hydrogen back pressure.

system. It is found that directly milling 2LiH−MgB2 with 3 mol % NbB2 additive exhibits no appreciable improvement on the dehydrogenation kinetics. Meanwhile, the dehydrogenation kinetics of hydrogenated 2LiH−MgB2−0.15LiF−0.03NbB2 is slightly superior to that of hydrogenated neat 2LiH−MgB2 but obviously inferior to that of hydrogenated 2LiH−MgB2− 0.03NbF5, indicating that the enhancement of dehydrogenation kinetics for the hydrogenated 2LiH−MgB2−0.03NbF5 could not be ascribed to the combination of 0.03NbB2 and functional F-anion, as shown in the inset of Figure 5. Figure 6 displays the SEM and EDX results of 2LiH−MgB2− 0.03NbF5 after dehydrogenation at 400 °C. It can be found that the dehydrogenated sample is composed of nanometer ovalshaped particles (∼50 nm) and planar structures (∼80 nm), with comparatively larger surface area, as shown in the magnifying SEM image of Figure 6b. As known from above XRD patterns, these nanometer particles are corresponding to the MgB2, LiH (LiH1‑xFx) and NbH phases. For heterogeneous catalyzed reaction, the practical catalytic efficiency relies not only on the intrinsic activity of the species but also on the distribution of the catalysts.19 EDX elemental mapping of the dehydrogenated 2LiH−MgB2−0.03NbF5 shows a homogeneous distribution of Nb atom throughout the sample (Figure S6), thus resulted in remarkable de/hydrogenation kinetics. Compared with the in situ formed nano NbH and LiF (LiH1‑xFx) active species in 2LiH−MgB2−0.03NbF5, the supposed bigger particle size and less homogeneous distribution of NbH might be the reason of the slightly inferior dehydrogenation kinetics of 2LiH−MgB2−0.15LiF−0.03NbH (Figure S7). Based on the above de/hydrogenation properties and phase analysis of the 2LiH−MgB2−0.03NbF5 system, it can be safely concluded that the NbF5 additive in improving reversible hydrogen storage properties are ascribed to the synergistic effect of in situ formed nano NbH acting as “active gateways” facilitating the diffusion of hydrogen through the diffusion barriers both in de/hydrogenation processes and the “favorable thermodynamic destabilization” from the reversible transition

Figure 6. SEM and EDX results of the 2LiH−MgB2−0.03NbF5 sample after the fifth dehydrogenation cycle at 400 °C. (a) Multiplier 30 k; (b) multiplier 100 k; (c) EDX spectrum corresponding to the red pane in panel a.

of LiH1−xFx caused by functionality of F-anion substitution in the Li−Mg−B−H system. This fundamental understanding is of significance for the design and optimization of catalytic method and species and thereby laying the foundation for further improving the hydrogen storage properties of the catalyzed Li−Mg−B−H system.

4. CONCLUSIONS In summary, the reversible hydrogen storage properties of the Li−Mg−B−H system can be notably improved by doping E



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Disordered Mesoporous Carbon: Hydrogen Storage Performances and Destabilization Mechanisms. Int. J. Hydrogen Energy 2007, 32, 3976− 3980. (7) Ngene, P.; van Zwienen, M.; de Jongh, P. E. Reversibility of the Hydrogen Desorption from LiBH4: A Synergetic Effect of Nanoconfinement and Ni Addition. Chem. Commun. 2010, 46, 8201−8203. (8) Fang, Z. Z.; Kang, X. D.; Luo, J. H.; Wang, P.; Li, H. W.; Orimo, S. Formation and Hydrogen Storage Properties of Dual-Cation (Li, Ca) Borohydride. J. Phys. Chem. C 2010, 114, 22736−22741. (9) Fang, F.; Li, Y. T.; Song, Y.; Sun, D. L.; Zhang, Q. G.; Ouyang, L. Z.; Zhu, M. Superior Destabilization Effects of MnF2 over MnCl2 in the Decomposition of LiBH4. J. Phys. Chem. C 2011, 115, 13528− 13533. (10) Arnbjerg, L. M.; Ravnsbæk, D. B.; Filinchuk, Y.; Vang, R. T.; Cerenius, Y.; Besenbacher, F.; Jørgensen, J. E.; Jakobsen, H. J.; Jensen, T. R. Structure and Dynamics for LiBH4−LiCl Solid Solutions. Chem. Mater. 2009, 21, 5772−5782. (11) Li, H. W.; Orimo, S.; Nakamori, Y.; Miwa, K.; Ohba, N.; Towata, S.; Züttel, A. Materials Designing of Metal Borohydrides: Viewpoints from Thermodynamical Stabilities. J. Alloys Compd. 2007, 446, 315−318. (12) Züttel, A.; Rentsch, S.; Fischer, P.; Wenger, P.; Sudan, P.; Mauron, P.; Emmenegger, C. Hydrogen Storage Properties of LiBH4. J. Alloys Compd. 2003, 356, 515−520. (13) Fan, M. Q.; Sun, L. X.; Zhang, Y.; Xu, F.; Zhang, J.; Chu, H. F. The Catalytic Effect of Additive Nb2O5 on the Reversible Hydrogen Storage Performances of LiBH4−MgH2 Composite. Int. J. Hydrogen Energy 2008, 33, 74−80. (14) Guo, Y.; Yu, X.; Gao, L.; Xia, G.; Guo, Z.; Liu, H. Significantly Improved Dehydrogenation of LiBH4 Destabilized by TiF3. Energy Environ. Sci. 2010, 3, 465−470. (15) Zhang, Y.; Zhang, W. S.; Fan, M. Q.; Liu, S. S.; Chu, H. L.; Zhang, Y. H.; Gao, X. Y.; Sun, L. X. Enhanced Hydrogen Storage Performance of LiBH4-SiO2-TiF3 Composite. J. Phys. Chem. C 2008, 112, 4005−4010. (16) Yang, J.; Sudik, A.; Wolverton, C. Destabilizing LiBH4 with a Metal (M = Mg, Al, Ti, V, Cr, or Sc) or Metal Hydride (MH2 = MgH2, TiH2, or CaH2). J. Phys. Chem. C 2007, 111, 19134−19140. (17) Shao, J.; Xiao, X. Z.; Chen, L. X.; Fan, X. L.; Li, S. Q.; Ge, H. W.; Wang, Q. D. Enhanced Hydriding−Dehydriding Performance of 2LiBH4−MgH2 Composite by the Catalytic Effects of Transition Metal Chlorides. J. Mater. Chem. 2012, 22, 20764−20772. (18) Vajo, J. J.; Skeith, S. L.; Mertens, F. Reversible Storage of Hydrogen in Destabilized LiBH4. J. Phys. Chem. B 2005, 109, 3719− 3722. (19) Bösenberg, U.; Doppiu, S.; Mosegaard, L.; Barkhordarian, G.; Eigen, N.; Borgschulte, A.; Jensen, T. R.; Cerenius, Y.; Gutfleisch, O.; Klassen, T.; Dornheim, M.; Bormann, R. Hydrogen Sorption Properties of MgH2-LiBH4 Composites. Acta Mater. 2007, 55, 3951−3958. (20) Zhang, Y.; Morin, F.; Huot, J. The Effects of Ti-based Additives on the Kinetics and Reactions in LiH/MgB2 Hydrogen Storage System. Int. J. Hydrogen Energy 2011, 36, 5425−5430. (21) Saldan, I.; Campesi, R.; Zavorotynska, O.; Spoto, G.; Baricco, M.; Arendarska, A.; Taube, K.; Dornheim, M. Enhanced Hydrogen Uptake/Release in 2LiH−MgB2 Composite with Titanium Additives. Int. J. Hydrogen Energy 2011, 37, 1604−1612. (22) Wang, P. J.; Ma, L. P.; Fang, Z. Z.; Kang, X. D.; Wang, P. Improved Hydrogen Storage Property of Li-Mg-B-H System by Milling with Titanium Trifluoride. Energy Environ. Sci. 2009, 2, 120− 123. (23) Kou, H. Q.; Xiao, X. Z.; Li, J. X.; Li, S. Q.; Ge, H. W.; Wang, Q. D.; Chen, L. X. Effects of Fluoride Additives on Dehydrogenation Behaviors of 2LiBH4+MgH2 System. Int. J. Hydrogen Energy 2012, 37, 1021−1026. (24) Xiao, X.; Shao, J.; Chen, L.; Kou, H.; Fan, X.; Deng, S.; Zhang, L.; Li, S.; Ge, H.; Wang, Q. Effects of NbF5 Addition on the De/ Rehydrogenation Properties of 2LiBH4/MgH2 Hydrogen Storage System. Int. J. Hydrogen Energy 2012, 37, 13147−13157.

2LiH/MgB2 mixture with small amounts of NbF5 additive. The as-synthesized 2LiH−MgB2−0.03NbF5 can obtain a reversible hydrogen storage capacity of ca. 9.0 wt % under the cyclic condition of dehydrogenation at 400 °C within 20 min and rehydrogenation at 350 °C within 20 min. It is found that NbF5 reacts with LiH to form LiH1−xFx and NbH during ball-milling and subsequent hydrogenation. The pronounced de/hydrogenation performances arising upon doping NbF5 could be ascribed to the novel synergistic effect of in situ formed nano NbH particles acting as “active gateways” facilitating the diffusion of hydrogen and the reversible transition of LiH1−xFx resulting in the “favorable thermodynamic destabilization” in Li−Mg−B−H system, thus promoting effect persists well in the subsequent de/hydrogenation cycles.

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ASSOCIATED CONTENT

S Supporting Information *

Figures of XRD pattern of as-prepared NbH and XRD patterns of 2LiH−MgB2 doped without and with different additives by ball-milling. Cycling isothermal hydrogenation curves of 2LiH− MgB2 doped without and with different additives by ballmilling. Figure of XRD pattern of the 2LiH−MgB2−0.03NbF5 sample after several dehydrogenation cycles. SEM image and EDX elemental mapping of 2LiH−MgB2−0.03NbF5 and 2LiH−MgB2−0.15LiF−0.03NbB2 sample dehydrogenation at 400 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86 571 8795 1152. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the supported by National Basic Research Program of China (2010CB631300), National High Technology Research & Development Program of China (2012AA051503), National Natural Science Foundation of China (51001090 and 51171173), China Postdoctoral Science Foundation (20090262 and 2012M521167), and Key Science and Technology Innovation Team of Zhejiang Province (2010R50013).

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