Mg nanocomposites synthesized by ball milling

International Journal of Hydrogen Energy 32 (2007) 2408 – 2411 www.elsevier.com/locate/ijhydene

Hydrogen storage in carbon/Mg nanocomposites synthesized by ball milling Hayao Imamura ∗ , Ichiro Kitazawa, Yusuke Tanabe, Yoshihisa Sakata Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan Received 25 August 2006; received in revised form 24 October 2006; accepted 24 October 2006 Available online 23 February 2007

Abstract Carbon nanocomposites obtained by ball milling of graphite and magnesium with organic additives (benzene or cyclohexane) under different conditions have been studied with the aim of preparing novel hydrogen storage materials. It has been proved by thermal desorption spectrometry (TDS) and neutron diffraction measurements that the hydrogen taken up by the nanocomposites exists in at least two states; the one is the hydrogen strongly associated with the carbon component and the other the hydride in the magnesium component. The ball milling resulted in the generation of large amounts of dangling carbon bonds in graphite, which acted as active sites to take up the hydrogen. When D2 gas was brought into contact with such composites, the isotope exchange reaction with the hydrogen in the magnesium hydride occurred at 453 K, and not with the hydrogen associated with the carbon. The properties of such hydrogen taken up were also discussed from the standpoint of isotope effects. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; Ball milling; Nanocomposites; Magnesium; Graphite; Thermal desorption spectrometry; Isotope effect

1. Introduction By taking advantages of the features of various carbon materials, much interest has been recently directed toward the applications to possible use in hydrogen storage [1–8]. On the other hand, a great effort has been made on studies of metal hydrides as effective techniques for hydrogen storage, especially in magnesium metal and magnesium-containing alloys [9,10]. We have proposed the application of nanocomposites obtained by highenergy ball milling of graphite and magnesium with organic additives (benzene, cyclohexene, cyclohexane or tetrahydrofuran) as promising candidates for hydrogen storage [11–18]. The use of the additives in the ball milling is crucial in determining the characteristics of the resulting nanocomposites. One of interesting features of the additives is that they act as a useful hydrogen donor; thus considerable dehydrogenation of the additives occurs in the formation processes of nanocomposites by the ball milling, and consequently, the hydrogen produced is taken up in different states in the composites. In this study, interest has centered on identifying the properties of such hydrogen species, compared with the hydrogen ∗ Corresponding author. Fax: +81 836 85 9601.

E-mail address: [email protected] (H. Imamura).

absorbed by the ordinary hydrogenation using H2 gas. This possibly leads to the preparation of highly effective storage materials. Moreover, for the composite samples prepared by the use of C6 H6 or C6 D6 as additives, the hydrogenation is carried out with D2 or H2 gas, respectively, followed by TDS measurements combined with mass spectrometer. This is useful in characterizing the hydrogen (deuterium) taken up with discrimination of the isotopes in the nanocomposites. 2. Experimental procedures Magnesium (99.95%; 100 mesh) and graphite (grade: SP-1) were used as obtained from Rare Metallic Co. Ltd. and Union Carbide Co., respectively. Benzene (C6 H6 ), deuteriobenzene (C6 D6 ) and cyclohexane (C6 H12 ) were commercially obtained and used as obtained. Titanium tetraisopropoxide from Wako Pure Chemical Ind., Ltd. was reagent grade. Ball milling was carried out with a planetary-type ball mill (High G: BX 254; Kurimoto Ltd.), being capable of operating at 863 rpm. In ball milling of graphite with additives, graphite (6 g) and benzene (8 cm3 ) were placed in a stainless steel container (160 cm3 ; lined with zirconia) with zirconia balls and the mixtures were subjected to ball milling for 20 h. The carbon/Mg nanocomposites were prepared by ball milling of

0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.10.058

H. Imamura et al. / International Journal of Hydrogen Energy 32 (2007) 2408 – 2411

3. Results and discussion We have shown important functions of organic additives such as benzene or cyclohexane in the ball milling technique of graphite [11–18]. Upon ball milling of graphite with benzene or cyclohexane, the hydrogen which was produced as a result of decomposition or dehydrogenation of the additives during ball milling was taken up by nanostructured graphite in fairly large quantities. As shown in TDS of Fig. 1(a), upon ball milling with benzene for 20 h the hydrogen thus taken up was detected as broad desorption peaks from temperatures above 670 K. When cyclohexane was used, relatively large amounts of hydrogen were taken up by the graphite (Fig. 1(b)), compared to the case for benzene. This is due to that cyclohexane (C6 H12 ) has higher hydrogen content than benzene (C6 H6 ). In addition, this is also

80 60

peak intensity / a.u.

523 K

m/e=2 (H2)

40

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373

20 0 273

b

696 K

473

673

873

1073

1273

473 573 673 temperature / K

773

873

Fig. 2. TDS of H2 for the sample upon ball milling of graphite (1.8 g)/Mg (4.2 g) with benzene (8 cm3 ) for 20 h.

80 m/e=2 (H2)

60 40 20 0 273

473

673 873 temperature/K

H2

peak intensity / a.u.

peak intensity / a.u.

a

associated with the ease of breaking of the C–H bond. Thus cyclohexane and benzene serve as an effective hydrogen donor in the ball-milling with graphite. The use of such hydrogen donors enables hydrogen storage by nanostructured graphite. Next, upon addition of Mg powder, the ball milling of graphite with benzene yielded nanocomposites, resulting in drastic changes in behavior of hydrogen uptake (Fig. 2). The quantities of hydrogen taken up increased and its desorption temperature significantly decreased compared to the results without Mg; the desorption of hydrogen was shifted more than 300 K to lower temperatures. Moreover, judging from TDS, it is evident that the hydrogen taken up by the nanocomposites exist in at least two states. The hydrogen was characterized as species desorbed around 520 and 700 K from the composites. Both desorption peaks tended to increase with ball-milling times (10–30 h) [11]. This indicates appreciable rupture of the C–H bond of benzene and cyclohexane during the ball milling. When deuteriobenzene (C6 D6 ) was used instead of normal benzene (C6 H6 ), similar TDS traces were obtained, but the desorption peak around 520 K was not pronounced compared to the results with C6 H6 [11]. However, there were significant isotope effects for the desorption of H2 and D2 observed around 700 K in TDS when C6 H6 or C6 D6 was used as additives, respectively. Moreover, similar isotope effects were confirmed when TDS was measured with the mixed samples of graphite/Mg nanocomposites separately milled with C6 H6 or C6 D6 (Fig. 3).

peak intensity / a.u.

graphite (1.8 g) and Mg powder (4.2 g) with additives (8 cm3 ) for 20 h. Ti-added nanocomposites were similarly prepared by ball milling of graphite (1.8 g), Mg (4.2 g) and titanium tetraisopropoxide solutions in benzene to obtain the titanium content of about 2.5 wt%. Thermal desorption spectrometry (TDS) measurements were made using a high vacuum system with a turbo molecular pump. Without exposure to air the nanocomposites were transferred to a sample tube, which was connected to the vacuum system. TDS spectra of gases desorbed from the samples were continuously recorded on a quadrupolar mass spectrometer (ANELVA Co., AQA-100R) with a heating rate of 2.5 Kmin−1 . To compare the amounts of desorbed hydrogen among the samples, the peak intensity of TDS traces was expressed in units per gram of sample.

2409

1073

D2

1273

Fig. 1. (a) TDS of H2 for graphite (6 g) milled with benzene (C6 H6 : 8 cm3 ) for 20 h; (b) TDS of H2 for graphite (6 g) milled with cyclohexane (C6 H12 : 8 cm3 ) for 20 h.

273

373

473 573 temperature / K

673

773

Fig. 3. TDS of H2 and D2 for the samples upon ball milling of graphite (1.8 g)/Mg (4.2 g) with C6 H6 and C6 D6 for 20 h, respectively.

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H. Imamura et al. / International Journal of Hydrogen Energy 32 (2007) 2408 – 2411

519 K

0.004 0.003 0.002 0.001 0

0

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100

150 200 time / min

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Fig. 4. Deuterium absorption at 453 K by the sample upon ball milling of graphite (1.8 g)/Mg (4.2 g) with C6 H6 (8 cm3 ) for 20 h. D2 : 500 Torr.

D2

peak intensity / a.u. 273

603 K

peak intensity / a.u.

D2 absorption (mol/g)

0.005

H2

HD

373

473

573 673 temperature / K

773

873

Fig. 5. TDS of H2 , HD and D2 for the sample after deuterium absorption. The sample was prepared by ball milling of graphite (1.8 g)/Mg (4.2 g) with C6 H6 (8 cm3 ) for 20 h.

To evaluate the reactivity and characteristics of such hydrogen species taken up by the composites, D2 gas was brought into contact with the graphite/Mg nanocomposites milled with C6 H6 . Deuterium absorption immediately occurred as shown in Fig. 4. For the sample absorbing D2 of 3.2 × 10−3 mol/g, the distributions of hydrogen and deuterium in the composites were examined with TDS measurements combined with mass spectrometer. As shown in TDS (Fig. 5), the desorption peaks around 570 K were composed of D2 as a principal component, HD and H2 , while the peaks around 700 K rarely contained deuterium. According to TDS data of Mg–H systems reported elsewhere [19,20], the hydrogen desorption observed around 473–523 K is assigned to hydrogen from the magnesium hydride. Thus it was proved by comparison of Figs. 4 and 5 that deuterium was absorbed by the magnesium metal and that the absorbed deuterium was detected as desorption peaks of D2 and HD at 575 and 580 K, respectively. Since hydrogen is dissociatively absorbed in magnesium to form the hydride, the desorption of HD observed in TDS (Fig. 5) also strongly suggests that the deuterium gas is absorbed by the magnesium and that recombination of absorbed deuterium with the hydrogen taken up in the Mg component during ball milling with C6 H6 occurs. The desorption temperature of hydrogen from magnesium hydride was raised to about 570 K through sintering, compared to that for as prepared sample. Since the desorption peak ob-

273

373

473 573 temperature / K

645 K

673

773

Fig. 6. Effects of titanium addition to graphite/Mg composites on TDS of H2 .

served around 700 K contained little deuterium, it is derived from sites other than magnesium, namely carbon-containing sites freshly formed by ball milling as evidenced by neutron diffraction [11]. The ball milling results in the generation of large amounts of dangling carbon bonds in graphite, which act as active sites to take up the hydrogen [21]. Thus gaseous D2 interacts with the hydrogen observed at 520 K in TDS (Fig. 2) and not the hydrogen observed at 700 K. For the nanocomposite milled with C6 D6 , H2 absorption was, on the contrary, carried out to reach 6.2 × 10−3 mol/g. In TDS the absorbed hydrogen was liberated around 580 K mostly as H2 because deuterium hardly existed in the magnesium site for the sample prepared with C6 D6 , as shown in Fig. 3. Moreover, the desorption observed around 700 K was mostly D2 , and not HD and H2 . This is the case for the results for the nanocomposite milled with C6 H6 ; the desorbed hydrogen observed around 700 K is derived from sites other than magnesium. An interesting feature of hydrogen desorption is that the isotope effect is different between the desorption observed around 520–580 and 700 K, due to arising from magnesiumand carbon-involved sites in the nanocomposites, respectively. The desorption of H2 , HD and D2 deriving from the magnesium was observed at 590, 580 and 575 K, respectively (Fig. 5), which showed an inverse isotope effect. As shown in Fig. 3, the desorption of H2 and D2 from the carbon sites occurred at 700 and 710 K, respectively, being the normal one. The Ti-added nanocomposites prepared by ball milling of graphite and magnesium with solutions of Ti(OC3 H7 )4 in benzene for 20 h exhibited interesting TDS results (Fig. 6). The addition of titanium resulted in drastic changes in sites for the hydrogen uptake by the nanocomposites. The amounts of hydrogen taken up by the magnesium increased markedly, while the desorption temperatures of hydrogen from the carbon-involved sites decreased more than 50 K. Further, different types of hydrogen other than them seem to be formed upon ball milling of graphite/Mg with titanium tetraisopropoxide. The precise roles of titanium in this system are unknown, but upon addition of titanium the hydrogen taken up by the carbon sites is activated to a significant extent. This provides effective information about the preparation of hydrogen storage materials composed of carbon.

H. Imamura et al. / International Journal of Hydrogen Energy 32 (2007) 2408 – 2411

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