Low-Temperature Reversible Hydrogen Storage Properties of LiBH4:A Synergetic Effect of Nanoconfinement and NanocatalysisJie Shao, Xuezhang Xiao, Xiulin Fan, Liuting Zhang, Shouquan Li, Hongwei Ge, Qidong Wang,and Lixin Chen*

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province,Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China

*S Supporting Information

ABSTRACT: LiBH4 has been loaded into a highly ordered mesoporous carbonscaffold containing dispersed NbF5 nanoparticles to investigate the possiblesynergetic effect of nanoconfinement and nanocatalysis on the reversible hydrogenstorage performance of LiBH4. A careful study shows that the onset desorptiontemperature for nanoconfined LiBH4@MC-NbF5 system is reduced to 150 °C, 225°C lower than that of the bulk LiBH4. The activation energy of hydrogen desorptionis reduced from 189.4 kJ mol−1 for bulk LiBH4 to 97.8 kJ mol−1 for LiBH4@MC-NbF5 sample. Furthermore, rehydrogenation of LiBH4 is achieved under mildconditions (200 °C and 60 bar of H2). These results are attributed to the active Nb-containing species (NbHx and NbB2) and the function of F anions as well as thenanosized particles of LiBH4 and high specific surface area of the MC scaffold. Thecombination of nanoconfinement and nanocatalysis may develop to become animportant strategy within the nanotechnology for improving reversible hydrogenstorage properties of various complex hydrides.

■ INTRODUCTION

Hydrogen, as a chemical energy carrier, is widely regarded as apotential cost-effective, renewable, and clean energy alternativeto fossil fuel, especially in the transportation sector. Thedevelopment of safe hydrogen storage technology with a highenergy density is a key prerequisite for the widespread usage ofhydrogen mobile applications.1 Recently, intensive interest hasbeen focused on complex hydrides,2−8 especially for lithiumborohydride (LiBH4), owing to its high gravimetric andvolumetric hydrogen capacities (18.5 wt % and 121 kg H2m−3).9 Unfortunately, the high desorption temperature (above400 °C), sluggish kinetics, and undesirable rehydrogenationconditions (350 bar of H2 at 600 °C) appear to limit itspractical application as an on-board hydrogen storagemedium.10 It is worthwhile to study strategies to improve thehydrogen sorption kinetics and reversibility in complex metalhydrides. To date several approaches, like partial cation/anionsubstitution,11−13 catalyst doping,14,15 and reactant destabiliza-tion,16,17 have been proposed to ease these problems. However,desorption and rehydrogenation temperatures are still too high.A recent promising strategy to significantly accelerate the

reaction kinetics is to reduce the particle size and formnanomaterials.18 By ball milling or a solvent evaporationprocess, nanoscale LiBH4 was formed with enhanced hydrogendesorption kinetics.19 Nanoconfinement into a mesoporousscaffold can be used to restrain the particle growth andagglomeration during desorption/absorption cycles.20−22

Thereby, kinetic enhancement and improved cycling stabilitywere achieved. Furthermore, nanoconfinement can even alter

the thermodynamical stability of metal hydrides as has beendemonstrated by theoretical calculations and experiments.23,24

In the case of LiBH4, Vajo’s group demonstrated that thedesorption temperature of LiBH4 was lowered by 75 °C, andrelatively mild absorption conditions of 100 bar of H2 and 400°C were obtained when incorporated into a carbon scaffold.25

Nevertheless, relying solely on this method is still difficult toachieve the rapidly reversible hydrogen absorption anddesorption under mild conditions.In our previous work, we demonstrated that the hydrogen

storage properties of LiBH4 can be substantially improved byintroducing NbF5 through the in situ formation of active Nb-containing catalysts and the function of F− anions.26−28 Morerecently, Heyn et al.29 and Jensen et al.30 both foundhydrogen−fluorine substitution in metal borohydrides, whichprovides a destabilization of metal borohydrides and mayfacilitate hydrogen uptake. Herein, we report a remarkableenhancement of the reversibility of hydrogen storage perform-ance in LiBH4 by a synergetic effect of nanoconfinement andNbF5 addition. By comparing the hydrogen de/absorptionkinetics with those of bulk LiBH4, nanoconfined LiBH4, andNbF5-doped LiBH4, we will show how NbF5 can improve thehydrogen storage properties of nanoconfined LiBH4. Moreover,in order to acquire detailed information about the kinetics ofthe reactions, we further investigated the kinetic mechanism of

Received: March 29, 2014Revised: May 7, 2014Published: May 9, 2014

Article

pubs.acs.org/JPCC

© 2014 American Chemical Society 11252 dx.doi.org/10.1021/jp503127m | J. Phys. Chem. C 2014, 118, 11252−11260

the LiBH4@MC-NbF5 system. At the end of the article, theactive species and their catalytic mechanisms are discussed.

■ EXPERIMENTAL SECTIONReagents. Commercially available LiBH4 (95%, Acros

Organics), NbF5 (98%, Aldrich), NbB2 (325 mesh, Aldrich),tetraethyl orthosilicate (TEOS, 98%, Aldrich), and Pluronicsurfactant P123 (Mw 5800, Aldrich) were used as received forthis research. All sample handling was performed in an MBraunglovebox maintained under a pure argon atmosphere with <1ppm of O2 and H2O vapor levels.Synthesis of Mesoporous Carbon Scaffold (MC) and

Loading of NbF5 Nanoparticles into MC Matrix (MC-NbF5). The mesoporous carbon scaffold was prepared by aninverse replica using SBA-15 as a solid template and sucrose asa carbon source described by Zhao’s group.31 NbF5 nano-particles loaded into MC matrix were prepared by the typicalmelt infiltration method via capillary action. And prior to theloading, the mesoporous carbon scaffold was degassed for 6 h at350 °C to remove the possible adsorbed moisture and gases. Amixture of 10 wt % NbF5 and the as-prepared MC matrix wasobtained by grinding for 10 min in a mortar and then loadingand sealing in a stainless steel autoclave in an Ar-filled glovebox.Then the sealed autoclave was transferred to a thermostat andheated to 90 °C for 30 min.Infiltration of LiBH4 into MC-NbF5 (LiBH4@MC-NbF5).

The predetermined amounts of LiBH4 and MC-NbF5 matrixwere ground for 10 min by hand in a glovebox. Then themixture was loaded into a sample holder and subsequentlyattached to a fixed-volume Sievert’s-type instrument withoutexposing the sample to air. The sample holder was heated to300 °C under about 140 bar of H2 and then kept at thistemperature for 30 min. The amount of infiltrated LiBH4 iscalculated from the bulk density of LiBH4 (ρ = 0.67 g cm−3)and the total pore volumes of MC matrix (obtained using BJH

methods, see Figure S2). In order to maximize the infiltrationamount of LiBH4, different pore volume fractions of LiBH4infiltration amounts have been conducted (see Figure S1), andthe optimal infiltration amount of LiBH4 we chose is 85 vol %.The premelted sample is hereafter referred to as LiBH4@MC-NbF5.

Preparation of Control Samples. The infiltrated LiBH4into MC and a ball-milled mixture of LiBH4 with NbF5 wereprepared as control samples. The infiltration procedure ofLiBH4@MC is consistent with that of the LiBH4@MC-NbF5.While 10 wt % NbF5 and LiBH4 were ball milled via a planetaryQM-3SP4 for 3 h. The ball-to-powder ratio was 40:1 with amilling speed of 400 rpm. Hereafter, the ball-milled sample islabeled as LiBH4-NbF5.

Characterization. X-ray diffraction (XRD) analyses wereperformed on an X’Pert Pro X-ray diffractometer (PANalytical,The Netherlands) with Cu Kα radiation at 40 kV and 40 mA.During the sample transfer and scanning, a lab-built argon filledcontainer was applied to prevent the samples from air andmoisture. The morphology of the sample was observed usingscanning electron microscopy (SEM, Hitachi SU-70) equippedwith an energy dispersive X-ray spectroscopy (EDX, HORIBAX-Max). An internal view of the specimen (∼12 × 10 × 1 μm3)was prepared by a focus ion beam technique (FIB) using a FEIQUATA 3D. The specimen was shot by gallium ion beam withthe energy of 30 kV. The microstructure was further examinedby transmission electron microscopy (TEM, JEOL JEM-1230working at 120 kV). Special caution had been taken to preventthe H2O/O2 contamination during the measurements.Physisorption isotherms were collected using a QuantacromeAutosorb-1-C system with nitrogen gas at 77 K. Prior tomeasurement, the samples were degassed for 24 h at 120 °C.The surface areas were calculated by the Brunauer−Emmett−Teller (BET) method. The pore size distributions were derivedfrom the adsorption branches of the isotherms using the

Figure 1. TEM (a) image of MC-NbF5 specimen, SEM (b, c) images of nanoconfined LiBH4@MC-NbF5 specimen shot by gallium ion beam, EDXmaps of B (d), C (e), Nb (f), and F (g) to the area in the red square (b), and the corresponding quantitative analyses of elements for the selectedareas shown in the inset.

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Barrett−Joyner−Halenda (BJH) model. The total pore volumewas obtained at P/P0 = 0.997.The differential scanning calorimetry and mass spectrometry

(DSC-MS) measurements were conducted on a Netzsch STA449F3 analyzer coupled with a Netzsch Q430C massspectrometer at different heating rates under flowing argonconditions (high purity, 50 mL min−1). The MS signals at themass-to-charge ratio (m/z) = 2, 18, 26, 32, and 49 wererecorded in order to detect H2, H2O, B2H6, O2, and BF3. Nosignificant quantity of H2O or O2 was detected. Fouriertransform infrared (FTIR) spectra were obtained with a BrukerTensor 27 unit in transmission mode. The testing samples wereprepared by cold-pressing a mixture of powder and potassiumbromide (KBr) at a weight ratio of 1:200 to form a pellet.Isothermal dehydrogenation and rehydrogenation of thesamples were conducted by using a carefully calibratedSievert’s-type apparatus. Specific steps detailed in our previousliterature.14 For comparison with the hydrogen capacity of bulkLiBH4, the hydrogen contents reported in this work arecalculated based only on the pure LiBH4, discounting theweight of the carbon scaffold and the additive. X-rayphotoelectron spectroscopy (XPS) was carried out on a VGESCALAB MARK II system with Mg Kα radiation (1253.6 eV)at a base pressure of 1 × 10−8 Torr. All binding energy (BE)values were referenced to the C 1s peak of contaminant carbonat 284.6 eV with an uncertainty of ±0.2 eV.

■ RESULTS AND DISCUSSIONCharacterization of the LiBH4@MC-NbF5 Composite.

In a typical synthesis process, the mesoporous carbon scaffold(MC) was synthesized using a hard template route frommesoporous silica particles (SBA-15).31 A highly orderedhexagonal arrangement of mesopores is easily observed bySEM and TEM images (shown in Figure S2). From the N2adsorption−desorption isotherm and BJH pore size distributioncurve of MC (Figure S2c), high mesoporous uniformity with apore size of about 3.8 nm is obtained, which is in agreementwith the TEM image. Figure 1a shows the SEM and TEMimages of the NbF5 nanoparticles loaded MC scaffold (MC-NbF5). NbF5 nanoparticles were homogeneously located in theMC scaffold without any aggregation, most of which were lessthan 10 nm in size. The corresponding energy dispersive X-rayspectroscopy (EDX) combined with focused ion beam (FIB)analyses are displayed in Figure S3. It is found that Nb and Fatoms are homogeneously dispersed inside the MC matrix, andthe detected amount of NbF5 is basically the same with theoriginal design (Table S1). This confirms that 10 wt % NbF5were successfully loaded in the MC scaffold.The surface morphology for the infiltrated LiBH4@MC-

NbF5 was obtained through SEM images. As shown in Figure1b,c, the highly ordered mesoporous channels were obviouslyfilled with LiBH4 after the infiltration, while the B, Nb, and Fmaps agree well with the C map from FIB-EDX for the loadedcomposite (Figure 1d−g), suggesting the good dispersions ofLiBH4 inside the pores of MC scaffold. Compared with thepure MC scaffold in Figure S2 and Table 1, after incorporationof NbF5 and LiBH4 into MC, the specific surface area (SBET)and the total pore volume were reduced from 1321 to 33.58 m2

g−1 and from 1.25 to 0.11 cm3 g−1, respectively, also stronglysuggesting that the LiBH4 particles had been successfullyconfined into the nanochannels of the MC scaffold.Figure S4 and Figure 2a show the XRD patterns of as-

prepared pure MC, MC-NbF5, and LiBH4@MC-NbF5. The

two major broad diffraction peaks of [002] and [100] of thegraphite structure can be observed for the MC scaffold (FigureS4a), in accordance with previous reports.32,33 For MC-NbF5(Figure S4b), some weak diffraction peaks of NbF5 emerge,suggesting that the NbF5 nanoparticles have incorporated intothe MC scaffold, in good agreement with the TEMobservations (Figure 1a). After the infiltration of LiBH4 intoMC-NbF5, the XRD pattern of the as-prepared LiBH4@MC-NbF5 shows some weak broadening LiBH4 diffractions (Figure2a(i)), which indicates a reduction of the average crystallite sizeand maybe increased stress and strain in the diminishedparticles of LiBH4 due to nanoconfinement.22 No pattern forNbF5 is evident, while there are reflections due to LiF, whichmeans a minor part of LiBH4 may react with NbF5 during theinfiltration. It is consistent with the XRD pattern for the ball-milled LiBH4-NbF5 in Figure S4c. In our previous work, wedemonstrated that a chemical reaction between LiBH4 andNbF5 took place to generate the active Nb- and F-containingspecies during ball milling.26,27 It was believed that both Nb-and F-containing species play critical roles for the improvedhydrogen storage performance of the NbF5-doped LiBH4.

28

However, these catalytic phases were not directly observedpresumably because of the low concentration of disorder stateor fine crystallites introduced by nanoconfinement, whichagrees well with other reports.34,35 The existence of [BH4]

− inthe infiltrated sample was confirmed by means of FTIR (Figure2b). For the as-prepared LiBH4@MC-NbF5 (Figure 2b(i)),there shows the strong characteristic peaks of B−H bendingmode at 1125 cm−1 and stretching modes around 2223−2384cm−1, confirming the existence of LiBH4.

15,36,37

Superior Hydrogen Storage Properties of the LiBH4@MC-NbF5 Composite. Hydrogen desorption from nano-confined LiBH4@MC-NbF5 composite and bulk LiBH4 wasstudied by simultaneous differential scanning calorimetry andmass spectrometry (DSC-MS). As shown in Figure 3, onlyhydrogen was detected from the thermal desorption measure-ment by MS (data of possible B2H6 and BF3 are shown in

Table 1. Surface Areas and the Total Pore Volumes of MCand LiBH4@MC-NbF5

sample SBET (m2 g−1) VP (cm3 g−1)

MC 1321 1.25LiBH4@MC-NbF5 33.58 0.11

Figure 2. XRD patterns (a) and FTIR spectra (b) of as-prepared (i)LiBH4@MC-NbF5; (ii) LiBH4@MC and (iii) LiBH4@MC-NbF5 afterdesorbed at 300 °C; (iv) MC-NbF5 and (v) LiBH4@MC-NbF5 afterabsorbed at 200 °C and 60 bar of H2 for 18 h.

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Figure S5). The DSC profile of the bulk LiBH4 shows theorthorhombic to hexagonal structure transition at 113 °C andmelting at 285 °C. A small amount of hydrogen comes out atthe melting point, and then majority of hydrogen releases above375 °C, exhibiting dehydrogenation peak at 467 °C. Themelting of LiBH4 in the ball-milled LiBH4-NbF5 shifted to thelower temperature (266 °C), which is in agreement with ourprevious work.26−28 Interestingly, the nanoconfined samples donot display the distinct melting point, indicating that LiBH4confined in MC matrix became more disordered. This isconsistent with the previous observations for LiBH4 confinedwithin small pores.22 Compared with the bulk LiBH4, thenanoconfined samples demonstrate pronouncedly reduceddehydrogenation temperatures and significantly improvedkinetics. The onset of hydrogen desorption for LiBH4@MCreduces to 205 °C, which is 170 °C lower than the bulk LiBH4,indicating a remarkable nanoeffect on the decomposition ofLiBH4. Moreover, the LiBH4@MC-NbF5 can even desorbhydrogen at 150 °C, remarkably lower than that of the ball-milled LiBH4-NbF5 (282 °C) and LiBH4@MC (205 °C). Aswe know, the hydrogen diffusivity and [BH4]

− mobility playimportant roles in the LiBH4 desorption and LiBH4 typicallydesorbs after melting.15,22 Liu’s group has found that [BH4]

−

anion mobility is significantly enhanced by the nanoconfine-ment.38 That is one of the reasons why the nanoscaled samplescan release hydrogen below the melting point of bulk LiBH4.On the other hand, the onset desorption temperature is directlydetermined by thermodynamics.32 By long duration volumetrichydrogen desorption measurements for nanoconfined LiBH4 at300 °C (Figure S6), it is believed that a synergetic effect ofnanoconfinement and the function of F anions introduced by

NbF5 nanoadditive can further modify the thermodynamics ofLiBH4@MC-NbF5 complex hydride, enabling hydrogen releaseunder the desirable conditions.Parallel to the thermal analyses, the desorption kinetics of the

first desorption cycle were also examined by using a volumetricmethod. As displayed in Figure 4a, the bulk LiBH4 did notshow any signals of hydrogen desorption and only a negligibleamount of hydrogen was released from ball-milled LiBH4-NbF5at 200 °C. When it comes to LiBH4@MC, it could desorb 0.99wt % of H2 at this temperature within 2.5 h, showing thatnanoconfinement can enhance the hydrogen desorptionkinetics favorably. More importantly, with additional NbF5dopant LiBH4 could release 3.13 wt % of H2 under the samecondition when confined in MC-NbF5, and the averagedesorption rate is 3.2 times compared to LiBH4@MC. Thisdemonstrates that the synergetic effect of nanoconfinement andnanocatalysis can improve the hydrogen desorption kinetics ofLiBH4 pronouncedly. Furthermore, LiBH4@MC-NbF5 canrelease up to about 6.52 wt % of H2 at 200 °C. To the bestof our knowledge, it is the highest desorption capacity forLiBH4 at such low temperature.As well as improved desorption kinetics, we also found that

the LiBH4 nanoparticles confined in MC-NbF5 exhibitedimproved absorption kinetics relative to LiBH4@MC. Thedehydrogenated LiBH4@MC-NbF5 sample could be rehydro-genated under moderate conditions at 200 °C and 60 bar of H2.We measured the subsequent dehydrogenation half-cycle toquantify the restored hydrogen amount (shown in Figure 4b),and the absorption of LiBH4 was confirmed by the subsequentXRD and FTIR measurements (shown in Figure 2). Figure 4bpresents a comparison of the first two desorption cycles ofLiBH4@MC and LiBH4@MC-NbF5 at 300 °C. It was observedthat the LiBH4@MC-NbF5 released the restored 10.65 wt %hydrogen rapidly in the second cycle, at a similar desorptionrate to that observed in the first cycle. However, the LiBH4@MC only released 3.33 wt % H2, and the ball-milled LiBH4-NbF5 merely absorbed a trace of hydrogen under theseconditions (Figure S7).For the desorbed LiBH4@MC-NbF5, only diffraction peaks

of LiF is discernible while some traces of LiBH4 are stilldetected in the dehydrogenated LiBH4@MC in Figure 2a,indicating that the LiBH4@MC-NbF5 could decomposecompletely at 300 °C and has a better desorption kinetics.No LiH and B could be observed in both of thedehydrogenated samples, suggesting that the LiH and B arein amorphous or disordered-nanoclaster state and most likelyconfined in the nanopores. The disappearance and reemergenceof the characteristic [BH4]

− group of LiBH4 in FTIR spectra(Figure 2b) clearly evidenced their reversible dehydrogenationreactions under the applied conditions.25,37 Moreover, from theintensity of [BH4]

− group, we can easily tell that the LiBH4@MC-NbF5 contains a better absorption kinetics at 200 °C and60 bar of H2. To the best of our knowledge, these are thelowest conditions reported to date for LiBH4 hydrogenation,which may be due to the fact that both the kinetics andthermodynamics of LiBH4 are altered by the combination ofnanoconfinement and NbF5 nanoadditive in this method.As the amount of hydrogen released from bulk LiBH4

decreases significantly with successive desorption/absorptioncycles,25 we further investigated the cycling stability of thenanoconfined samples. The normalized reversible hydrogenstorage capacities during first five desorption−absorption cyclesfor the samples LiBH4@MC and LiBH4@MC-NbF5 are

Figure 3. DSC-MS profiles of the (a) bulk LiBH4, (b) ball-milledLiBH4-NbF5, (c) LiBH4@MC, and (d) LiBH4@MC-NbF5 at a heatingrate of 5 °C min−1. DSC profiles and MS spectra (m/z = 2) are shownas black and blue curves, respectively.

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compared in Figure 4c, and their specific cycling desorptioncurves are displayed in Figure S8. The reversible hydrogenstorage capacity tends to decrease possibly due to the formationof some more stable closo-boranes, e.g. Li2B12H12.

30 Only 47%initial hydrogen is released from the LiBH4@MC at the fifthcycle. However, more than 66% hydrogen for LiBH4@MC-NbF5 is desorbed at the same cycle. It is obvious that theLiBH4@MC-NbF5 shows a higher capacity retention ratio thanthe LiBH4@MC, indicating that NbF5 addition can greatlyimprove the hydrogen release and uptake reversibility ofnanoconfined LiBH4.As the XRD could not provide a direct observation of the

Nb-containing species in cycling products, X-ray photoelectronspectroscopy (XPS) method was employed to characterize andidentify the Nb-containing species. The goal was to furtherelucidate the reaction mechanism of the LiBH4@MC-NbF5system. Figure 5 presents the XPS spectra of the LiBH4@MC-NbF5 samples before and after the first and fifth desorptioncycle at 260 °C under 0.02 bar of H2. The Nb 3d spectra of theas-prepared LiBH4@MC-NbF5 sample (Figure 5a) can beresolved fairly well with two spin−orbit split doublets from twochemically different Nb entities. The peaks at 203.2 eV (Nb3d5/2) and 205.9 eV (Nb 3d3/2) should be assigned to theNbHx,

39,40 revealing that the NbF5 nanoparticles transformedinto NbHx after LiBH4 infiltrated into the MC-NbF5 matrix.The peaks at 207.2 and 210 eV should be assigned to theNb2O5,

41 which might be due to the oxidation during thesample transfer. After the first desorption cycle, the shape of theNb 3d spectrum have no obvious change. However, the sample

collected at the end of the fifth desorption cycle exhibites asmall but significant positive shift in the Nb 3d core levelbinding energy compared to the other samples seen in Figure5c. It is noteworthy that a new Nb entity of NbB2 (203.5 and206.2 eV)42,43 can be easily identified. A combination of the B1s and Nb 3d spectra could further confirm this observation. Asdisplayed on the right of Figure 5, the B 1s spectra of the as-prepared LiBH4@MC-NbF5 samples show the peaks of LiBH4(188.4 eV) and amorphous B (187.2 eV) for desorption

Figure 4. (a) Hydrogen desorption curves for the prepared samples at 200 °C under 0.02 bar of H2; (b) the first two desorption cycles of LiBH4@MC and LiBH4@MC-NbF5 at 300 °C under 0.02 bar of H2, the absorption was carried out at 200 °C and 60 bar of H2 for 18 h; (c) normalizedcyclic hydrogen storage capacities for LiBH4@MC and LiBH4@MC-NbF5. The heating rate is 5 °C min−1.

Figure 5. XPS spectra in the energy levels of Nb 3d (left) and B 1s(right) for the LiBH4@MC-NbF5 samples (a) before the firstdesorption cycle, (b) after the first desorption cycle, and (c) afterthe fifth desorption cycle at 260 °C under 0.02 bar of H2. Forcomparison, the commercial NbB2 was used as a reference.

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product before and after the first desorption cycle,respectively.44 While after the fifth cycle, the sample exhibitesa negative shift in the binding energy and shows a new weak B1s peak at 186 eV, which should be assigned to the NbB2.

43

The oxide phase at 192.0 eV corresponds to B2O3, which is inagreement with the literature values.45

A careful analysis of the XPS spectra revealed that theemergence of NbB2 is accompanied by the attenuation of theNbHx signal. By the XPS area ratios of Nb species for thecorresponding LiBH4@MC-NbF5 samples shown in Table S2,we were able to clearly identify the decrease of NbHx. Itsuggests that the formed NbHx was unstable and mostlyconverted to NbB2 in the subsequent desorption cycles via thefollowing reaction 1 that is thermodynamically feasible (seeTable S3). Our previous works have found that manynanotransition metal borides (such as NiB, ZrB2, CeB6,NbB2, and TiB2) can not only act as the active species forthe hydrogen storage of LiBH4 system but also enhance thecycling stability of LiBH4 system by improving the rehydroge-nation property.15,37,46,47 Therefore, it is reasonable to believethat the newly formed NbB2 is the key factor to improve thecycling stability for LiBH4@MC-NbF5.

+ → + + +

Δ = −=

x

G

2LiBH NbH 2LiH NbB (3 /2)H

113.97 kJx

x

4 2 2

298, 148

(1)

Kinetic Mechanism of the LiBH4@MC-NbF5 Compo-site. To identify the kinetics mechanism for the dehydrogen-ation of confined LiBH4, we also measured the isothermaldehydrogenation kinetic curves of LiBH4@MC and LiBH4@

MC-NbF5 in the temperatures range of 280−310 °C (Figure6a). It is obvious that with additional NbF5 nanoparticlesconfined in MC matrix the LiBH4@MC-NbF5 exhibits asuperior desorption kinetics over the MC-NbF5. To furtherinvestigate the kinetics mechanism, it is necessary to select aproper model first. A practical approach for the rapid selectionof an appropriate rate equation for the kinetics has been madeby Sharp49 and Jone’s group.50 The experimental values of (t/t0.5)exp are plotted against the theoretical ones (t/t0.5)theo toproduce a linear plot, where t is the time and t0.5 means thetime when α = 0.5. The value of the linear slope for anacceptable model should be very close to 1. Figure 6b showsthe relationship of (t/t0.5)exp versus (t/t0.5)theo for nine differentkinetic mechanisms (listed in Table S451), and the linear slopesof each model are listed in the figure. For LiBH4@MC, the R3model exhibits a good linear relationship with slope close to 1,whereas for LiBH4@MC-NbF5, it should be the D3 model.This finding indicates that the hydrogen desorption reaction ofLiBH4@MC correlates closely to the three-dimensional phaseboundary controlled kinetic mechanism represented by the R3model. When additional NbF5 nanoparticles were incorporatedinto the LiBH4@MC system, the kinetic mechanism waschanged to the three-dimensional diffusion-controlled model.Figure S9 depicts the plots of proposed kinetics mechanism

versus time for the nanoconfined LiBH4. It shows that all curvesexhibit a good linearity with linear coefficient R2 > 0.99; theirreaction behaviors can be reasonably interpreted by the three-dimensional phase boundary controlled model and the three-dimensional diffusion controlled model, respectively. Accordingto the rate constants derived from Figure S9, the apparent

Figure 6. (a) Normalized isothermal dehydrogenation curves of LiBH4@MC and LiBH4@MC-NbF5 at different temperature range of 280−310 °C.(b) (t/t0.5)theo vs (t/t0.5)exp of LiBH4@MC and LiBH4@MC-NbF5 for various kinetic models. (c) Kissinger plots of the bulk LiBH4 (squares),LiBH4@MC (circles), and LiBH4@MC-NbF5 (triangles).

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activation energy, Ea, of the hydrogen desorption can becalculated using the Arrhenius equation K = A exp(−Ea/RT)(2). The results are presented in Figure S10 and Table 2. It is

found that Ea is only 102.0 ± 3.8 kJ mol−1 for hydrogendesorption from LiBH4@MC-NbF5, which is drastically lowerthan that of LiBH4@MC (Ea = 140.9 ± 6.2 kJ mol−1). Inaddition, we use Kissinger’s method to further investigate theapparent activation energy.52 Experimental data collected underdifferent heating rates of 3.5, 5, 6.5, 8, and 10 °C min−1 were fitusing the Kissinger equation ln(β/Tp

2) = −Ea/RTp + ln(AR/Ea)(3), and the Ea was calculated from the slope of the fitting line.As shown in Figure 6c and Table 2, the data derived fromKissinger’s method are very close to the value that determinedby Arrhenius equation. The Ea for hydrogen release of LiBH4@MC-NbF5 is 91.6 kJ mol−1 lower than that of bulk LiBH4 (Ea =189.4 ± 3.9 kJ mol−1), indicating that the Ea is significantlyaffected by nanoconfinement and nanocatalysis, which thusenable superior kinetics under the same conditions.25,53,54

Discussion of the Enhanced Hydrogen StorageProperties. On the basis of the above observations, theresults show that the hydrogen desorption kinetics, reversibility,and cycling stability are all improved considerably by confiningLiBH4 into the NbF5 loaded MC scaffold as compared to theLiBH4@MC sample. This is evident that the combination ofnanoconfinement and NbF5 addition does bring a favorablesynergistic catalytic effect. It is well-known that the practicalcatalytic efficiency relies not only on the intrinsic activity of thespecies but also on the distribution of the catalyst particles.15,55

The more homogeneously the catalyst particles disperse andthe more tightly the catalyst bind to the reactants, the bettercatalytic effect that is obtained. This is the reason why the ball-milling method is usually effective for the mixing of materialsand catalyst particles, and nanosized particles are much moreefficient in catalysis than microsized particles.46,56 In thispresent work, NbF5 nanoparticles are incorporated into the MCmatrix with small pore size (3.8 nm) and high surface area,resulting in a well distribution and strong contact withreactants. Meanwhile, the NbHx and F-containing catalyticspecies formed during the LiBH4 infiltration are also confinedin the nanopores, which are small enough and provide a hugenumber of active sites for the catalytic decomposition of LiBH4.They may also facilitate the dissociation and recombination ofhydrogen molecules on their surface and the atomic hydrogendiffusion along the grain boundaries and inside the grains.Furthermore, the unstable intermediate NbHx will mostlyconvert to NbB2 heterogeneous nucleation agent in thesubsequent desorption cycles, which substantial improves thecycle stability of the LiBH4@MC-NbF5 system.46,47 Moreimportantly, additional NbF5 doping can further modify thethermodynamics of nanoconfined LiBH4 the function of Fanions, which has been theoretically and experimentallydemonstrated in the study of LiBH4,

28,57,58 enabling hydrogencycle under the desirable conditions. In general, the mechanism

of hydrogen storage property enhancement by nanoconfine-ment and catalysis is very complicated and not sufficientlyunderstood. Further research on this topic is needed.

■ CONCLUSIONS

In this work, we have reported a simple strategy to significantlyimprove the hydrogen storage properties of LiBH4, bycombining the NbF5 catalyst doping and the nanoconfinement.The results show that the onset desorption temperature fornanoconfined LiBH4@MC-NbF5 system reduces to 150 °C,225 °C lower than that of the bulk LiBH4. More importantly,rehydrogenation of LiBH4 is achieved under mild conditions(200 °C and 60 bar of H2), which are the lowest conditionsreported to date. The activation energy of hydrogen desorptionis reduced from 189.4 kJ mol−1 for bulk LiBH4 to 97.8 kJ mol

−1

for the LiBH4@MC-NbF5 sample, indicating a reduced kineticbarrier. All these have demonstrated that there is a favorablesynergetic effect between nanoconfinement and NbF5 addition.A full understanding of this synergetic effect may help tosignificantly enhance the reversible hydrogen storage propertiesof LiBH4. The combination of nanoconfinement and function-alized nanoporous scaffolds may develop to become animportant protocol within the nanotechnology for improvingproperties of variety complex hydrides.

■ ASSOCIATED CONTENT

*S Supporting InformationDSC curves for samples infiltrated with different pore volumefractions of LiBH4. SEM and TEM images of as-preparedmesoporous carbon scaffold; pore size distributions for the MCand the loaded LiBH4@MC-NbF5 composite from the BETresults; SEM and EDX maps of MC-NbF5 specimen; XRDpatterns of as-prepared pure MC, MC-NbF5, and ball-milledLiBH4-NbF5; long duration volumetric hydrogen desorption fornanoconfined LiBH4 at 300 °C under 3.60 bar of H2; hydrogencyclic desorption curves for LiBH4@MC and LiBH4@MC-NbF5 at 260 °C under 0.02 bar of H2; time dependence ofkinetic modeling equations f(α) and Arrhenius plots for thedesorption kinetics of LiBH4@MC and LiBH4@MC-NbF5.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail lxchen@zju.edu.cn; Tel +86 571 8795 1152; Fax +86571 8795 1152 (L.C.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support forthis research from the National High Technology Research &Development Program of China (2012AA051503), from theNational Basic Research Program of China (2010CB631300),from the National Natural Science Foundation of China(51171173 and 51001090), from the Program for InnovativeResearch Team in University of Ministry of Education of China(IRT13037), and from the Key Science and TechnologyInnovation Team of Zhejiang Province (2010R50013).

Table 2. Apparent Activation Energies (Ea) for As-PreparedSamples by Different Methods

Ea (kJ mol−1)

sample Arrhenius Kissinger

LiBH4 189.4 ± 3.9LiBH4@MC 140.9 ± 6.2 143.1 ± 3.7LiBH4@MC-NbF5 102.0 ± 3.8 97.8 ± 3.7

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