Microstructures and Hydrogen Desorption Properties of the MgH2

Jul 28, 2014 - These Nb/NbH layers will act as an impediment to grain growth of Mg/MgH2 and as gateway for hydrogen diffusion during the cycling ...
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Microstructures and Hydrogen Desorption Properties of the MgH2− AlH3 Composite with NbF5 Addition Haizhen Liu,† Xinhua Wang,*,†,‡ Yongan Liu,† Zhaohui Dong,§ Shouquan Li,‡ Hongwei Ge,† and Mi Yan*,† †

State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, and ‡Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Zhejiang University, Hangzhou, Zhejiang 310027, China § Zhejiang Metallurgical Research Institute, Hangzhou, Zhejiang 310017, China ABSTRACT: Niobium fluoride (NbF5) is introduced into the MgH2 + 1/4AlH3 hydride composite by ball milling to improve the hydrogen desorption properties of the Mg−Al−H system. It is found that after being ball milled with 1 mol % of NbF5, AlH3 in the composite has almost fully decomposed and forms metallic Al, which indicates that NbF5 can significantly destabilize AlH3. DSC results show that NbF5 addition also helps reduce the peak desorption temperature of MgH2 in the composite from 324 to 280 °C. Isothermal desorption measurements demonstrate that MgH2 in the doped composite can rapidly release 98% of its hydrogen after desorption at 300 °C for 1 h, while the valued is only 46% for the undoped composite. The activation energy for hydrogen desorption of MgH2 in the doped composite is calculated to be 104.5 kJ/mol, much lower than that in the undoped composite (127.4 kJ/mol). These results suggest that NbF5 addition dramatically improves the hydrogen desorption kinetics of the MgH2 + 1/4AlH3 composite. Dehydrogenation− hydrogenation measurements reveal that the hydrogen desorption kinetics of the undoped composite declines with cycle number, whereas the NbF5-doped composite maintains good cycling stability. Microstructure studies indicate that the decline of the kinetics is attributed to the grain growth and particle agglomeration of MgH2 during hydrogen sorption cycling. However, NbF5 addition can suppress this grain growth through the formation of Nb/NbH layers surrounding the particles of MgH2 and acting both as the impediment to grain growth of Mg/MgH2 and as the gateway for hydrogen diffusion. Finally, the role that AlH3 plays in the hydrogen desorption process of the Mg−Al−H composites is discussed. materials.20−24 Mixing with suitable additives is another strategy to improve the hydrogen storage properties of MgH2. These additives include metals,17,25−32 oxides,33−40 halides,41−44 hydrides,45−47 or nonmetals.48,49 Recently, strategies for simultaneous tuning of the thermodynamics and kinetics of Mg-based hydrogen storage alloys have been reported.50,51 By the above methods, the hydrogen desorption and absorption properties of MgH2 have been remarkably enhanced over the past years. Aluminum hydride (AlH3) is another lightweight metal hydride, and it possesses a theoretical hydrogen capacity of 10.01 wt %. AlH3 is an ideal off-board hydrogen storage material because it can start to release hydrogen at a temperature below 100 °C.52 AlH3 is typically prepared by the ether reaction of LiAlH4 and AlCl3 forming AlH3·nEt2O and LiCl followed by the separation of LiCl and ether.53 Depending on the synthesis process, AlH3 will crystallize in different structures. α-AlH3 is the most stable, whereas the β, γ, and α′ polymorphs are less stable.53 However, AlH3 can be kinetically stabilized under ambient conditions because of the surface oxide layer.52 Concerning the reversibility, it was shown that a

1. INTRODUCTION Energy and environment crises are forcing humans to search for new energy sources and carriers. Hydrogen is considered to be an ideal energy carrier for both stationary and mobile applications because of its high energy density, high abundance, good performance in fuel cells, and nontoxicity. However, hydrogen storage is still a bottleneck technique and the development of a lightweight, effective, and safe hydrogen storage system is essential for on-board applications.1,2 One of the most promising hydrogen storage methods is in the form of metal hydrides.3−5 Among the various metal hydrides studied, magnesium hydride (MgH2) is widely acknowledged as a promising hydrogen storage material because it possesses a high hydrogen capacity of 7.66 wt % and magnesium is low cost, lightweight, and abundant in the earth.6,7 However, the hydrogen sorption kinetics of the MgH2 is too slow and its thermal stability is unfavorably high (ΔH = −74 kJ/mol).8−11 It has been shown that a temperature higher than 300 °C is needed for MgH2 to start hydrogen desorption at 0.1 MPa H2.12 In recent years, many methods have been reported to enhance the hydrogen storage properties of MgH2. Reducing the particle or the grain size of MgH2 to nanoscale is one of the strategies,13 which can be achieved by high-energy ball milling12,14−19 or by confinement within nanostructured © 2014 American Chemical Society

Received: May 23, 2014 Revised: July 14, 2014 Published: July 28, 2014 18908

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measurements were conducted using a lab-built Sieverts-type apparatus. For the TPD measurements, the samples were heated gradually from room temperature to about 500 °C with a heating rate of 4 °C/min. For the isothermal desorption measurements, the samples were quickly heated to 300 °C and the temperature was kept constant at about 300 °C. With regard to the cycling hydrogen desorption measurements, the dehydrogenated samples were rehydrogenated under 300 °C and 5 MPa H2 for 1 h prior to the next desorption cycle. Powder X-ray diffraction (XRD) tests were conducted on a PANalytical X-ray diffractometer (X’Pert Pro) with Cu Kα radiation. To avoid exposure to the air or any moisture, the samples were sealed with an amorphous membrane during XRD measurements. Synchronous thermal analyses were conducted using a differential scanning calorimeter (DSC, Netzsch STA449F3) equipped with a mass spectrometer (MS, Netzsch QMS403C). This mass spectrometer was utilized to collect hydrogen deosorption spectrum synchronously during the DSC measurement. The samples were heated from room temperature to about 500 °C with a set heating rate under flowing argon of 50 mL/min. Scanning electron microscopy (SEM, FEI SIRION 100 or Hitachi S4800) was applied to study the morphology of the sample. The elemental distributions were analyzed by an attached energy dispersive spectroscopy (EDS) instrument.

hydrogen pressure higher than 2.5 GPa is required to recover AlH3 from the spent Al at room temperature.54,55 Nevertheless, AlH3 has been successfully synthesized by an electrochemical method56 or by the formation of triethylamine alane using the direct hydrogenation of dimethylethylamine and catalyzed aluminum followed by transamination with triethylamine.57 Graetz et al. have reviewed the research progress on AlH3 as an energy and hydrogen storage material.58 By combining two or more hydrides to form the so-called reactive hydride composites (RHCs), the overall reaction enthalpy can be changed through formation of new compounds upon decomposition.59,60 Inspired by this idea, we have prepared a MgH2−AlH3 composite in our previous work.45,61 In this composite, a mutual destabilization has been observed: the hydrogen sorption kinetics of the MgH2 is significantly improved and the hydrogen desorption temperature of γ-AlH3 is slightly reduced. In addition, it was shown that MgH2 in the MgH2−AlH3 composite exhibits hydrogen storage properties better than that in the MgH2−Al system in that AlH3 improves not only the desorption−absorption kinetics but also the desorption capacity of the MgH2, while the metallic Al only slightly enhances the desorption kinetics.45 It was suggested that the superiority of AlH3 over metallic Al is attributed both to the brittleness feature of aluminum hydride and to the fact that AlH3 will decompose to form oxide-free Al*. The former characteristic will lead to more uniform elemental distributions when preparing the MgH2−AlH3 composite by short-term ball milling, and the latter characteristic will produce Al* with high chemical activity, which will benefit the kinetics of the reaction between MgH2 and Al.45 It was also found that during the desorption process of the MgH2−AlH3 composite, AlH3 will first decompose to form an oxide-free Al*, and then this in situ formed oxide-free Al* will further react with MgH2 to form Mg17Al12 alloy,45,61 thus improving the desorption properties of MgH2.45,59 Despite the good results that the hydrogen sorption properties of MgH2 have been significantly improved by ball milling with AlH3, the performance of this composite is still far from practical application. Thus, further improvement in the hydrogen desorption properties of the MgH2−AlH3 composite is highly desired. Niobium fluoride (NbF5) has been utilized to improve the hydrogen storage properties of Mg-based hydrogen storage materials and shows positive results.62−71 In this work, NbF5 was introduced into the MgH2 + 1/4AlH3 composite by ball milling to improve the hydrogen desorption properties of the MgH2 + 1/4AlH3 composite. Then, the microstructures and the hydrogen desorption properties were studied and possible catalysis mechanisms were discussed.

3. RESULTS AND DISCUSSION The XRD pattern of the as-prepared AlH3 is shown in Figure 1a, which shows the AlH3 sample contains mainly the γ-AlH3

Figure 1. XRD patterns of (a) as-prepared AlH3, (b) as-milled undoped MgH2 + 1/4AlH3 composite, (c) as-milled NbF5-doped MgH2 + 1/4AlH3 composite, (d) undoped MgH2 + 1/4AlH3 composite after isothermal desorption at 300 °C, and (e) NbF5doped MgH2 + 1/4AlH3 composite after isothermal desorption at 300 °C.

2. EXPERIMENTS MgH2 (Alfa Aesar, 98%) and NbF5 (Alfa Aesar, 99%) were used as received. AlH3 was prepared by an organometallic method, which has been described in detail elsewhere.53,72,73 The composites were prepared by ball milling using a QM3SP4 planetary ball mill (Nanjing Nanda Instrument Plant). Generally, 1 g of the sample together with 50 g of stainless steel balls was sealed in a 100 mL stainless steel vial in an argon-filled glovebox. Then milling was conducted at 400 rpm for certain durations. To prevent temperature increase due to long-term milling, the milling process was paused every 6 min for heat dissipation. Temperature-programmed desorption (TPD) (i.e., the nonisothermal desorption) and the isothermal desorption

phase. The NbF5-doped MgH2 + 1/4AlH3 composite was prepared by ball milling the MgH2 with 1 mol % NbF5 for 10 h followed by further ball milling with 1/4AlH3 for an additional 30 min. For comparison, the undoped MgH2 + 1/4AlH3 composite was milled for the same duration of 10.5 h. Their XRD patterns are shown in panels b and c of Figure 1, respectively. It is observed that the diffraction peaks of MgH2 have broadened intensively, indicating that the grain sizes of MgH2 have been refined after ball milling. Scherrer’s equation, expressed by eq 1, was used to estimate the average grain sizes of MgH2 in the as-prepared composites74 18909

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Figure 2. Schematic diagram of hand-milling the AlH3 with 1 mol % NbF5 during TPD measurements at room temperature.

D=

Kλ B cos θ

(1)

where D is the average grain size, λ the wavelength of the incident X-ray, B the full width at half-maximum of a diffraction peak, θ the Bragg angle, and K a constant depending on the shape of the grains (generally assumed to be 0.89). The average grain sizes of MgH2 in the as-milled undoped MgH2 + 1/4AlH3 composite and the NbF5-doped MgH2 + 1/4AlH3 composite are estimated to be 11.3 and 11.9 nm, respectively, which suggests that ball milling can effectively reduce the grain size of MgH2 to nanoscale. It is found that after ball milling, AlH3 in the NbF5-doped MgH2 + 1/4AlH3 composite has almost fully decomposed because the XRD pattern in Figure 1c shows the absence of AlH3 phase. Instead, the metallic Al is present in the ball-milled composite. However, AlH3 is still present in the as-milled undoped MgH2 + 1/4AlH3 composite. This indicates that γAlH3 is significantly destabilized by NbF5 and has decomposed to form metallic Al and H2 during ball milling. To reveal the detailed desorption behavior of AlH3 with NbF5 addition, 0.9405 g of AlH3 and 0.0595 g of NbF5 (i.e., the composition of AlH3 + 1 mol % NbF5), together with 50 g of balls, were loaded into a 100 mL vial. Then, the vial was connected to a Sieverts type apparatus for TPD measurements. It is noted that the operation was carried out carefully to ensure that AlH3 did not contact NbF5 before the TPD measurement. The TPD measurement was started by shaking the vial by hand (we call it hand-milling) for 10 min at room temperature; in this way AlH3 will contact NbF5 and begins decomposing to release hydrogen gas. A schematic diagram of hand-milling the AlH3 with 1 mol % NbF5 during TPD measurements at room temperature is shown in Figure 2. The hydrogen desorption curve is shown in Figure 3a. The SEM image and the elemental distributions of the NbF5-doped AlH3 after desorption for about 15 h is displayed in Figure 3b. It can be clearly seen that the elemental Nb and F are uniformly distributed in the composite; this indicates that hand milling for 10 min is sufficient to mix AlH3 with NbF5 well. It is seen from Figure 3a that AlH3 releases hydrogen immediately after contacting NbF5, and a hydrogen capacity of 5.0 wt % is obtained after desorption for 1 h. It was suggested in previous work that AlH3 is metastable at room temperature, and this metastability is generally assumed to be ascribed to the surface Al2O3 layer surrounding the AlH3 particle and serving to

Figure 3. (a) TPD curves of the NbF5-doped AlH3. Desorption is carried out at room temperature. (b) SEM image and elemental distributions of the NbF5-doped AlH3 after isothermal desorption at room temperature for about 15 h.

encapsulate the contained hydrogen.75 The hydrogen desorption starts only when the surface oxide layer breaks and free AlH3 surface gets in contact with the gas phase.76 Therefore, in our present work, NbF5 may be able to break the surface oxide layer easily, thus promoting the desorption process of AlH3 at room temperature. Figure 4 presents the synchronous DSC-MS(H2) curves of the undoped and NbF5-doped MgH2 + 1/4AlH3 composites. For comparison, the curves of the pure MgH2 milled under the same conditions are also shown. An endothermic desorption peak starting at about 300 °C and peaking at 347.7 °C is found in the DSC-MS(H2) curves of the as-milled MgH2. As for the undoped MgH2 + 1/4AlH3 composite, two desorption peaks are found in the MS(H2) curve with the low-temperature peak ascribed to the hydrogen desorption of AlH3 and the hightemperature peak related to the hydrogen desorption of MgH2. The onset desorption temperature and the peak desorption temperature of MgH2 in the undoped MgH2 + 1/4AlH3 composite are reduced to 245 and 323.8 °C, respectively, 18910

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To further study the hydrogen desorption kinetics of the composites with or without NbF5 addition, Kissinger’s equation77 was used to estimate the apparent activation energy (Ea) for the hydrogen desorption reaction of MgH2. Kissinger’s equation can be written as ln

E c =− a +A 2 RTP TP

(2)

where c is the heating rate used in the DSC test and TP is the absolute temperature at which the rate of a desorption reaction is the highest. R is the universal gas constant, and A is a constant. DSC curves with various heating rates for the MgH2 + 1/4AlH3 composites with or without NbF5 addition are shown in Figure 6a,b. From the estimation of the hydrogen desorption activation energies in Figure 6c, it is indicated that the hydrogen desorption activation energy of MgH2 in the NbF5doped MgH2 + 1/4AlH3 composite is reduced from 127.4 kJ/ mol for the undoped composite to 104.5 kJ/mol. This reduction in the desorption activation energy is believed to contribute to the improvement in the hydrogen desorption kinetic property of the MgH2 in the NbF5-doped MgH2 + 1/ 4AlH3 composite. The cycling hydrogen desorption properties of the undoped and NbF5-doped MgH2 + 1/4AlH3 composites were studied at 300 °C for the initial 3 cycles. Prior to the next dehydrogenation, the rehydrogenations of the dehydrogenated samples were carried out under conditions of 300 °C and 5 MPa H2 for 1 h. The cycling hydrogen desorption curves are displayed in Figure 7. As can be seen, the hydrogen desorption kinetics of the NbF5-doped MgH2 + 1/4AlH3 composite is generally faster than that of the undoped MgH2 + 1/4AlH3 composite. In addition, for the undoped MgH2 + 1/4AlH3 composite, the hydrogen desorption kinetics decreases with cycling. However, for the composite with NbF5 addition, the hydrogen desorption kinetics first declines in the second cycle and then is maintained in the third cycle. This indicates that NbF5 addition benefits the cycling desorption kinetics of the MgH2 + 1/4AlH3 composite. From Figure 7, it is observed that about 4.5 wt % of hydrogen can be reversibly stored in the MgH2 + 1/4AlH3 composite doped with 1 mol % NbF5. It is noted that the hydrogen desorption capacity of the doped composite is somewhat lower than the undoped one; this may be attributed to the dead weight of the NbF5 addition. We then conducted XRD measurements of the composites in the hydrided state at the second cycle, and the XRD profiles are shown in Figure 8. It can be seen that the diffraction peak of MgH2 in the composite with NbF5 addition is weaker and broader than that without NbF5 addition. According to Scherrer’s equation (eq 1), the average grain sizes of MgH2 in the composites with and without NbF5 addition are calculated to be 47.8 and 64.8 nm, respectively; both which are larger than their original size of 11 nm. This grain growth of MgH2 may contribute to the degradation of the hydrogen desorption kinetics of MgH2 in the second cycle, shown in Figure 7. However, the grain size of MgH2 in the NbF5-doped composite is smaller than that in the undoped composite; this suggests that NbF5 addition can restrain the grain growth to some extent during the absorption−desorption cycling, thus improving the cycling hydrogen desorption kinetics of the composite. Jin et al.65 and Kim et al.63,64 have studied the hydrogen sorption properties of MgH2 with NbF5 as catalyst. They

Figure 4. DSC (a) and MS-H2 (b) curves of the as-milled MgH2 and the undoped and NbF5-doped MgH2 + 1/4AlH3 composites. Heating rate is 4 °C/min.

compared to those of the as-milled MgH2. When mixed with 1 mol % NbF5, the onset and the peak desorption temperatures of MgH2 in the MgH2 + 1/4AlH3 are further reduced to 225 and 279.9 °C, respectively, which are 75 and 67.8 °C lower than those of the as-milled MgH2. It is found that the desorption peak of AlH3 is absent in the DSC-MS(H2) curves of the NbF5-doped MgH2 + 1/4AlH3 composite; this is because AlH3 has almost fully decomposed during ball milling, as shown in Figures 1c and 3a. In a word, NbF5 can significantly reduce the desorption temperature of the MgH2 + 1/4AlH3 composite. To study the hydrogen desorption kinetics of MgH2 in the composites, the isothermal desorption measurements were carried out at 300 °C; the fractional isothermal desorption curves of MgH2 are shown in Figure 5. As can be seen, the

Figure 5. Fractional isothermal desorption curves of MgH2 in the asmilled MgH2, undoped MgH2 + 1/4AlH3 composites, and NbF5doped MgH2 + 1/4AlH3 composites. Desorption temperature is 300 °C.

hydrogen desorption kinetics of MgH2 is significantly improved after NbF5 addition. Within the initial 1 h, only 16% and 46% of MgH2 have decomposed for the as-milled MgH2 and the undoped MgH2 + 1/4AlH3 composite, respectively, while the value reaches nearly 98% for the NbF5-doped composite. This suggests that NbF5 addition can significantly improve the hydrogen desorption kinetics of the MgH 2 + 1/4AlH 3 composite. 18911

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Figure 6. DSC curves of the undoped (a) and NbF5-doped (b) MgH2 + 1/4AlH3 composites. (c) Estimation of the activation energy for the hydrogen desorption reaction of MgH2 in the composites.

Therefore, the formation of MgF2 and the Nb/NbH layers both contribute to the improvement in the hydrogen desorption properties of the NbF5-doped MgH2 + 1/4AlH3 composite. Recently, Ouyang et al.79 reported that the grain growth of MgH2 upon hydrogen sorption cycling can be suppressed by formation of some CeH2/CeH2.73 layers, which is the same effect of the Nb/NbH layers generated from NbF5 in the present work. Figure 9 contains the SEM images of the undoped MgH2 + 1/4AlH3 composite in the hydrided state before and after

Figure 7. Cycling isothermal desorption curves of the undoped and the NbF5-doped MgH2 + 1/4AlH3 composites. Desorption temperature is 300 °C.

Figure 9. SEM images of the undoped MgH2 + 1/4AlH3 composite: (a) in the as-prepared state (upper three images) and (b) in the hydrided state of the fifth sorption cycle (lower three images).

cycling absorptions. It can be seen that the particles in the composite tend to aggregate after several sorption cycles, which will result in reductions of the specific surface area and the gateways for hydrogen diffusions. This may also contribute to the decline of cycling desorption kinetics of the undoped MgH2 + 1/4AlH3 composite. As has been shown from XRD results in Figure 8a, the grains of MgH2 in the undoped MgH2 + 1/4AlH3 composite grow with cycling sorption. Therefore, grain growth and particle aggregation may both lead to the decline of the desorption kinetics of the undoped MgH 2 + 1/4AlH 3 composite. Figure 10 presents the SEM images of the desorbed MgH2 + 1/4AlH3 composites with or without NbF5 addition. Different morphologies of the composites after desorption are evident in that the particles of the composite without NbF5 addition are generally compact and closed, while the particles of the composite with NbF5 addition are loosened and porous with many small holes distributed throughout the particle surfaces. This porous structures are believed to benefit the hydrogen diffusion through the particles during absorption−desorption process, thus improving the hydrogen sorption kinetics. We suppose that the porous structures originate from the unique film-like, network-structured Nb/NbH layers,63−65 which serve

Figure 8. XRD patterns of the undoped MgH2 + 1/4AlH3 composite (a) and the NbF5-doped MgH2 + 1/4AlH3 composite (b) in hydrided state at the second cycle.

suggested that NbF5 will melt during high-energy ball milling, and this promotes the formation of extremely fine, film-like, and network-structured Nb/NbH layers preferentially along the grain boundaries of MgH2. These Nb/NbH layers will act as an impediment to grain growth of Mg/MgH2 and as gateway for hydrogen diffusion during the cycling absorption−desorption process. The role NbF5 plays on the hydrogen sorptions of MgH2 was studied by TEM analysis in their work. However, in our present work, we have used XRD analysis to further confirm the benefit of NbF5 on suppressing the grain growth of MgH2. In addition, they also showed the formation of the byproduct MgF2 by a selected area diffraction (SAD) technique, which is also observed in our XRD pattern of the NbF5-doped MgH2 + 1/4AlH3 composite after desorption (Figure 1e). MgF2 is believed to improve the initial activation of NbF5-doped MgH2 in the hydrogen deosorption process.78 18912

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composites ball milled for different durations, as shown in Figure 11, supports our opinion that the predecomposed AlH3 would not effectively improve the hydrogen desorption properties of MgH2. Figure 12a contains the XRD patterns of the MgH2 + 2AlH3 composites prepared by high-energy ball milling for different

Figure 10. SEM images of the undoped (a) and the NbF5-doped (b) MgH2 + 1/4AlH3 composites after desorption at 300 °C.

as impediments to grain growth of Mg/MgH2 and prevent the particles from contacting each other. We had expected that the hydrogen desorption properties of the MgH2−AlH3−NbF5 composite should be better than that of the MgH2−NbF5 composite (i.e., a synergy effect of AlH3 and NbF5 on the MgH2). However, the TPD curves in Figure 11 indicate that the hydrogen desorption temperature of the Figure 12. XRD patterns (a) and DSC curves (b) of the MgH2 + 2AlH3 composites ball milled under conditions of 400 rpm and 1 bar Ar for different durations (5−100 h).

durations (5−100 h). From Figure 12a, we can see that milling durations significantly affect the phase structures of the MgH2 + 2AlH3 composites. The MgH2 + 2AlH3 composite milled for 5 h consists of MgH2 and AlH3, suggesting both that the two original components do not react with each other and that AlH3 did not begin to decompose after milling for 5 h. When the milling duration increases to 20 h, some traces of Mg(AlH4)2 and metallic Al appear. This indicates that a small part of AlH3 has reacted with MgH2 to form the new phase, Mg(AlH4)2. Moreover, the appearance of the metallic Al indicates that some traces of AlH3 have decomposed after milling for 20 h. A further increase in the milling duration to 50 h results both in the formation of more Mg(AlH4)2 and in the decomposition of more AlH3. After being ball milled for 100 h, the MgH2 + 2AlH3 composite has changed to a mixture of MgH2 and Al, which demonstrates that AlH3 or Mg(AlH4) has fully decomposed during such long-term milling of 100 h. We then carried out the DSC measurements of the MgH2 + 2AlH3 composites ball milled for different durations, as shown in Figure 12b, to study their desorption behaviors. There are two endothermic desorption peaks in the DSC curves of the 5− 50 h milled composites. The low-temperature peak (100−180 °C) is related either to the decomposition reaction of AlH3 to form Al45,61 or to the first desorption step of Mg(AlH4)2 to form MgH2 and Al.80−82 In other words, these two reactions are nearly overlapping in the DSC curves. The hightemperature peak (200−350 °C) is ascribed to the decomposition reaction of MgH2. It is observed that the peak desorption temperatures of MgH2 in the composites milled for

Figure 11. TPD curves of MgH2 with AlH3 addition, NbF5 addition, or with a coaddition of AlH3 and NbF5. Heating rate is 4 °C/min.

MgH2−AlH3−NbF5 composite shows almost no difference with that of the MgH2−NbF5 composite except that the onset desorption temperature of the MgH2−AlH3−NbF5 composite is slightly lower. We attributed this phenomenon to the extremely good catalysis effect of NbF5 on AlH3 and the metastability of AlH3 used in our present work. It was found that AlH3 will decompose immediately after contacting NbF5, as shown in Figure 3. Therefore, when preparing the MgH2− AlH3−NbF5 composite by high-energy ball milling, AlH3 may have decomposed to form ductile Al metal at the beginning of the milling process. As a result, the surface of the metallic Al may be easily oxidized during subsequent milling process and sample transformation. In other words, AlH3 in the MgH2− AlH3−NbF5 composite will be changed to the common Al after milling. This metallic Al is proved to be much weaker than AlH3 regarding its ability to improve the hydrogen desorption properties of MgH2 in the Mg−Al−H composites.45 In fact, a study on the thermal decomposition of the MgH2 + 2AlH3 18913

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The Journal of Physical Chemistry C 5−50 h generally locates at 280−290 °C. Then, the interesting feature is that the peak desorption temperature of MgH2 in the 100 h milled composite is 338 °C, much higher than that of those composites milled for 5−50 h. It is shown in Figure 12a that AlH3 in the 100 h milled composite has fully decomposed to form the metallic Al during milling, which is the main difference from those 5−50 h milled composites. This predecomposed metallic Al may be easily oxidized on the surface layer during long-term milling and subsequent sample transformation because the formation heat of Al2O3 is −1675 kJ/mol,83 much larger than that of Mg17Al12 (−2.9 kJ/mol84). In addition, Al is a ductile metal and would suffer from cold welding during high-energy milling, which would lead to inefficient mixing between MgH2 and Al. These two features of the metallic Al may weaken its ability to improve the hydrogen desorption properties of MgH2. The state of AlH3 in the 100 h milled MgH2 + 2AlH3 composite is the same as that in the previously studied NbF5-doped MgH2 + 1/4AlH3 composite, as both have decomposed fully during the milling process. Therefore, it is not surprising that the desorption properties of the NbF5-doped MgH2 + 1/4AlH3 composite are not appreciably better than those of the MgH2−NbF5 composite. We suggest that in the MgH2−AlH3 systems, the decomposition of AlH3 must occur during the TPD process, instead of the preparation process, so that AlH3 is able to undergo the desorption reaction to generate the oxide-free Al* in situ, thus improving the hydrogen desorption properties of MgH2.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Basic Research Program of China (973 Program) (2010CB631304), National Natural Science Foundation of China (51171168), Public Project of Zhejiang Province (2013C31033), Key Science and Technology Innovation Team of Zhejiang Province (2010R50013), and Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

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4. CONCLUSIONS NbF5 addition can significantly reduce the hydrogen desorption temperature and meanwhile improve the hydrogen desorption kinetics of MgH2 in the MgH2 + 1/4AlH3 composite. In addition, NbF5 also remarkably destabilizes AlH3 and will lead to the rapid decomposition of AlH3 at room temperature. The cycling desorption kinetics of the NbF5-doped MgH2 + 1/ 4AlH3 composite is greatly improved when compared with that of the undoped composite. In addition, the desorption kinetics of the undoped composite declines with cycle number, while it first declines at the second cycle and is maintained at the third cycle for the NbF5-doped composite. The XRD and SEM analyses indicated that NbF5 addition can restrain the grain growth and the particle agglomeration of the Mg−Al−H system. We find that in the MgH2−AlH3 systems, the decomposition of AlH3 taking place during the TPD process, instead of the preparation process, is crucial in improving the hydrogen desorption properties of MgH2. Therefore, the ideal catalysts for the MgH2−AlH3 systems should be those additives that are able to improve the hydrogen desorption properties of MgH2 effectively but do not easily decompose AlH3 during the preparation process. Alternatively, the more stable α-AlH3 may be utilized to replace the less stable γ-AlH3 used in the present work when preparing the MgH2−AlH3 composite.

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*E-mail: [email protected]. Tel/Fax: +86-571-8795-2716. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 18914

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