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Hydrogen storage properties of the Mg–Ni–CrCl3 nanocomposite
Journal of Alloys and Compounds 333 (2002) 207–214
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Hydrogen storage properties of the Mg–Ni–CrCl 3 nanocomposite Yu Zhenxing, Liu Zuyan*, Wang Erde School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Received 17 April 2001; accepted 15 June 2001
Abstract The effects of the addition of the transition metal chloride CrCl 3 on the hydriding and dehydriding behavior of Mg–3 wt.% Ni hydrogen storage material were investigated. The characteristics of the ball-milled nanocomposite Mg–3 wt.% Ni–1CrCl 3 such as hydriding capacity in the ball-milling process and the kinetics of hydriding / dehydriding were examined. The hydrogen absorption capacity of the composite is greatly increased in the ball-milling process under hydrogen pressure (0.5 MPa). The absorption rate of the composite is fast and the hydrogen storage capacity is more than that of the sample without CrCl 3 . 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Energy storage materials; Mechanical alloying; Gas–solid reaction
1. Introduction In metal–hydrogen systems, Mg-based alloys are of tremendous interest from both a fundamental and an applied perspective because of their high hydrogen storage capacity. Pure magnesium has a hydrogen capacity of 7.6 wt.% in the form of MgH 2 . Pure magnesium, however, is prone to oxidation and has a low catalytic activity for the dissociative chemisorption of hydrogen. Even if the diameter of the magnesium powder grain is less than 50 nm, it still has a hydride dissociation temperature of 3308C [1], which is marginal for prospective applications. Some Mg-based alloys, for example Mg 2 Ni, are more stable in air than Mg, and have a lower dissociation temperature of 3008C [2]. Because nickel improves the catalytic activity significantly, the hydriding and dehydriding rates are consequently increased. However, Mg 2 Ni has a lower theoretical capacity than pure magnesium at 3.6%. Magnesium–nickel, iron, vanadium alloys with composition as low as 2–5% prepared by mechanical alloying have been reported [3–9]. Such compositions have high hydrogen capacities and high hydrogen reaction rates compared with Mg. A small concentration of dispersed nickel particles prepared by chemical vapor deposition enhances the hydrogen storage of magnesium [10]. The nanocrystalline or amorphous structure produced by *Corresponding author. E-mail address: [email protected] (L. Zuyan).
ball-milling results in dramatic changes in the hydrogen properties [11–13], especially an improvement of the hydriding / dehydriding kinetics. Another way of affecting the hydrogenation properties by ball-milling is to prepare a mixture or composite of storage hydrogen materials. In this paper, Mg–3Ni–1CrCl 3. (wt.%) was ball-milled under hydrogen pressure, and the effects of CrCl 3 on the Mg–Ni system were studied with respect to their hydrogenation properties.
2. Experimental Magnesium powder (.99%), nickel powder (.99.5%) and chromium(III) chloride (CrCl 3 ) were provided by the Shanghai Chemical Reagent Company. The average particle size of Ni and Mg were 20 mm as reported by the manufacture. The Mg, Ni and CrCl 3 powders were mixed in a stainless vial in the proportion 96:3:1 (wt.%), respectively. Milling was carried out with a QM-1SP4 planetary ball-milling machine constructed at Najing University with stainless steel balls with a ball-to-powder weight ratio of 10:1. The diameters of the balls were 20 and 10 mm, respectively. Grinding was performed under hydrogen atmosphere (about 0.5 MPa). Because the samples absorb hydrogen in the ball-milling process, the hydrogen pressure cannot be kept constant. The hydrogen of the vial must be recharged to 0.5 MPa at intervals. Since Mg–Ni– CrCl 3 samples can rapidly absorb hydrogen, the vial was
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01723-6
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refilled at intervals of 3–4 h; the Mg–Ni vial was refilled with hydrogen at 8–12 h. The rate of hydriding of both samples will decrease in the later part of the ball-milling process. A small amount of powder was taken for analysis and for hydrogen desorption / absorption experiments. From the hydrogen desorption experiment, we can measure the amount of hydrogen absorbed by the sample at particular milling times. All handling was carried out in a glove box under argon. The hydriding / dehydriding cyclic experiments were carried out using in-house designed apparatus. Fig. 1 shows a schematic diagram of the apparatus and the reactor. The apparatus can automatically record useful data such as temperature (temperature of the sample in the reactor and the temperature of the reactor wall), pressure and dehydriding hydrogen volume. The amount of absorbed hydrogen was measured using the pressure change in a constant volume; the dashed frame shows how to measure the desorbed hydrogen from the volume change using a displacement sensor. The position of water vessel 8 can be adjusted in order to keep the water level of vessel 8 consistent with that of vessel 10. If we do not adjust the position of vessel 8, the error is only 1.5%. By adjusting the bellows of sealed valve 6, we can measure the amount of hydrogen desorbed under any hydrogen pressure (0.1– 0.68 MPa). X-ray diffraction was performed on a Rigaku apparatus
with Cu Ka radiation. SEM was carried out on an Hitachi S-570.
3. Results
3.1. Mechanical milling process The evolution of the X-ray diffraction spectra as a function of milling time for two samples, Mg–3Ni and Mg–3Ni–1CrCl 3 (wt.%), is shown in Figs. 2 and 3, respectively. From these figures it can clearly be seen that the intensity of the diffraction peaks of magnesium is reduced and the width of the peaks is increased with increasing milling time. At the same time, the intensity and width of the peaks of MgH 2 increase rapidly. However, there are some obvious differences in Figs. 2 and 3, for example the peaks of MgH 2 in Fig. 3 increase more quickly than those in Fig. 2. The peaks of magnesium for Mg–3 wt.% Ni in Fig. 2 appear even at a ball-milling time of 160 h, but the peaks for Mg–3Ni–1CrCl 3 (wt.%) in Fig. 3 disappear at 98 h. This indicates that CrCl 3 is a strong catalyst for the hydriding process. The relative intensities of the various peaks calculated using the X-ray diffraction apparatus are shown in Table 1. The addition of CrCl 3 not only promotes the hydrogen
Fig. 1. Schematic diagram of the hydriding / dehydriding apparatus.
Y. Zhenxing et al. / Journal of Alloys and Compounds 333 (2002) 207 – 214
Fig. 2. XRD spectra of Mg–3Ni (wt.%) ball-milled for various times.
Fig. 3. XRD spectra of Mg–3Ni1–CrCl 3 (wt.%) ball-milled for various times.
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210 Table 1 Relative intensities of various phases Milling time (h) 29 45 66 98 130 160
Mg–3Ni
Mg–3Ni–1CrCl 3
Mg
MgH 2
Ni
Mg
MgH 2
Ni
85 72 66 48 20 21
9 18 22 34 48 47
5 9 11 17 30 30
71 55 37 – – –
17 29 40 64 62 –
10 14 21 35 37 –
absorption kinetics, but also reduces the crystalline grain size of Mg, MgH 2 and Ni. The changes in the grain size of Mg, Ni and MgH 2 can be calculated according to Scherrer’s expression [14]. The results are shown in Table 2. In Table 2, the corresponding grain size of each phase in Mg–3Ni–1CrCl 3 (wt.%) is much smaller than that in Mg–3Ni (wt.%). Fig. 4 shows the SEM patterns of samples Mg–Ni and Mg–Ni–CrCl 3 milled for 29 h. It can be seen that the average powder size of Mg–Ni–CrCl 3 is much smaller than that of Mg–Ni. The smaller crystalline size and powder size is one reason why Mg–3Ni–1CrCl 3 (wt.%) has better hydriding and dehydriding kinetics.
3.2. Hydrogen storage properties The third absorption curves of Mg–3Ni (wt.%) and Mg–3Ni–CrCl 3 (wt.%) at 473 K and under 2.0 MPa of hydrogen (in fact, this pressure is at the beginning of absorption; at the end of the hydriding process, the hydrogen pressure is about 1.2 MPa) are shown in Figs. 5 and 6, respectively. It is clear that the Mg–3Ni–1CrCl 3 (wt.%) composite absorbs hydrogen more rapidly and has a greater hydrogen storage capacity than Mg–3Ni (wt.%). In Fig. 6, curves 1 and 2 show that Mg–3Ni–1CrCl 3 (wt.%) can absorb hydrogen to 6.2 wt.% in 50 s at 2008C
Table 2 Grain sizes of phases at various milling times Sample
Milling time (h)
Phases
Grain size (nm)
Mg–3Ni (wt.%)
29
Mg Ni MgH 2 Mg Ni MgH 2
33.2 30.6 22.1 31.9 27.6 19.0
Mg Ni MgH 2 Mg Ni MgH 2
25.5 30.6 18.6 10.2 26.8 9.6
66
Mg–3Ni–1CrCl 3 (wt.%)
29
66
Fig. 4. SEM patterns of the samples milled for 29 h: (a) Mg–3Ni (wt.%); (b) Mg–3Ni–1CrCl 3 (wt.%).
and to 5.8 wt.% even at 1608C. Such a rate of absorption is amazing. However, the hydrogen absorption of Mg–3 wt.% Ni is also rapid before reaching 3.5 wt.%, but after that point the rate of absorption declines. The desorption curve of Mg–3Ni–1CrCl 3 (wt.%) at 598 K is shown in Fig. 7. This shows that the desorption speed is also rapid and that the dehydriding process is almost completed in about 800 s at 3208C under 0.1 MPa. The PCT curve of Mg–3Ni–1CrCl 3 (wt.%) at 3208C is shown in Fig. 8. There is a plateau at 0.25 MPa. From the hydrogenation and dehydrogenation experiments for both samples we can state that when the milling time exceeds 66 h, the storage properties of Mg–3Ni– 1CrCl 3 cannot be improved, but, in contrast, the storage properties of Mg–3Ni can be improved with increasing ball-milling time.
Y. Zhenxing et al. / Journal of Alloys and Compounds 333 (2002) 207 – 214
211
Fig. 5. Hydrogen absorption curve of Mg–3Ni at 2008C under 2.0 MPa hydrogen pressure.
4. Discussion
4.1. Temperature changes during the hydriding process The temperature changes of the samples during the absorption process are shown in Figs. 9 and 10, which indicate that the samples can quickly absorb hydrogen and the temperature can increase rapidly to 4108C in a few seconds. The high temperature will lead to a fast hydriding
process. We may define the process of rapidly rising temperature as the ignition process. Many experiments on the storage of Mg-based materials have shown that ignition takes place in fast hydriding processes. Because the enthalpy of MgH 2 (74.4 kJ / mol) is larger than that of other hydrogen materials, and in a rapid absorption process a great deal of heat can be produced but cannot be transfered to the environment in a short time, an ignition phenomenon must occur. This phenomenon is very im-
Fig. 6. Hydrogen absorption curve of Mg–3Ni–1CrCl 3 at (1) 2008C, (2) 1608C and (3) 1008C under 2.0 MPa hydrogen pressure.
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Fig. 7. Sixth desorption curve of Mg–3Ni–1CrCl 3 under 0.1 MPa.
portant for the hydrogen absorption process in practical applications; it can provide the hydrogen atoms with sufficient energy to cross the energy barrier and to penetrate the MgH 2 layer. In general, the key step in the absorption process is the diffusion of hydrogen in the hydride MgH 2 . If a dense hydride layer covers the whole surface of the magnesium particles, diffusion of hydrogen through this hydride layer will be very difficult and the absorption process will be
stopped. But if ignition takes place during the absorption process, the speed of absorption will be increased. There are many factors that can influence ignition phenomena, such as catalysis, the heat-transfer coefficient of the reactor, the level of activation of Mg-based materials, etc. In Fig. 9, the temperature is raised rapidly to 4108C, and in Fig. 8 the temperature is only raised to 2708C, which shows that CrCl 3 has a strong catalytic effect on the absorption of Mg–Ni alloys.
Fig. 8. Pressure–concentration isotherm of Mg–3Ni–1CrCl 3 at 3208C.
Y. Zhenxing et al. / Journal of Alloys and Compounds 333 (2002) 207 – 214
213
Fig. 9. Temperature and absorption curves of the Mg–3 wt.% Ni sample.
4.2. Catalytic effect of CrCl3 According to the experimental results, it can be concluded that CrCl 3 has a catalytic effect on the hydrogen absorption and desorption process of Mg–Ni system materials. But the catalytic mechanism of CrCl 3 is not completely clear. We believe that the effect may have two aspects. The first is that CrCl 3 is very helpful in peeling off the membrane MgO from the Mg grain surface. We all know that MgO plays an important role in the hydrogenation or dehydrogenation process. If the membrane MgO is peeled off from the Mg grain surface, the kinetics
of the hydriding / dehydriding process of the Mg-based composite will be promoted. Second, the Cr 31 of CrCl 3 enhances the chemisorption of hydrogen and establishes a rapid dynamic equilibrium between molecular hydrogen and hydrogen atoms [14]. This equilibrium between molecular hydrogen and hydrogen atoms will promote the kinetics the hydriding and dehydriding process. Two CrCl 3 samples, one with no crystalline water and another with crystalline water (CrCl 3 ?6H 2 O), were used in cyclic hydrogenation and dehydrogenation experiments. We found that CrCl 3 ?6H 2 O (experimental sample with 1 wt.% CrCl 3 ?6H 2 O) also has a catalytic effect on the
Fig. 10. Temperature and absorption curves of the Mg–3Ni–1CrCl 3 (wt.%) sample.
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absorption / desorption process. In the ball-milling process, CrCl 3 ?6H 2 O decomposes into CrCl 3 and H 2 O. The H 2 O will react with the MgO that covers the surface of the Mg grains to produce Mg(OH) 2 . Since Mg(OH) 2 is more loosely attached than MgO, it will be broken off more easily during the ball-milling process so that more fresh surfaces of Mg grains will be produced. Therefore, the hydrogenation speed will be promoted. The fact that traces of water can activate Mg-based storage materials has been demonstrated by Hampton [15,16]. Other transition metal chlorides, such as TiCl 4 , FeCl 2 and NiCl 2 , have been tested as new kinds of catalysts and their catalytic effects on Mg-based storage materials are clear and significant.
5. Conclusions The addition of CrCl 3 and other transition metal chlorides to Mg–Ni storage materials can greatly increase the hydriding / dehydriding rate and reduce the hydriding temperature. Mg can absorb hydrogen even during the ballmilling process. The hydrogen storage capacity of the composite Mg–3Ni–1CrCl 3 can reach 6.3 wt.%. The grain size of Mg can clearly be reduced. When the ball-milling time reaches 66 h, the average Mg crystalline grain size of Mg–3Ni–1CrCl 3 (wt.%) is about 10.2 nm, whereas the grain size of the sample without CrCl 3 is about 31.9 nm. The Mg–3Ni–CrCl 3 nanocomposite produced by ballmilling exhibits good kinetic behavior in the hydriding / dehydriding process, which offers a new opportunity for the selection and preparation of new kinds of storage materials.
References [1] B. Bogdanovi, B. Spliethoff, Int. J. Hydrogen Energy 12 (1987) 863. [2] Z. Dehouche, R. Djiaozandry, J. Goyette, T.K. Bose, J. Alloys Comp. 288 (1999) 269–276. ¨ [3] M.Y. Song, E. Ivanov, B. Darriet, M. Pezat, P. Hagenmuller, J. Less-Common Met. 131 (1987) 71–79. [4] L. Guoxian, W. Erde, Fangshoushi, J. Alloys Comp. 223 (1995) 111–114. [5] R.L. Holtz, M.A. Iman, J. Mater. Sci. 34 (1999) 2655–2663. [6] I.G. Konstanchuk, E.Yu. Ivanov, M. Pezat, B. Darriet, V.V. Boldyrev, ¨ P. Hagenmuller, J. Less-Common Met. 131 (1987) 181–189. [7] S.S. Sai Raman, O.N. Srivastava, J. Alloys Comp. 241 (1996) 167–174. [8] S. Orimo, K. Ikeda, H. Fujii, Y. Fujikawa, Y. Ritano, K. Yamamoto, Structural and hydriding properties of the Mg–Ni–H system with nano-and / or amorphous structures, Acta Mater. 45 (6) (1997) 2271–2278. [9] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Comp. 291 (1999) 295–299. [10] J. Chen, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Alloys Comp. 280 (1998) 290–293. [11] M. Khrussanova, P. Peshev, J. Less-Common Met. 131 (1987) 379–383. [12] K. Junaki, S.-i. Orimo, H. Hirosuke Sumida, J. Alloys Comp. 270 (1998) 160–163. [13] Y. Tsushio, H. Enoki, E. Akiba, J. Alloys Comp. 285 (1999) 298–301. [14] D. Jingfa (Ed.), Introduction of the Catalytic Effect Principle, 1st Edition, Jilin People’s Publishing Company, 1980, p. 490, in Chinese. [15] M.D. Hampton, J.K. Lomness, Int. J. Hydrogen Energy 24 (1999) 175–187. [16] M.D. Hampton, R. Juturn, J. Lomness, Int. J. Hydrogen Energy 24 (1999) 981–988.
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Hydrogen storage properties of the Mg–Ni–CrCl 3 nanocomposite Yu Zhenxing, Liu Zuyan*, Wang Erde School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Received 17 April 2001; accepted 15 June 2001
Abstract The effects of the addition of the transition metal chloride CrCl 3 on the hydriding and dehydriding behavior of Mg–3 wt.% Ni hydrogen storage material were investigated. The characteristics of the ball-milled nanocomposite Mg–3 wt.% Ni–1CrCl 3 such as hydriding capacity in the ball-milling process and the kinetics of hydriding / dehydriding were examined. The hydrogen absorption capacity of the composite is greatly increased in the ball-milling process under hydrogen pressure (0.5 MPa). The absorption rate of the composite is fast and the hydrogen storage capacity is more than that of the sample without CrCl 3 . 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Energy storage materials; Mechanical alloying; Gas–solid reaction
1. Introduction In metal–hydrogen systems, Mg-based alloys are of tremendous interest from both a fundamental and an applied perspective because of their high hydrogen storage capacity. Pure magnesium has a hydrogen capacity of 7.6 wt.% in the form of MgH 2 . Pure magnesium, however, is prone to oxidation and has a low catalytic activity for the dissociative chemisorption of hydrogen. Even if the diameter of the magnesium powder grain is less than 50 nm, it still has a hydride dissociation temperature of 3308C [1], which is marginal for prospective applications. Some Mg-based alloys, for example Mg 2 Ni, are more stable in air than Mg, and have a lower dissociation temperature of 3008C [2]. Because nickel improves the catalytic activity significantly, the hydriding and dehydriding rates are consequently increased. However, Mg 2 Ni has a lower theoretical capacity than pure magnesium at 3.6%. Magnesium–nickel, iron, vanadium alloys with composition as low as 2–5% prepared by mechanical alloying have been reported [3–9]. Such compositions have high hydrogen capacities and high hydrogen reaction rates compared with Mg. A small concentration of dispersed nickel particles prepared by chemical vapor deposition enhances the hydrogen storage of magnesium [10]. The nanocrystalline or amorphous structure produced by *Corresponding author. E-mail address: [email protected] (L. Zuyan).
ball-milling results in dramatic changes in the hydrogen properties [11–13], especially an improvement of the hydriding / dehydriding kinetics. Another way of affecting the hydrogenation properties by ball-milling is to prepare a mixture or composite of storage hydrogen materials. In this paper, Mg–3Ni–1CrCl 3. (wt.%) was ball-milled under hydrogen pressure, and the effects of CrCl 3 on the Mg–Ni system were studied with respect to their hydrogenation properties.
2. Experimental Magnesium powder (.99%), nickel powder (.99.5%) and chromium(III) chloride (CrCl 3 ) were provided by the Shanghai Chemical Reagent Company. The average particle size of Ni and Mg were 20 mm as reported by the manufacture. The Mg, Ni and CrCl 3 powders were mixed in a stainless vial in the proportion 96:3:1 (wt.%), respectively. Milling was carried out with a QM-1SP4 planetary ball-milling machine constructed at Najing University with stainless steel balls with a ball-to-powder weight ratio of 10:1. The diameters of the balls were 20 and 10 mm, respectively. Grinding was performed under hydrogen atmosphere (about 0.5 MPa). Because the samples absorb hydrogen in the ball-milling process, the hydrogen pressure cannot be kept constant. The hydrogen of the vial must be recharged to 0.5 MPa at intervals. Since Mg–Ni– CrCl 3 samples can rapidly absorb hydrogen, the vial was
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refilled at intervals of 3–4 h; the Mg–Ni vial was refilled with hydrogen at 8–12 h. The rate of hydriding of both samples will decrease in the later part of the ball-milling process. A small amount of powder was taken for analysis and for hydrogen desorption / absorption experiments. From the hydrogen desorption experiment, we can measure the amount of hydrogen absorbed by the sample at particular milling times. All handling was carried out in a glove box under argon. The hydriding / dehydriding cyclic experiments were carried out using in-house designed apparatus. Fig. 1 shows a schematic diagram of the apparatus and the reactor. The apparatus can automatically record useful data such as temperature (temperature of the sample in the reactor and the temperature of the reactor wall), pressure and dehydriding hydrogen volume. The amount of absorbed hydrogen was measured using the pressure change in a constant volume; the dashed frame shows how to measure the desorbed hydrogen from the volume change using a displacement sensor. The position of water vessel 8 can be adjusted in order to keep the water level of vessel 8 consistent with that of vessel 10. If we do not adjust the position of vessel 8, the error is only 1.5%. By adjusting the bellows of sealed valve 6, we can measure the amount of hydrogen desorbed under any hydrogen pressure (0.1– 0.68 MPa). X-ray diffraction was performed on a Rigaku apparatus
with Cu Ka radiation. SEM was carried out on an Hitachi S-570.
3. Results
3.1. Mechanical milling process The evolution of the X-ray diffraction spectra as a function of milling time for two samples, Mg–3Ni and Mg–3Ni–1CrCl 3 (wt.%), is shown in Figs. 2 and 3, respectively. From these figures it can clearly be seen that the intensity of the diffraction peaks of magnesium is reduced and the width of the peaks is increased with increasing milling time. At the same time, the intensity and width of the peaks of MgH 2 increase rapidly. However, there are some obvious differences in Figs. 2 and 3, for example the peaks of MgH 2 in Fig. 3 increase more quickly than those in Fig. 2. The peaks of magnesium for Mg–3 wt.% Ni in Fig. 2 appear even at a ball-milling time of 160 h, but the peaks for Mg–3Ni–1CrCl 3 (wt.%) in Fig. 3 disappear at 98 h. This indicates that CrCl 3 is a strong catalyst for the hydriding process. The relative intensities of the various peaks calculated using the X-ray diffraction apparatus are shown in Table 1. The addition of CrCl 3 not only promotes the hydrogen
Fig. 1. Schematic diagram of the hydriding / dehydriding apparatus.
Y. Zhenxing et al. / Journal of Alloys and Compounds 333 (2002) 207 – 214
Fig. 2. XRD spectra of Mg–3Ni (wt.%) ball-milled for various times.
Fig. 3. XRD spectra of Mg–3Ni1–CrCl 3 (wt.%) ball-milled for various times.
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210 Table 1 Relative intensities of various phases Milling time (h) 29 45 66 98 130 160
Mg–3Ni
Mg–3Ni–1CrCl 3
Mg
MgH 2
Ni
Mg
MgH 2
Ni
85 72 66 48 20 21
9 18 22 34 48 47
5 9 11 17 30 30
71 55 37 – – –
17 29 40 64 62 –
10 14 21 35 37 –
absorption kinetics, but also reduces the crystalline grain size of Mg, MgH 2 and Ni. The changes in the grain size of Mg, Ni and MgH 2 can be calculated according to Scherrer’s expression [14]. The results are shown in Table 2. In Table 2, the corresponding grain size of each phase in Mg–3Ni–1CrCl 3 (wt.%) is much smaller than that in Mg–3Ni (wt.%). Fig. 4 shows the SEM patterns of samples Mg–Ni and Mg–Ni–CrCl 3 milled for 29 h. It can be seen that the average powder size of Mg–Ni–CrCl 3 is much smaller than that of Mg–Ni. The smaller crystalline size and powder size is one reason why Mg–3Ni–1CrCl 3 (wt.%) has better hydriding and dehydriding kinetics.
3.2. Hydrogen storage properties The third absorption curves of Mg–3Ni (wt.%) and Mg–3Ni–CrCl 3 (wt.%) at 473 K and under 2.0 MPa of hydrogen (in fact, this pressure is at the beginning of absorption; at the end of the hydriding process, the hydrogen pressure is about 1.2 MPa) are shown in Figs. 5 and 6, respectively. It is clear that the Mg–3Ni–1CrCl 3 (wt.%) composite absorbs hydrogen more rapidly and has a greater hydrogen storage capacity than Mg–3Ni (wt.%). In Fig. 6, curves 1 and 2 show that Mg–3Ni–1CrCl 3 (wt.%) can absorb hydrogen to 6.2 wt.% in 50 s at 2008C
Table 2 Grain sizes of phases at various milling times Sample
Milling time (h)
Phases
Grain size (nm)
Mg–3Ni (wt.%)
29
Mg Ni MgH 2 Mg Ni MgH 2
33.2 30.6 22.1 31.9 27.6 19.0
Mg Ni MgH 2 Mg Ni MgH 2
25.5 30.6 18.6 10.2 26.8 9.6
66
Mg–3Ni–1CrCl 3 (wt.%)
29
66
Fig. 4. SEM patterns of the samples milled for 29 h: (a) Mg–3Ni (wt.%); (b) Mg–3Ni–1CrCl 3 (wt.%).
and to 5.8 wt.% even at 1608C. Such a rate of absorption is amazing. However, the hydrogen absorption of Mg–3 wt.% Ni is also rapid before reaching 3.5 wt.%, but after that point the rate of absorption declines. The desorption curve of Mg–3Ni–1CrCl 3 (wt.%) at 598 K is shown in Fig. 7. This shows that the desorption speed is also rapid and that the dehydriding process is almost completed in about 800 s at 3208C under 0.1 MPa. The PCT curve of Mg–3Ni–1CrCl 3 (wt.%) at 3208C is shown in Fig. 8. There is a plateau at 0.25 MPa. From the hydrogenation and dehydrogenation experiments for both samples we can state that when the milling time exceeds 66 h, the storage properties of Mg–3Ni– 1CrCl 3 cannot be improved, but, in contrast, the storage properties of Mg–3Ni can be improved with increasing ball-milling time.
Y. Zhenxing et al. / Journal of Alloys and Compounds 333 (2002) 207 – 214
211
Fig. 5. Hydrogen absorption curve of Mg–3Ni at 2008C under 2.0 MPa hydrogen pressure.
4. Discussion
4.1. Temperature changes during the hydriding process The temperature changes of the samples during the absorption process are shown in Figs. 9 and 10, which indicate that the samples can quickly absorb hydrogen and the temperature can increase rapidly to 4108C in a few seconds. The high temperature will lead to a fast hydriding
process. We may define the process of rapidly rising temperature as the ignition process. Many experiments on the storage of Mg-based materials have shown that ignition takes place in fast hydriding processes. Because the enthalpy of MgH 2 (74.4 kJ / mol) is larger than that of other hydrogen materials, and in a rapid absorption process a great deal of heat can be produced but cannot be transfered to the environment in a short time, an ignition phenomenon must occur. This phenomenon is very im-
Fig. 6. Hydrogen absorption curve of Mg–3Ni–1CrCl 3 at (1) 2008C, (2) 1608C and (3) 1008C under 2.0 MPa hydrogen pressure.
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Fig. 7. Sixth desorption curve of Mg–3Ni–1CrCl 3 under 0.1 MPa.
portant for the hydrogen absorption process in practical applications; it can provide the hydrogen atoms with sufficient energy to cross the energy barrier and to penetrate the MgH 2 layer. In general, the key step in the absorption process is the diffusion of hydrogen in the hydride MgH 2 . If a dense hydride layer covers the whole surface of the magnesium particles, diffusion of hydrogen through this hydride layer will be very difficult and the absorption process will be
stopped. But if ignition takes place during the absorption process, the speed of absorption will be increased. There are many factors that can influence ignition phenomena, such as catalysis, the heat-transfer coefficient of the reactor, the level of activation of Mg-based materials, etc. In Fig. 9, the temperature is raised rapidly to 4108C, and in Fig. 8 the temperature is only raised to 2708C, which shows that CrCl 3 has a strong catalytic effect on the absorption of Mg–Ni alloys.
Fig. 8. Pressure–concentration isotherm of Mg–3Ni–1CrCl 3 at 3208C.
Y. Zhenxing et al. / Journal of Alloys and Compounds 333 (2002) 207 – 214
213
Fig. 9. Temperature and absorption curves of the Mg–3 wt.% Ni sample.
4.2. Catalytic effect of CrCl3 According to the experimental results, it can be concluded that CrCl 3 has a catalytic effect on the hydrogen absorption and desorption process of Mg–Ni system materials. But the catalytic mechanism of CrCl 3 is not completely clear. We believe that the effect may have two aspects. The first is that CrCl 3 is very helpful in peeling off the membrane MgO from the Mg grain surface. We all know that MgO plays an important role in the hydrogenation or dehydrogenation process. If the membrane MgO is peeled off from the Mg grain surface, the kinetics
of the hydriding / dehydriding process of the Mg-based composite will be promoted. Second, the Cr 31 of CrCl 3 enhances the chemisorption of hydrogen and establishes a rapid dynamic equilibrium between molecular hydrogen and hydrogen atoms [14]. This equilibrium between molecular hydrogen and hydrogen atoms will promote the kinetics the hydriding and dehydriding process. Two CrCl 3 samples, one with no crystalline water and another with crystalline water (CrCl 3 ?6H 2 O), were used in cyclic hydrogenation and dehydrogenation experiments. We found that CrCl 3 ?6H 2 O (experimental sample with 1 wt.% CrCl 3 ?6H 2 O) also has a catalytic effect on the
Fig. 10. Temperature and absorption curves of the Mg–3Ni–1CrCl 3 (wt.%) sample.
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absorption / desorption process. In the ball-milling process, CrCl 3 ?6H 2 O decomposes into CrCl 3 and H 2 O. The H 2 O will react with the MgO that covers the surface of the Mg grains to produce Mg(OH) 2 . Since Mg(OH) 2 is more loosely attached than MgO, it will be broken off more easily during the ball-milling process so that more fresh surfaces of Mg grains will be produced. Therefore, the hydrogenation speed will be promoted. The fact that traces of water can activate Mg-based storage materials has been demonstrated by Hampton [15,16]. Other transition metal chlorides, such as TiCl 4 , FeCl 2 and NiCl 2 , have been tested as new kinds of catalysts and their catalytic effects on Mg-based storage materials are clear and significant.
5. Conclusions The addition of CrCl 3 and other transition metal chlorides to Mg–Ni storage materials can greatly increase the hydriding / dehydriding rate and reduce the hydriding temperature. Mg can absorb hydrogen even during the ballmilling process. The hydrogen storage capacity of the composite Mg–3Ni–1CrCl 3 can reach 6.3 wt.%. The grain size of Mg can clearly be reduced. When the ball-milling time reaches 66 h, the average Mg crystalline grain size of Mg–3Ni–1CrCl 3 (wt.%) is about 10.2 nm, whereas the grain size of the sample without CrCl 3 is about 31.9 nm. The Mg–3Ni–CrCl 3 nanocomposite produced by ballmilling exhibits good kinetic behavior in the hydriding / dehydriding process, which offers a new opportunity for the selection and preparation of new kinds of storage materials.
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