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dehydriding properties of magnesium
Journal of Alloys and Compounds 460 (2008) 559–564
Kinetic investigation of the catalytic effect of a body centered cubic-alloy TiV1.1Mn0.9 (BCC) on hydriding/dehydriding properties of magnesium A.L. Yonkeu a,∗ , I.P. Swainson a , J. Dufour b , J. Huot b a
The Canadian Neutron Beam Centre, National Research Council of Canada, Building 459, Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada b IRH, Universit´ e du Qu´ebec a` Trois-Rivi`eres, 3351 des Forges, P.O. Box 500, Trois-Rivi`eres, Qu´ebec G9A 5H7, Canada Received 10 March 2007; received in revised form 3 June 2007; accepted 5 June 2007 Available online 12 June 2007
Abstract In this work, the morphology, crystal structure and hydrogen absorption–desorption kinetics of ball-milled Mg–TiV1.1 Mn0.9 composites were investigated. It was found that a low proportion of BCC-catalyst has a drastic effect on the sorption kinetics of MgH2 . Hydrogen sorption kinetics and capacities of the composite are correlated with milling time. The composite MgH2 –2 mol% BCC milled for 40 h can absorb 6.2 wt% hydrogen within 2 min, and desorbs the same amount in about 7 min at 573 K. Extending the milling period to 80 h reduces the storage capacity of the composite. The absorption behavior is best described by a three-dimensional growth of the hydride phase from the surface to the bulk with hydrogen diffusing through the hydride phase, whereas desorption kinetics are controlled by the two-dimensional growth of magnesium (in the bulk and surface) with a constant velocity of the Mg/MgH2 interface. BCC alloy catalyst is particularly effective for desorption kinetics when the composite is milled for a long period of time. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Hydrogen storage; Magnesium; BCC alloy; Composite; Mechanical milling
1. Introduction Hydrogen is a promising energy carrier in the foreseeable future, capable of replacing fossil fuels in many applications. Storing hydrogen is considered to be one of the most challenging aspects for the introduction of hydrogen in transportation. The storage of hydrogen in solid materials is a promising solution for stationary and mobile applications. Among the various materialbased hydrogen storage system now considered, Mg could be a good candidate because of its high hydrogen storage capacity, with a theoretical value of 7.6 wt%, and its low cost. However, its high temperature of operation and slow sorption kinetics can limit the commercial application of magnesium. One of the powerful ways of improving the hydrogen sorption properties is by high-energy ball milling of hydrides [1]. It has been reported that ball milling Mg or MgH2 with transition metals, as well as intermetallics, oxides or chlorides improved hydrogen sorption kinetics [2–8]. Recently, Mg–BCC composites have been ∗
Corresponding author. Tel.: +1 613 584 3311x4478; fax: + 1 613 584 4040. E-mail address: [email protected] (A.L. Yonkeu).
investigated [9,10]. Gu et al. reported that ball milling magnesium with Ti1.0 V1.1 Mn0.9 alloy [6.4 mol% (or 30 wt%)] resulted in a composite with good hydrogen sorption properties [10]. In the present investigation, a lower proportion (2 mol%) of BCC alloy Ti1.0 V1.1 Mn0.9 was used. Also, milling was performed under argon atmosphere on a mixture of magnesium hydride and BCC alloy, while in reference [10] a mixture of magnesium and BCC alloy was milled under a hydrogen atmosphere. The BCC alloy Ti1.0 V1.1 Mn0.9 was chosen because of its good hydrogen sorption kinetics and high hydrogen storage capacity [11]. A tentative explanation of the hydrogen sorption mechanism of the composite is proposed. 2. Experimental procedures The magnesium hydride powder MgH2 (95 wt% MgH2 , 5 wt% Mg) was purchased from Th. Goldschmidt AG. The BCC Ti1.0 V1.1 Mn0.9 alloy was prepared by induction melting of the constituent pure metals (Ti sponge, 99.9%; Mn chunks, 99.98%; V chunks, 99.7%). The mixture was turned over several times and remelted in order to ensure high homogeneity of the alloy. The ingot was manually crushed and ground into powder. The powder mixture of MgH2 (98 mol%) and BCC alloy catalyst (2 mol%) were mechanically milled with a high-energy Spex 8000 Mixer/Mill using a stainless steel milling jar and three
0925-8388/$ – see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.016
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hardened steel grinding balls (two balls of 11.1 mm diameter and one ball of 15.9 mm diameter) under argon atmosphere for several hours. The initial ballto-powder mass ratio was 10:1 for a charge of 2.71 g of powder. A small amount of powder was extracted at selected times for monitoring the structural change. All handling of samples were performed inside a glove box with a continuously purified argon atmosphere in order to avoid the oxidation of samples. The hydrogen absorption/desorption behaviors of the mechanically milled samples were evaluated by using an automated PCT apparatus. After milling, about 150 mg of the specimen sample was loaded into a reactor. Before measurements start, the system was set under vacuum and the reactor containing the sample was heated to 573 K for hydrogen desorption (first desorption of the sample). The absorption and desorption kinetics measurements were performed at 573 K with an initial pressure of 10 and 0.05 bar for absorption and desorption, respectively. The X-ray diffraction (XRD) analysis of samples was carried out with a Phillips PW 1729 diffractometer using Cu K␣ radiation. A Hitachi S-4800 equipped with an energy-dispersive X-ray detector (EDX) was used for electron scanning electron microscopy (SEM). The backscattered electron micrograph was performed on a JEOL JSM-820. The thermal properties of the as-milled samples were studied by pressurized differential scanning calorimetry (TA Q10P) using a heating rate of 10 K min−1 and under 2 bar of hydrogen.
3. Results and discussion 3.1. Morphology and microstructure of samples The morphology of unmilled and milled composite is shown in Fig. 1. The unmilled composite has an average grain size of ∼35 m. After 40 h of milling, the grains are an agglomeration of smaller particles (Fig. 1B). The BCC-catalyst particles are easily visible on the backscattered electron (BSE) micrograph of Fig. 1C. It can be clearly seen that small particles of BCC-catalyst (<0.5 m) are embedded on the surface of MgH2 particles after milling. However, as this is ball-milled material, we expect each grain to be a mixture of MgH2 and BCC particles. Therefore, BCC particles are on the surface and in the interior of grains. BCC particles are of the same size as in reference [10]. Fig. 2 shows the X-ray diffraction patterns of milled and unmilled MgH2 with and without BCC-catalyst. The X-ray diffraction pattern of the unmilled MgH2 –2 mol% BCC shows -MgH2 and Mg phases. The Mg phase found in the composite originates from the purchased MgH2 used in this work as a starting product, and is due to the partial dehydrogenation of MgH2 with time. Contrary to reference [10], the catalyst phase could not be observed due to its low quantity (2% molar) in the composite. XRD patterns of the ball-milled composites show peaks indicating the presence of -MgH2 , Mg, ␥-MgH2 and MgO phases. The mechanical milling introduces change in the microstructure of the composite with the appearance of a new phase, which is identified to be the orthorhombic ␥-MgH2 , as previously described by Schulz et al. [12]. Diffraction peaks of the composite are significantly broadened, due to the nanocrystallinity of the composite. Also a H-desorbed composite (milled for 40 h) XRD pattern confirms that magnesium is the major phase after the desorption process. The formation of MgO was observed even though all the handling of samples was done under a protective atmosphere. Periodic openings of the milling jar inside the glove box could lead to the appearance of MgO phase according to the study of Varin et al. [13]. It was showed by these authors that by
Fig. 1. (A) SEM micrographs of MgH2 –2 mol% BCC composite unmilled and (B) milled for 40 h. (C) BSE micrograph of MgH2 –2 mol% BCC composite milled for 40 h: gray areas are MgH2 particles whereas bright spot indicates BCC particles.
continuously milling magnesium (milling jar was never opened for sampling), no MgO phase could be detected. 3.2. Thermal behaviors Fig. 3 shows traces of Pressurized Differential Scanning Calorimetry (PDSC) measurements of MgH2 milled with and
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Table 1 Pressurized Differential Scanning Calorimetry (PDSC) temperature peaks: TON = temperature of desorption onset, LT = low temperature peak, HT = high temperature peak Samples (milling time)
MgH2 [0 h] (starting material) MgH2 (40 h) MgH2 –BCC (40 h) MgH2 –BCC (80 h)
Fig. 2. X-ray powder diffraction of unmilled MgH2 –2 mol% BCC composite, H-desorbed composite and composite after ball milling for several hours.
without catalyst. The endotherms correspond to the release of hydrogen. The PDSC curve of the MgH2 starting material possesses a single endothermic hydrogen desorption peak at 433.3 ◦ C, whereas the PDSC curves of milled samples show two distinct peaks. The onset temperature of desorption (TON ), the peak temperature for low temperature (LT) and high temperature (HT) peaks, as indicated in Fig. 3, are summarized in Table 1. It shows that temperature values of both desorption onset and high temperature peak decrease when MgH2 is milled without catalyst. An additional decrease of these values is observed when MgH2 is milled together with the catalyst, while the high temperature desorption peak for composites milled at 40 and 80 h is stable at around 383 ◦ C. The reduction of the onset temperature of desorption strongly indicates that even a low quantity of BCC alloy TiV1.1 Mn0.9 , along with the milling process, greatly improves hydrogen desorption properties of magnesium. A mechanical ball milling produces a nanocomposite with particles of various sizes (see
Fig. 3. PDSC trace of MgH2 with and without BCC alloy, under 2 bar of hydrogen.
PDSC peaks (◦ C) TON
LT
HT
429.2 386.5 368.4 362.3
– 390.9 375.6 368.3
433.3 410.0 383.4 383.7
SEM micrographs) and a metastable ␥-MgH2 phase. According to a recent study of Varin et al., the observed desorption peak doublet is to a great extent associated with the distribution of reduced particulate sizes and the presence of the ␥-MgH2 phase, which might occupy the finest particle fraction [14]. Still, the present PDSC measurements show that hydrogen is more easily released from magnesium hydride when it is milled in the presence of the BCC-catalyst than from the magnesium hydride milled without catalyst. 3.3. Sorption properties of the composites Figs. 4 and 5, respectively, present the hydriding and dehydriding curves at 573 K of MgH2 with and without BCC-catalyst for different milling times. It shows that the sorption kinetics of MgH2 is improved by the presence of a small quantity (2 mol%) of BCC alloy catalyst. Without catalyst, the sorption kinetics are slow even after 40 h of milling. Addition of the BCC alloy TiV1.1 Mn0.9 has a drastic effect on the hydrogen sorption kinetics. After only 2 h of milling, the composite absorbs faster than the pure MgH2 milled for 40 h. Hydriding/dehydriding kinetics enhancement become much more noticeable when the composite is milled for 20 h or more. The composite MgH2 –2 mol% BCC milled for 40 h absorbs 6.2 wt% hydrogen within 2 min, and desorbs the same quantity in about 7 min at 573 K. Gu et al.
Fig. 4. Absorption kinetics curves of milled MgH2 with and without 2 mol% BCC alloy at various milling times. Applied pressure: 10 bar, temperature: 573 K.
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Fig. 5. Desorption kinetics curves of milled MgH2 with and without 2 mol% BCC alloy at various milling times. Applied pressure: 0.05 bar, temperature: 573 K.
also observed improved sorption kinetics with increased milling time when milling Mg with a high concentration TiV1.1 Mn0.9 [30 wt% or 6.4 mol%] in hydrogen atmosphere [10], however, the sorption kinetics were not as good as those we obtained in the present study especially for the desorption kinetics at 573 K. The mechanism controlling the hydrogen sorption kinetics in milled composites was determined by comparing the hydrogenation data with theoretical rate equations related to different rate-limiting steps. Derived kinetics equations used in this work and descriptions of corresponding models are listed in Table 2. These functions are derived for a single spherical particle, or alternatively for an ideal assembly of identical spherical particles that start the reaction simultaneously, at the same time [15,16]. For real-life powders, functions describing the reactions kinetics mechanisms should consider the following factors: (i) particle size distributions, (ii) particle shape variations and (iii) time distributions for the beginning of the reaction on each one of the particles. A quantitative estimate for using the ideal single-particle analysis functions for “real-life” powders has been presented by Mintz and Zeiri [16], with the conclusion that in spite of the above-mentioned distributions (in size, shape and initiation times), correct reaction mechanisms are still obtained by such analysis as long as the particles start to react simultane-
Fig. 6. Plots of the resulting curves of different kinetic equations applied to the experimental absorption (A) and desorption (B) data of MgH2 –2 mol% TiV1.1 Mn0.9 composite milled for 40 h.
ously and the initial thickness of reacted phase is much smaller than the particle size. The plot [transformed phase fraction (left side of equation) versus time] giving the best linearity determines the controlling mechanism. The rate-limiting step for all composites was determined by fitting the transformed fraction over the range 0–80%. (Fig. 6) presents examples of the determination of the kinetic
Table 2 Model-derived functions related to different rate-limiting step [15–17] f: reacted fraction, t: reaction time, k: reaction rate constant, D: dimension, JMA: Johnson–Mehl–Avrami and CV: contracting volume Kinetic equations
Description of the model
f = kt [−In(1 − f )]1/2 = kt [−In(1 − f )]1/3 = kt 1 − (1 − f )1/2 = kt
(1) (2) (3) (4)
1 − (1 − f )1/3 = kt
(5)
1−
2f 3
− (1 − f )2/3 = kt
(6)
Surface controlled (chemisorption) Random bulk nucleation/surface and 2D-growth with a constant interface velocity (JMA model) Random bulk nucleation/surface and 3D-growth with a constant interface velocity (JMA model) Contracting envelope and 2D-growth of the new phase from the surface to the bulk with a constant interface velocity (CV model) Contracting envelope and 3D-growth of the new phase from the surface to the bulk with a constant interface velocity (CV model) Contracting envelope with decreasing interface velocity controlled by 3D-diffusion growth through the transformed phase (CV model)
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Fig. 7. Sorption kinetic rates constant as a function of milling time for ballmilled MgH2 –2 mol% TiV1.1 Mn0.9 composite. Open symbols correspond to absorption (Eq. (6) of Table 2) and solid symbols correspond to desorption (Eq. (2) of Table 2).
rate-limiting step for absorption (6A) and desorption (6B) reactions for the composite milled for 40 h. The hydrogen absorption reactions for all composites in this study are best described by a three-dimensional diffusion-controlled CV model (Eq. (6), Table 2), whereas the rate-limiting step for hydrogen desorption reactions is governed by a two-dimensional interface growth JMA model (Eq. (2), Table 2). The hydrogen absorption reaction is controlled by the diffusion of hydrogen through a protective hydride layer. The hydrogen desorption reaction is controlled by JMA bulk nucleation and growth of the transformed phase, which is Mg in this case. In the case where MgH2 was milled with Nb2 O5 catalyst, Barkhordarian et al. [18] found the same kinetic rate-limiting step for absorption (three-dimensional diffusion-controlled CV model) whereas chemisorption and a two-dimensional interface growth CV model were rate-limiting steps for H-desorption from Mg–Nb2 O5 composites. The hydrogen absorption–desorption mechanisms of Mg–TiV1.1 Mn0.9 composite in this study are associated with bulk-related processes. BCC particles play the role of catalyst by improving the hydrogen sorption properties of the composite. After mechanically milling, nano-structured BCC alloy phase are uniformly mixed with Mg particles, thus giving bulk sorption mechanisms. The evolution of rate constants with milling time is presented in Fig. 7. Absorption is a diffusion-controlled mechanism. Its rate constant increases with milling time and seems to reach a maximum value after 40 h of milling. No significant increase of the absorption rate constant is obtained for further milling time. In the hydrogen desorption case, increasing the milling time of the composite produces a better dispersion of the nanosized catalyst particles, which leads to an increasing amount of available nucleation sites within the composite. As desorption is a bulk reaction controlled by nucleation, it is expected that the reaction rate will increase with milling time. This is confirmed by our measurement of the desorption rate constant as a function of milling time shown in Fig. 7.
563
Fig. 8. Pressure-composition isotherms at 573 K for MgH2 –2 mol% TiV1.1 Mn0.9 composite milled for 40 h.
Milling time has a positive impact specifically on the desorption kinetics of the composite. However, extending the milling time of the composite to 80 h causes a decrease in the hydrogen capacity of the composite. Wu et al. observed similar reduction in hydrogen storage capacity of a ball-milled MgH2 /single-walled carbon nanotube (SWNT) composite [19]. They showed that the reduction of the storage capacity is related to the breakdown of the structure of the SWNT catalyst. In our case, the nanocrystallinity of the hydride phases was stable while the composite was milled for 80 h. A systematic analysis of the microstructure of the catalyst in the ball-milled composite was unfortunately not possible due to its low quantity (only 2 mol%). Further study for a direct characterization of the microstructure of the BCC alloy within a composite milled for a longer period of time is underway. The present results suggest that a controlled milling time is critical to achieve a favourable catalytic enhancement in the composite while maintaining the storage capacity of the system. In this study, a mechanical milling of the composite for 40 h yields optimum sorption properties. The Pressure–Composition–Temperature (PCT) curve of catalyzed MgH2 milled for 40 h is shown in Fig. 8. The plateau of absorption and desorption are flat and there is a small hysteresis. This measurement shows that the thermodynamic properties of MgH2 milled for 40 h are not altered by the presence of a small quantity of the solid solution BCC (TiV1.1 Mn0.9 ). 4. Conclusion The catalytic effect of a BCC (TiV1.1 Mn0.9 ) alloy on magnesium hydride has been investigated. By milling 2 mol% of solid solution BCC alloy (TiV1.1 Mn0.9 ) together with magnesium hydride, the hydrogen sorption kinetics were improved. Sorption kinetics of the composite strongly depend on the milling time of the composite, particularly in the desorption case. The system MgH2 /BCC mechanically milled for 40 h achieved optimal hydrogen sorption properties, as it absorbs 6.2 wt% of hydrogen within a period of 2 min and desorbs the same amount of hydrogen within a period of 7 min at 573 K. Further milling of the
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composite up to 80 h leads to a reduction of storage capacity, even though the desorption kinetics properties are improved. Within the milling time frame considered for this study, the absorption kinetics of the composite are controlled by a threedimensional growth of the hydride phase and the diffusion of hydrogen through the hydride phase whereas the desorption kinetics are governed by the bulk/surface nucleation and twodimensional growth of the magnesium phase with a constant velocity of the Mg/MgH2 interface. Acknowledgments A.L.Y. wishes to thank Dr. R. Lastra (NRCan) and J. Margeson (NRC-IRC) for performing the scanning electron micrographs, and A. Buyers (AECL, Chalk River) for XRD measurements. R. Sammon, T. Dodds, S. Alavi, T. Whan, M. Watson and L. Cranswick are also acknowledged for their technical assistance in the Hydrogen Research Program at NRC-CNBC, Chalk River. References [1] A. Zaluska, L. Zaluski, J.O. Strom-Olsen, R. Schulz, US Patent #6080381 (2000).
[2] G. Liang, J. Huot, S. Boily, et al., J. Alloys Compd. 292 (1999) 247– 252. [3] J. Huot, J.F. Pelletier, L.B. Lurio, et al., J. Alloys Compd. 348 (2003) 319. [4] J.-L. Bobet, E. Akiba, Y. Nakamura, et al., Int. J. Hydrogen Energy 25 (2000) 987–996. [5] P. Wang, A.M. Wang, B.Z. Ding, et al., J. Alloys Compd. 334 (2002) 243. [6] Y.Q. Hu, C. Yan, H.F. Zhang, et al., J. Alloys Compd. 375 (2004) 265–269. [7] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Compd. 315 (2001) 237–242. [8] E. Ivanov, I. Konstanchuk, B. Bokhonov, et al., J. Alloys Compd. 359 (2003) 320–325. [9] X. Liu, Z. Huang, L. Jiang and S. Wang, Int. J. Hydrogen Energy (2006), doi:10.1016/j.ihydene.2006.08.030. [10] H. Gu, Y. Zhu, L. Li, J. Alloys Compd. 424 (2006) 382–387. [11] H. Iba, E. Akiba, J. Alloys Compd. 253/254 (1997) 21–24. [12] R. Schulz, J. Huot, G. Liang, et al., Mater. Sci. Eng. A 267 (1999) 240. [13] R.A. Varin, S. Li, Z. Wronski, O. Morozova, et al., J. Alloys Compd. 390 (2005) 282. [14] R.A. Varin, T. Czujko, Ch. Chiu, et al., J. Alloys Compd. 424 (2006) 356–364. [15] M.H. Mintz, J. Bloch, Prog. Solid State Chem. 16 (1985) 163–194. [16] M.H. Mintz, Y. Zeiri, J. Alloys Compd. 216 (1994) 159–175. [17] J. Avrami, J. Chem. Phys. 7 (1939) 1103–1112; J. Avrami, J. Chem. Phys. 8 (1940) 212–224; J. Avrami, J. Chem. Phys. 9 (1941) 171–184. [18] G. Barkhordarian, T. Klassen, R. Bormann, J. Alloys Compd. 407 (2006) 249–255. [19] C.Z. Wu, P. Wang, X. Yao, et al., J. Alloys Compd. 420 (2006) 278–282.
Kinetic investigation of the catalytic effect of a body centered cubic-alloy TiV1.1Mn0.9 (BCC) on hydriding/dehydriding properties of magnesium A.L. Yonkeu a,∗ , I.P. Swainson a , J. Dufour b , J. Huot b a
The Canadian Neutron Beam Centre, National Research Council of Canada, Building 459, Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada b IRH, Universit´ e du Qu´ebec a` Trois-Rivi`eres, 3351 des Forges, P.O. Box 500, Trois-Rivi`eres, Qu´ebec G9A 5H7, Canada Received 10 March 2007; received in revised form 3 June 2007; accepted 5 June 2007 Available online 12 June 2007
Abstract In this work, the morphology, crystal structure and hydrogen absorption–desorption kinetics of ball-milled Mg–TiV1.1 Mn0.9 composites were investigated. It was found that a low proportion of BCC-catalyst has a drastic effect on the sorption kinetics of MgH2 . Hydrogen sorption kinetics and capacities of the composite are correlated with milling time. The composite MgH2 –2 mol% BCC milled for 40 h can absorb 6.2 wt% hydrogen within 2 min, and desorbs the same amount in about 7 min at 573 K. Extending the milling period to 80 h reduces the storage capacity of the composite. The absorption behavior is best described by a three-dimensional growth of the hydride phase from the surface to the bulk with hydrogen diffusing through the hydride phase, whereas desorption kinetics are controlled by the two-dimensional growth of magnesium (in the bulk and surface) with a constant velocity of the Mg/MgH2 interface. BCC alloy catalyst is particularly effective for desorption kinetics when the composite is milled for a long period of time. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Hydrogen storage; Magnesium; BCC alloy; Composite; Mechanical milling
1. Introduction Hydrogen is a promising energy carrier in the foreseeable future, capable of replacing fossil fuels in many applications. Storing hydrogen is considered to be one of the most challenging aspects for the introduction of hydrogen in transportation. The storage of hydrogen in solid materials is a promising solution for stationary and mobile applications. Among the various materialbased hydrogen storage system now considered, Mg could be a good candidate because of its high hydrogen storage capacity, with a theoretical value of 7.6 wt%, and its low cost. However, its high temperature of operation and slow sorption kinetics can limit the commercial application of magnesium. One of the powerful ways of improving the hydrogen sorption properties is by high-energy ball milling of hydrides [1]. It has been reported that ball milling Mg or MgH2 with transition metals, as well as intermetallics, oxides or chlorides improved hydrogen sorption kinetics [2–8]. Recently, Mg–BCC composites have been ∗
Corresponding author. Tel.: +1 613 584 3311x4478; fax: + 1 613 584 4040. E-mail address: [email protected] (A.L. Yonkeu).
investigated [9,10]. Gu et al. reported that ball milling magnesium with Ti1.0 V1.1 Mn0.9 alloy [6.4 mol% (or 30 wt%)] resulted in a composite with good hydrogen sorption properties [10]. In the present investigation, a lower proportion (2 mol%) of BCC alloy Ti1.0 V1.1 Mn0.9 was used. Also, milling was performed under argon atmosphere on a mixture of magnesium hydride and BCC alloy, while in reference [10] a mixture of magnesium and BCC alloy was milled under a hydrogen atmosphere. The BCC alloy Ti1.0 V1.1 Mn0.9 was chosen because of its good hydrogen sorption kinetics and high hydrogen storage capacity [11]. A tentative explanation of the hydrogen sorption mechanism of the composite is proposed. 2. Experimental procedures The magnesium hydride powder MgH2 (95 wt% MgH2 , 5 wt% Mg) was purchased from Th. Goldschmidt AG. The BCC Ti1.0 V1.1 Mn0.9 alloy was prepared by induction melting of the constituent pure metals (Ti sponge, 99.9%; Mn chunks, 99.98%; V chunks, 99.7%). The mixture was turned over several times and remelted in order to ensure high homogeneity of the alloy. The ingot was manually crushed and ground into powder. The powder mixture of MgH2 (98 mol%) and BCC alloy catalyst (2 mol%) were mechanically milled with a high-energy Spex 8000 Mixer/Mill using a stainless steel milling jar and three
0925-8388/$ – see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.016
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hardened steel grinding balls (two balls of 11.1 mm diameter and one ball of 15.9 mm diameter) under argon atmosphere for several hours. The initial ballto-powder mass ratio was 10:1 for a charge of 2.71 g of powder. A small amount of powder was extracted at selected times for monitoring the structural change. All handling of samples were performed inside a glove box with a continuously purified argon atmosphere in order to avoid the oxidation of samples. The hydrogen absorption/desorption behaviors of the mechanically milled samples were evaluated by using an automated PCT apparatus. After milling, about 150 mg of the specimen sample was loaded into a reactor. Before measurements start, the system was set under vacuum and the reactor containing the sample was heated to 573 K for hydrogen desorption (first desorption of the sample). The absorption and desorption kinetics measurements were performed at 573 K with an initial pressure of 10 and 0.05 bar for absorption and desorption, respectively. The X-ray diffraction (XRD) analysis of samples was carried out with a Phillips PW 1729 diffractometer using Cu K␣ radiation. A Hitachi S-4800 equipped with an energy-dispersive X-ray detector (EDX) was used for electron scanning electron microscopy (SEM). The backscattered electron micrograph was performed on a JEOL JSM-820. The thermal properties of the as-milled samples were studied by pressurized differential scanning calorimetry (TA Q10P) using a heating rate of 10 K min−1 and under 2 bar of hydrogen.
3. Results and discussion 3.1. Morphology and microstructure of samples The morphology of unmilled and milled composite is shown in Fig. 1. The unmilled composite has an average grain size of ∼35 m. After 40 h of milling, the grains are an agglomeration of smaller particles (Fig. 1B). The BCC-catalyst particles are easily visible on the backscattered electron (BSE) micrograph of Fig. 1C. It can be clearly seen that small particles of BCC-catalyst (<0.5 m) are embedded on the surface of MgH2 particles after milling. However, as this is ball-milled material, we expect each grain to be a mixture of MgH2 and BCC particles. Therefore, BCC particles are on the surface and in the interior of grains. BCC particles are of the same size as in reference [10]. Fig. 2 shows the X-ray diffraction patterns of milled and unmilled MgH2 with and without BCC-catalyst. The X-ray diffraction pattern of the unmilled MgH2 –2 mol% BCC shows -MgH2 and Mg phases. The Mg phase found in the composite originates from the purchased MgH2 used in this work as a starting product, and is due to the partial dehydrogenation of MgH2 with time. Contrary to reference [10], the catalyst phase could not be observed due to its low quantity (2% molar) in the composite. XRD patterns of the ball-milled composites show peaks indicating the presence of -MgH2 , Mg, ␥-MgH2 and MgO phases. The mechanical milling introduces change in the microstructure of the composite with the appearance of a new phase, which is identified to be the orthorhombic ␥-MgH2 , as previously described by Schulz et al. [12]. Diffraction peaks of the composite are significantly broadened, due to the nanocrystallinity of the composite. Also a H-desorbed composite (milled for 40 h) XRD pattern confirms that magnesium is the major phase after the desorption process. The formation of MgO was observed even though all the handling of samples was done under a protective atmosphere. Periodic openings of the milling jar inside the glove box could lead to the appearance of MgO phase according to the study of Varin et al. [13]. It was showed by these authors that by
Fig. 1. (A) SEM micrographs of MgH2 –2 mol% BCC composite unmilled and (B) milled for 40 h. (C) BSE micrograph of MgH2 –2 mol% BCC composite milled for 40 h: gray areas are MgH2 particles whereas bright spot indicates BCC particles.
continuously milling magnesium (milling jar was never opened for sampling), no MgO phase could be detected. 3.2. Thermal behaviors Fig. 3 shows traces of Pressurized Differential Scanning Calorimetry (PDSC) measurements of MgH2 milled with and
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Table 1 Pressurized Differential Scanning Calorimetry (PDSC) temperature peaks: TON = temperature of desorption onset, LT = low temperature peak, HT = high temperature peak Samples (milling time)
MgH2 [0 h] (starting material) MgH2 (40 h) MgH2 –BCC (40 h) MgH2 –BCC (80 h)
Fig. 2. X-ray powder diffraction of unmilled MgH2 –2 mol% BCC composite, H-desorbed composite and composite after ball milling for several hours.
without catalyst. The endotherms correspond to the release of hydrogen. The PDSC curve of the MgH2 starting material possesses a single endothermic hydrogen desorption peak at 433.3 ◦ C, whereas the PDSC curves of milled samples show two distinct peaks. The onset temperature of desorption (TON ), the peak temperature for low temperature (LT) and high temperature (HT) peaks, as indicated in Fig. 3, are summarized in Table 1. It shows that temperature values of both desorption onset and high temperature peak decrease when MgH2 is milled without catalyst. An additional decrease of these values is observed when MgH2 is milled together with the catalyst, while the high temperature desorption peak for composites milled at 40 and 80 h is stable at around 383 ◦ C. The reduction of the onset temperature of desorption strongly indicates that even a low quantity of BCC alloy TiV1.1 Mn0.9 , along with the milling process, greatly improves hydrogen desorption properties of magnesium. A mechanical ball milling produces a nanocomposite with particles of various sizes (see
Fig. 3. PDSC trace of MgH2 with and without BCC alloy, under 2 bar of hydrogen.
PDSC peaks (◦ C) TON
LT
HT
429.2 386.5 368.4 362.3
– 390.9 375.6 368.3
433.3 410.0 383.4 383.7
SEM micrographs) and a metastable ␥-MgH2 phase. According to a recent study of Varin et al., the observed desorption peak doublet is to a great extent associated with the distribution of reduced particulate sizes and the presence of the ␥-MgH2 phase, which might occupy the finest particle fraction [14]. Still, the present PDSC measurements show that hydrogen is more easily released from magnesium hydride when it is milled in the presence of the BCC-catalyst than from the magnesium hydride milled without catalyst. 3.3. Sorption properties of the composites Figs. 4 and 5, respectively, present the hydriding and dehydriding curves at 573 K of MgH2 with and without BCC-catalyst for different milling times. It shows that the sorption kinetics of MgH2 is improved by the presence of a small quantity (2 mol%) of BCC alloy catalyst. Without catalyst, the sorption kinetics are slow even after 40 h of milling. Addition of the BCC alloy TiV1.1 Mn0.9 has a drastic effect on the hydrogen sorption kinetics. After only 2 h of milling, the composite absorbs faster than the pure MgH2 milled for 40 h. Hydriding/dehydriding kinetics enhancement become much more noticeable when the composite is milled for 20 h or more. The composite MgH2 –2 mol% BCC milled for 40 h absorbs 6.2 wt% hydrogen within 2 min, and desorbs the same quantity in about 7 min at 573 K. Gu et al.
Fig. 4. Absorption kinetics curves of milled MgH2 with and without 2 mol% BCC alloy at various milling times. Applied pressure: 10 bar, temperature: 573 K.
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Fig. 5. Desorption kinetics curves of milled MgH2 with and without 2 mol% BCC alloy at various milling times. Applied pressure: 0.05 bar, temperature: 573 K.
also observed improved sorption kinetics with increased milling time when milling Mg with a high concentration TiV1.1 Mn0.9 [30 wt% or 6.4 mol%] in hydrogen atmosphere [10], however, the sorption kinetics were not as good as those we obtained in the present study especially for the desorption kinetics at 573 K. The mechanism controlling the hydrogen sorption kinetics in milled composites was determined by comparing the hydrogenation data with theoretical rate equations related to different rate-limiting steps. Derived kinetics equations used in this work and descriptions of corresponding models are listed in Table 2. These functions are derived for a single spherical particle, or alternatively for an ideal assembly of identical spherical particles that start the reaction simultaneously, at the same time [15,16]. For real-life powders, functions describing the reactions kinetics mechanisms should consider the following factors: (i) particle size distributions, (ii) particle shape variations and (iii) time distributions for the beginning of the reaction on each one of the particles. A quantitative estimate for using the ideal single-particle analysis functions for “real-life” powders has been presented by Mintz and Zeiri [16], with the conclusion that in spite of the above-mentioned distributions (in size, shape and initiation times), correct reaction mechanisms are still obtained by such analysis as long as the particles start to react simultane-
Fig. 6. Plots of the resulting curves of different kinetic equations applied to the experimental absorption (A) and desorption (B) data of MgH2 –2 mol% TiV1.1 Mn0.9 composite milled for 40 h.
ously and the initial thickness of reacted phase is much smaller than the particle size. The plot [transformed phase fraction (left side of equation) versus time] giving the best linearity determines the controlling mechanism. The rate-limiting step for all composites was determined by fitting the transformed fraction over the range 0–80%. (Fig. 6) presents examples of the determination of the kinetic
Table 2 Model-derived functions related to different rate-limiting step [15–17] f: reacted fraction, t: reaction time, k: reaction rate constant, D: dimension, JMA: Johnson–Mehl–Avrami and CV: contracting volume Kinetic equations
Description of the model
f = kt [−In(1 − f )]1/2 = kt [−In(1 − f )]1/3 = kt 1 − (1 − f )1/2 = kt
(1) (2) (3) (4)
1 − (1 − f )1/3 = kt
(5)
1−
2f 3
− (1 − f )2/3 = kt
(6)
Surface controlled (chemisorption) Random bulk nucleation/surface and 2D-growth with a constant interface velocity (JMA model) Random bulk nucleation/surface and 3D-growth with a constant interface velocity (JMA model) Contracting envelope and 2D-growth of the new phase from the surface to the bulk with a constant interface velocity (CV model) Contracting envelope and 3D-growth of the new phase from the surface to the bulk with a constant interface velocity (CV model) Contracting envelope with decreasing interface velocity controlled by 3D-diffusion growth through the transformed phase (CV model)
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Fig. 7. Sorption kinetic rates constant as a function of milling time for ballmilled MgH2 –2 mol% TiV1.1 Mn0.9 composite. Open symbols correspond to absorption (Eq. (6) of Table 2) and solid symbols correspond to desorption (Eq. (2) of Table 2).
rate-limiting step for absorption (6A) and desorption (6B) reactions for the composite milled for 40 h. The hydrogen absorption reactions for all composites in this study are best described by a three-dimensional diffusion-controlled CV model (Eq. (6), Table 2), whereas the rate-limiting step for hydrogen desorption reactions is governed by a two-dimensional interface growth JMA model (Eq. (2), Table 2). The hydrogen absorption reaction is controlled by the diffusion of hydrogen through a protective hydride layer. The hydrogen desorption reaction is controlled by JMA bulk nucleation and growth of the transformed phase, which is Mg in this case. In the case where MgH2 was milled with Nb2 O5 catalyst, Barkhordarian et al. [18] found the same kinetic rate-limiting step for absorption (three-dimensional diffusion-controlled CV model) whereas chemisorption and a two-dimensional interface growth CV model were rate-limiting steps for H-desorption from Mg–Nb2 O5 composites. The hydrogen absorption–desorption mechanisms of Mg–TiV1.1 Mn0.9 composite in this study are associated with bulk-related processes. BCC particles play the role of catalyst by improving the hydrogen sorption properties of the composite. After mechanically milling, nano-structured BCC alloy phase are uniformly mixed with Mg particles, thus giving bulk sorption mechanisms. The evolution of rate constants with milling time is presented in Fig. 7. Absorption is a diffusion-controlled mechanism. Its rate constant increases with milling time and seems to reach a maximum value after 40 h of milling. No significant increase of the absorption rate constant is obtained for further milling time. In the hydrogen desorption case, increasing the milling time of the composite produces a better dispersion of the nanosized catalyst particles, which leads to an increasing amount of available nucleation sites within the composite. As desorption is a bulk reaction controlled by nucleation, it is expected that the reaction rate will increase with milling time. This is confirmed by our measurement of the desorption rate constant as a function of milling time shown in Fig. 7.
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Fig. 8. Pressure-composition isotherms at 573 K for MgH2 –2 mol% TiV1.1 Mn0.9 composite milled for 40 h.
Milling time has a positive impact specifically on the desorption kinetics of the composite. However, extending the milling time of the composite to 80 h causes a decrease in the hydrogen capacity of the composite. Wu et al. observed similar reduction in hydrogen storage capacity of a ball-milled MgH2 /single-walled carbon nanotube (SWNT) composite [19]. They showed that the reduction of the storage capacity is related to the breakdown of the structure of the SWNT catalyst. In our case, the nanocrystallinity of the hydride phases was stable while the composite was milled for 80 h. A systematic analysis of the microstructure of the catalyst in the ball-milled composite was unfortunately not possible due to its low quantity (only 2 mol%). Further study for a direct characterization of the microstructure of the BCC alloy within a composite milled for a longer period of time is underway. The present results suggest that a controlled milling time is critical to achieve a favourable catalytic enhancement in the composite while maintaining the storage capacity of the system. In this study, a mechanical milling of the composite for 40 h yields optimum sorption properties. The Pressure–Composition–Temperature (PCT) curve of catalyzed MgH2 milled for 40 h is shown in Fig. 8. The plateau of absorption and desorption are flat and there is a small hysteresis. This measurement shows that the thermodynamic properties of MgH2 milled for 40 h are not altered by the presence of a small quantity of the solid solution BCC (TiV1.1 Mn0.9 ). 4. Conclusion The catalytic effect of a BCC (TiV1.1 Mn0.9 ) alloy on magnesium hydride has been investigated. By milling 2 mol% of solid solution BCC alloy (TiV1.1 Mn0.9 ) together with magnesium hydride, the hydrogen sorption kinetics were improved. Sorption kinetics of the composite strongly depend on the milling time of the composite, particularly in the desorption case. The system MgH2 /BCC mechanically milled for 40 h achieved optimal hydrogen sorption properties, as it absorbs 6.2 wt% of hydrogen within a period of 2 min and desorbs the same amount of hydrogen within a period of 7 min at 573 K. Further milling of the
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composite up to 80 h leads to a reduction of storage capacity, even though the desorption kinetics properties are improved. Within the milling time frame considered for this study, the absorption kinetics of the composite are controlled by a threedimensional growth of the hydride phase and the diffusion of hydrogen through the hydride phase whereas the desorption kinetics are governed by the bulk/surface nucleation and twodimensional growth of the magnesium phase with a constant velocity of the Mg/MgH2 interface. Acknowledgments A.L.Y. wishes to thank Dr. R. Lastra (NRCan) and J. Margeson (NRC-IRC) for performing the scanning electron micrographs, and A. Buyers (AECL, Chalk River) for XRD measurements. R. Sammon, T. Dodds, S. Alavi, T. Whan, M. Watson and L. Cranswick are also acknowledged for their technical assistance in the Hydrogen Research Program at NRC-CNBC, Chalk River. References [1] A. Zaluska, L. Zaluski, J.O. Strom-Olsen, R. Schulz, US Patent #6080381 (2000).
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