Synergetic Effect of in Situ Formed Nano NbH and LiH1−xFx forImproving Reversible Hydrogen Storage Properties of the Li−Mg−B−H SystemXuezhang 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 andEngineering, Zhejiang University, Hangzhou 310027, China

*S Supporting Information

ABSTRACT: A significant improvement in hydrogenation/dehydro-genation properties of 2LiH/MgB2 can be achieved by adding NbF5.The results show that the NbF5 additive is effective for enhancing thede/hydrogenation kinetics of the Li−Mg−B−H system and reducingthe desorption temperatures of MgH2 and LiBH4. For the 2LiH−MgB2−0.03NbF5 sample, About 9.0 wt % hydrogen capacity is obtainedrapidly under cyclic conditions of rehydrogenation within 20 min at 350°C and dehydrogenation within 20 min at 400 °C; thus, catalyticimprovement persists well in the subsequent reversible dehydrogenationcycles. Moreover, the sample could reversibly reabsorb and release morethan 9.0 wt % hydrogen even at 250 and 375 °C, respectively.Microstructure analyses reveal that the NbF5 additive in improving thede/hydrogenation properties of Li−Mg−B−H system could be ascribedto 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 reversibletransition of LiH1−xFx caused by functionality of F-anion substitution. This fundamental understanding provides us with insightsinto 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 importantenergy carriers for future mobile applications due to theadvantages of high energy density, high elemental abundance,and environmentally benign product.1,2 Among hydrogenstorage media, solid-state hydrides offer several advantageswith respect to high volumetric hydrogen capacity, compact-ness, and safety despite its relatively low gravimetric hydrogencapacity.2 Currently, many research groups have paid moreattention to light metal complex hydrides because of their highvolumetric and gravimetric hydrogen capacities which aresuitable for fuel cell vehicles. Lithium borohydride (LiBH4) isregarded as one of the promising candidates for safe andefficient hydrogen storage materials due to its high reversiblehydrogen storage capacity (13.8 wt % H2). However, the strongand highly conjunct covalent and ionic bonds of LiBH4

extremely impose its problematic H-exchange thermodynamicsand kinetics for reversible hydrogen storage.3,4 Up to now, avariety of novel approaches have been developed to address thethermodynamics and kinetics limitations of LiBH4, includingnanoporous scaffolds,5−7 partial cation/anion substitution,8−11

thermodynamic destabilization of LiBH4 by transition metaloxide, fluoride, and hydride.12−17 The thermodynamicdestabilization of LiBH4 displays great potential for adjusting

de/hydrogenation thermodynamics and enhancing its kinetics.Vajo et al.18 reported that the dehydrogenation enthalpy of2LiBH4 + MgH2 is −46 kJ mol−1 H2, which is around 21 kJmol−1 H2 lower than that of pure LiBH4. The theoreticalhydrogen capacity of this Li−Mg−B−H system is still as highas 11.8 wt %, which undergoes the reversible hydrogen storagereactions under appropriate H2 back pressure by reaction 1.19

+ ↔ + +

↔ + +

2LiBH MgH 2LiBH Mg H

2LiH MgB 4H

4 2 4 2

2 2 (1)

Although the reaction enthalpy of 2LiBH4/MgH2 is lowered,dehydrogenation process still occurs at high temperature with arelatively slow two-step kinetics, even doped with a few TiCl3catalyst.18 Therefore, extensive efforts have been mainlyfocused on improving de/hydrogenation rate and reducingreaction temperature for the Li−Mg−B−H system by exploringhigh-performance catalysts. Sun et al. doped Nb2O5 into the2LiBH4/MgH2 system and found Nb2O5 could significantlyimprove the de/hydrogenation kinetics due to the formation of

Received: April 16, 2013Revised: May 15, 2013Published: May 22, 2013

Article

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NbH2, which facilitates hydrogen diffusion.13 Recently, a seriesof Ti-based additives (such as Ti, TiH2, TiB2, TiCl3, TiF3, TiF4,TiO2, TiN, and TiC) were investigated for their effects on theimprovements of de/hydrogenation properties of the LiH/MgB2 mixture, and these additives were mostly responsible fordecreasing hydrogen desorption temperature and increasingreaction kinetics of the Li−Mg−B−H system.20,21 Particularly,Wang et al. further demonstrated the advantage of TiF3 over itsanalogue TiCl3 on the decrease of desorption temperature ofthe Li−Mg−B−H system, resulting from F-substitution in theLiBH4 lattice.22 Quite recently, we investigated the effects oftransition-metal fluoride additives (NbF5, TiF3, CeF3, LaF3, andFeF3) on dehydrogenation properties of the Li−Mg−B−Hsystem.23 Among these transition-metal fluoride additives,NbF5 exhibited the most prominent performances in terms oflow desorption temperature and fast kinetics for the Li−Mg−B−H system.23,24 Moreover, Mao et al. also found that NbF5could improve the cycling de/rehydrogenation kinetics of2LiBH4/MgH2, resulting from the suppression of formingLi2B12H12.

25 Goudy et al. also confirmed that NbF5 is vastlysuperior to Nb2O5 and Mg2Ni for catalyzing the hydrogendesorption rates of the 2LiBH4/MgH2 system.26 However, thede/hydrogenation kinetics of the Li−Mg−B−H system is stillrelatively slow and the catalytic mechanism of NbF5 is not yetclear. A detailed understanding of the roles of NbF5 transition-metal additive is indispensable for further improvements inreversible de/hydrogenation of the Li−Mg−B−H system. Inaddition, as far as we know, very few papers reported the directcatalytic enhancement of dehydrogenated produces (2LiH/MgB2) in the Li−Mg−B−H system. Here we report a largedegree of improvement in reversible de/hydrogenation proper-ties of the Li−Mg−B−H system by doping a 2LiH/MgB2mixture with NbF5 catalyst precursor, which provides a novelsynergetic catalytic effect of in situ formed nano NbH andLiH1−xFx.

2. EXPERIMENTAL SECTIONCommercial LiH (98%), MgB2 (99%), NbF5 (98%), NbCl5(99%), LiF (99.99%), NbB2 (99%), and Nb (99.9%) powderswere all purchased from Sigma-Aldrich Corp. and used withoutfurther purification. The NbH was synthesized by ball-milledNb powder in the Planetary mill under a hydrogen pressure of1 MPa for 3 h, which XRD pattern is shown in Figure S1.Approximately 1.5 g mixture of LiH/MgB2 with a mole ratio of2:1 was ball-milled under Ar atmosphere for 6 h at 400 rpm in astainless 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 identicalconditions. The ball-milling process was paused for 0.1 hevery 0.2 h to avoid an increasing temperature of the sample.The samples of neat 2LiH−MgB2 and 2LiH−MgB2 doped withdifferent catalytic additives (NbF5, NbCl5, LiF, NbH, andNbB2) were prepared through ball-milling method. The MgB2and LiH main phases are detected in all XRD patterns of ball-milled samples (Figure S2). All experimental operations wereperformed in a high-pure Ar-filled glovebox, which wasequipped with a recirculation system to keep the H2O andO2 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 of0.02° from 10° to 90° (2θ). Scanning electron microscopy(SEM, Hitachi SU-70) equipped with energy dispersive X-ray

spectroscopy was performed to examine the microstructure andelemental distribution of the samples. A special sample holderfilled with argon was used to prevent the air and moisturecontamination during transfer and measurement.The differential scanning calorimetry and mass spectrometer

(DSC-MS) measurements were conducted on a synchronousthermal analysis (Netzsch STA 449F3 analyzer/NetzschQ403C mass spectrometer) with a heating rate of 3 °C/minfrom room temperature to 500 °C under flowing argoncondition (high purity, 50 mL/min). Before DSC-MS measure-ments, 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 acarefully calibrated Sievert’s type apparatus. The as-synthesizedsamples were rehydrogenated at 250−350 °C and dehydro-genated at 355−400 °C with an initial hydrogen back pressureof 8 and 0.4 MPa, respectively. During the typical cyclicexperiment, before dehydrogenation, all of the samples werepreviously hydrogenated at 350 °C with an initial hydrogenback pressure of 8 MPa for 15 h; before hydrogenation, allsamples were subsequently dehydrogenated at 400 °C with aninitial hydrogen back pressure of 0.4 MPa for 18 h. Forcomparison, the weight percent of hydrogen capacity wascalculated on the basis of the total weight of the samplesincluding the 2LiH−MgB2 and additives.

3. RESULTS AND DISCUSSIONThe DSC-MS characteristics of hydrogenated 2LiH−MgB2with and without fluoride additives are shown in Figure 1. All

DSC curves exhibit four distinct endothermic peaks, whichcorrespond 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 releasedfrom all samples is pure H2 without B2H6, and the MSdesorption peak temperatures are in good agreement with theDSC results. It can be found that the improved dehydrogen-ation of MgH2 and LiBH4 depends on the variations inconcentrations of NbF5 additive. Compared with the neat2LiH−MgB2, 2LiH−MgB2−0.01NbF5 shows slightly decreas-

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

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ing desorption temperatures of MgH2 and LiBH4. However, thedehydrogenations of MgH2 and LiBH4 dramatically shifttoward lower peak temperatures at 333.2 and 422.2 °C for2LiH−MgB2−0.03NbF5, which are 5 and 29.8 °C lower thanthe corresponding values of neat 2LiH−MgB2, respectively.The decreasing hydrogen desorption temperatures may be dueto the F-anion substitution in Li−Mg−B−H system.22,27

Interestingly, excepted for the unchanged LiBH4 structuretransition temperature at 113.6 °C, the sample doped withNbF5 additive exhibits a lower melting temperature than that ofneat 2LiH−MgB2. The more NbF5 additive doped, the lowermelting temperature appears. This phenomenon is likely due tothe 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 ofthe Li−Mg−B−H system, the isothermal de/hydrogenationcurves of samples are presented in Figure 2. The samples were

dehydrogenated and rehydrogenated with an initial hydrogenback pressure of 0.4 and 8 MPa, respectively. We noticed thatthe de/hydrogenation kinetics of all samples could completeactivation process and obtain stable cycling kinetics after thefirst hydrogenation (Figure S3 and S4). The de/hydrogenationkinetics of neat 2LiH−MgB2 is quite slow: it takes 10 h toabsorb 10.56 wt % H2 at 350 °C and desorbs only 4.00 wt % H2for 80 min at 400 °C, which is consistent with the previousreport.21 In contrast, the 2LiH−MgB2 doped with NbF5additive displays an improvement of reaction kinetics. Notably,the dehydrogenation rate becomes faster upon doping NbF5,especially for the second dehydrogenation step, indicating itsbeneficial effect on reducing incubation period of MgB2.

28 InFigure 2, the hydrogenated 0.01NbF5 and 0.03NbF5 doped2LiH−MgB2 can release 3.97 and 8.47 wt % hydrogen within15 min at 400 °C, respectively. However, the neat 2LiH−MgB2sample just releases 2.60 wt % hydrogen under identicalmeasurement conditions. This result is also in good agreementwith the MS analysis above. It can be found that the catalyticimprovement of 2LiH−MgB2−0.03NbF5 persists well in itscycling capacity as the subsequent reversible dehydrogenation,

which rapidly releases 1 equiv. of hydrogen within 3 min, andfurther 3 equiv. of hydrogen within 30 min in the fifth cycle, asshown in the inset of left-hand side. Moreover, the promotingcatalytic effect of NbF5 additive is also confirmed in thehydrogenation 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 undersame measurement condition. As shown in the inset of theright-hand side for Figure 2, the 2LiH−MgB2−0.03NbF5rapidly absorbs more than 9.0 wt % hydrogen within 20 minat 350 °C. So far as we know, such enhanced de/hydrogenationproperties of 2LiH−MgB2−0.03NbF5 have achieved greatimprovement in the Li−Mg−B−H system, compared withthe previous reports.13,20−22,26

Figure 3 shows the isothermal hydrogenation/dehydrogen-ation curves of 2LiH−MgB2−0.03NbF5 sample at different

temperatures. The hydrogenation curves exhibit one-stephydrogenation 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 theLiBH4 and MgH2 formed simultaneously under fairly moderateconditions.19 It is believed that the relative unstable MgB2 withlayer crystal structure, where each B atom is connected to amaximum of three other B atoms, is easy to react with LiH andH2 and simultaneously form LiBH4 and MgH2 under 8 MPahydrogen back pressure. For the hydrogenation curves, the2LiH−MgB2−0.03NbF5 sample can absorb 9.30 wt % H2within 1200 min even with the hydrogenation temperaturelowered to 250 °C. While as the hydrogenation temperatureincreases to 300 °C, the hydrogenation rate could be obviouslyenhanced and completely absorb 9.55 wt % H2 within 600 min.It takes only 65 min to absorb 90% of total hydrogenationcapacity at 300 °C. As for the dehydrogenation curves, thesample displays obvious two-step reversible dehydrogenationreactions at 375 °C: releasing 2.43 wt % H2 in 7 min andfurther 6.85 wt % H2 in 600 min, respectively. As expected, thedehydrogenation rate is accelerated and the incubation periodof MgB2 is dramatically shortened when the temperature isincreased. Although the 2LiH−MgB2−0.03NbF5 sample only

Figure 2. Isothermal dehydrogenation and hydrogenation curves ofball-milled (a) neat 2LiH−MgB2, (b) 2LiH−MgB2−0.01NbF5, and (c)LiH−MgB2−0.03NbF5 samples. The cycling isothermal dehydrogen-ation curves are presented in the inset of the left-hand side, and themagnified isothermal hydrogenation curves are presented in the insetof 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.

Figure 3. Isothermal hydrogenation/dehydrogenation curves of2LiH−MgB2−0.03NbF5 sample. (a) Hydrogenation at 250 and 300°C under 8 MPa hydrogen back pressure; (b) dehydrogenation at 355and 375 °C under 0.4 MPa hydrogen back pressure.

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desorbs ∼3.38 wt % H2 within 600 min at 355 °C, it can beconcluded that the second dehydrogenation step (2LiBH4 +Mg↔ 2LiH + MgB2 + 4H2) indeed occurs under the conditionof dehydrogenation at 355 °C and 0.4 MPa H2, due to thecapacity of the first dehydrogenation step is about 2.5 wt %. Ina word, the 2LiH−MgB2−0.03NbF5 could reversibly absorbmore than 9.0 wt % hydrogen even at 250 °C and mostlycomplete dehydrogenation below 375 °C.In order to clarify the pronounced effect of 2LiH−MgB2−

0.03NbF5 system, the XRD patterns in different hydrogenationand dehydrogenation stages are displayed in Figure 4. For theball-milled 2LiH−MgB2−0.03NbF5, LiH and MgB2 mainphases as well as Nb minor phase are detected. Afterhydrogenation, the main phases change to LiBH4 and MgH2together with LiF minor phase. After dehydrogenation, themain phases turn back into LiH and MgB2. These findingsconfirm the completely reversible de/hydrogenation reaction of2LiH−MgB2−0.03NbF5 system according to reaction 1.However, a few LiOH and MgO impurities are also observedin all samples, which maybe come from the raw powder itself oroxidation during measurement. Careful examination found thatLiH peak at 2θ = 44.5° shifts slightly to the high-angle side dueto the formation of LiH1−xFx phase by the solid-phaseinteraction between LiH and LiF, as discussed in Figure S2.10

It is important to note from Figure 4 (b) and (c) that, NbHphase could remain stable in the subsequent de/hydrogenationcycles. Even after 5 cycles, the NbH still maintains its phasestructure (Figure S5). Based on the above results, it ispostulated that the occurrences of reaction 2 in the ball-millingprocess and reaction 3 in the subsequent hydrogenationprocess.

+ → + +5LiH NbF 5LiF Nb 2.5H5 2 (2)

+ →2Nb H 2NbH2 (3)

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 thereaction 2 and 3 are calculated as −817.766 and −9.906 kJmol−1, suggesting the formations of Nb and NbH arethermodynamically favorable. As for the 2LiH−MgB2−

0.03NbF5 system, NbH can be formed after first hydrogenationand remain unchanged upon further de/hydrogenation cycles,thus improves the de/hydrogenation kinetics of Li−Mg−B−Hsystem by facilitating hydrogen diffusion reported by previousstudy.13 Additionally, the asymmetric shoulder peak at 2θaround 44.8° could be further detected in the dehydrogenatedsample even after several cycles (Figure S5). This resultsuggests that the reversible transition from LiF to LiH1‑xFx istaking place with the de/hydrogenation processes. Combinedwith the DSC-MS analysis above, the reversible transition ofLiH1‑xFx is consistent with the hypothesis regarding function-ality of F-anion substitution, which is responsible forthermodynamically destabilizing in the NbF5 catalyzed Li−Mg−B−H system.26,31

In an effort to understand whether or not NbH and LiH1−xFxmentioned above play the role in enhancement in dehydrogen-ation kinetics of hydrogenated 2LiH−MgB2−0.03NbF5 system,several additives such as NbF5, NbCl5, LiF, NbH, and NbB2were used individually and/or together as additives in the Li−Mg−B−H system. Figure 5 illustrates the isothermaldehydrogenation curves of hydrogenated 2LiH−MgB2 dopedwith different additives. Before dehydrogenation, all of thesamples were previously hydrogenated at 350 °C with an initialhydrogen back pressure of 8 MPa for 15 h. It can be found thatthe dehydrogenation kinetics of hydrogenated 2LiH−MgB2−0.03NbCl5 is inferior to that of hydrogenated 2LiH−MgB2−0.03NbF5, which further confirms the F-anion substitutionresulted in favorable thermodynamics modification of Li−Mg−B−H system, as discussed in Figure 1. However, the individualLiF or NbH additive shows no remarkable effect in improvingthe dehydrogenation kinetics, while codoping LiF and NbH in2LiH−MgB2 considerably improves the dehydrogenationkinetics 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 forthe improved dehydrogenation kinetics of hydrogenated LiH−MgB2−0.03NbF5. Additionally, it was ever reported that thetransition-metal boride can be served as the effective catalyst inthe improved dehydrogenation kinetics.20 So we furtherinvestigate the effect of NbB2 individual dopant and LiF−NbB2 codopants on the dehydrogenation of the Li−Mg−B−H

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 at350 °C; (c) after dehydrogenation of the first cycle at 400 °C.

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system. It is found that directly milling 2LiH−MgB2 with 3 mol% NbB2 additive exhibits no appreciable improvement on thedehydrogenation kinetics. Meanwhile, the dehydrogenationkinetics of hydrogenated 2LiH−MgB2−0.15LiF−0.03NbB2 isslightly superior to that of hydrogenated neat 2LiH−MgB2 butobviously inferior to that of hydrogenated 2LiH−MgB2−0.03NbF5, indicating that the enhancement of dehydrogenationkinetics for the hydrogenated 2LiH−MgB2−0.03NbF5 couldnot be ascribed to the combination of 0.03NbB2 and functionalF-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 thatthe dehydrogenated sample is composed of nanometer oval-shaped particles (∼50 nm) and planar structures (∼80 nm),with comparatively larger surface area, as shown in themagnifying SEM image of Figure 6b. As known from aboveXRD patterns, these nanometer particles are corresponding tothe MgB2, LiH (LiH1‑xFx) and NbH phases. For heterogeneouscatalyzed reaction, the practical catalytic efficiency relies notonly on the intrinsic activity of the species but also on thedistribution of the catalysts.19 EDX elemental mapping of thedehydrogenated 2LiH−MgB2−0.03NbF5 shows a homoge-neous distribution of Nb atom throughout the sample (FigureS6), 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, thesupposed bigger particle size and less homogeneous distribu-tion of NbH might be the reason of the slightly inferiordehydrogenation 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 safelyconcluded that the NbF5 additive in improving reversiblehydrogen storage properties are ascribed to the synergisticeffect of in situ formed nano NbH acting as “active gateways”facilitating the diffusion of hydrogen through the diffusionbarriers both in de/hydrogenation processes and the “favorablethermodynamic destabilization” from the reversible transition

of LiH1−xFx caused by functionality of F-anion substitution inthe Li−Mg−B−H system. This fundamental understanding isof significance for the design and optimization of catalyticmethod and species and thereby laying the foundation forfurther improving the hydrogen storage properties of thecatalyzed Li−Mg−B−H system.

4. CONCLUSIONSIn summary, the reversible hydrogen storage properties of theLi−Mg−B−H system can be notably improved by doping

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

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

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2LiH/MgB2 mixture with small amounts of NbF5 additive. Theas-synthesized 2LiH−MgB2−0.03NbF5 can obtain a reversiblehydrogen storage capacity of ca. 9.0 wt % under the cycliccondition of dehydrogenation at 400 °C within 20 min andrehydrogenation at 350 °C within 20 min. It is found that NbF5reacts with LiH to form LiH1−xFx and NbH during ball-millingand subsequent hydrogenation. The pronounced de/hydro-genation performances arising upon doping NbF5 could beascribed to the novel synergistic effect of in situ formed nanoNbH particles acting as “active gateways” facilitating thediffusion of hydrogen and the reversible transition of LiH1−xFxresulting in the “favorable thermodynamic destabilization” inLi−Mg−B−H system, thus promoting effect persists well in thesubsequent de/hydrogenation cycles.

■ ASSOCIATED CONTENT*S Supporting InformationFigures of XRD pattern of as-prepared NbH and XRD patternsof 2LiH−MgB2 doped without and with different additives byball-milling. Cycling isothermal hydrogenation curves of 2LiH−MgB2 doped without and with different additives by ball-milling. Figure of XRD pattern of the 2LiH−MgB2−0.03NbF5sample after several dehydrogenation cycles. SEM image andEDX elemental mapping of 2LiH−MgB2−0.03NbF5 and2LiH−MgB2−0.15LiF−0.03NbB2 sample dehydrogenation at400 °C. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel/Fax: +86 571 8795 1152. E-mail: lxchen@zju.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe would like to acknowledge the supported by National BasicResearch Program of China (2010CB631300), National HighTechnology Research & Development Program of China(2012AA051503), National Natural Science Foundation ofChina (51001090 and 51171173), China Postdoctoral ScienceFoundation (20090262 and 2012M521167), and Key Scienceand Technology Innovation Team of Zhejiang Province(2010R50013).

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