- Email: [email protected]
Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R = La, Nd and Sm) alloys
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 0 9 e1 0 7 1 4
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R [ La, Nd and Sm) alloys Q.A. Zhang*, C.J. Jiang, D.D. Liu School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, PR China
article info
abstract
Article history:
The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg10NiR
Received 23 January 2012
(R ¼ La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg10NiNd
Received in revised form
and Mg10NiSm alloys are in amorphous state but the Mg10NiLa alloy is composed of an
22 March 2012
amorphous phase and minor crystalline La2Mg17 after melt-spinning. The alloys can be
Accepted 6 April 2012
hydrogenated into MgH2, Mg2NiH4 and a rare earth metal hydride RHx. The rare earth metal
Available online 16 May 2012
hydride and Mg2NiH4 synergistically provide a catalytic effect on the hydrogen absorption edesorption reactions in the MgH2 system. The hydrogen storage kinetics is not influ-
Keywords:
enced by different rare earth metal hydrides but by the particle size of the rare earth metal
Magnesium alloy
hydrides.
Rare earth metal hydride
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Hydrogen storage
1.
Introduction
Magnesium hydride has been extensively studied because it can store 7.6 wt.% hydrogen [1]. However, the slow kinetics and high temperature of hydrogen desorption limit the practical applications of MgH2. To improve the hydrogen absorptionedesorption kinetics, ball-milling is usually used to prepare Mg-based hydrogen storage materials [2e8]. Nevertheless, the ball-milled Mg-based materials easily suffer from serious oxidation during preparation due to high reactivity. Hence, another method for accelerating the hydrogen absorptionedesorption rate of MgeH2 system is to alloy Mg with some catalytic elements such as Ni or/and rare earth elements [9e11]. An early investigation indicated that the hydrogen absorptionedesorption kinetics of MgeNieR (R ¼ rare earth metals) alloys can be improved with increasing the Ni and R contents [12]. Considered the requirement of hydrogen storage capacity, however, the Ni and R contents
reserved.
should be kept in a reasonable range [13]. Therefore, the recent studies of MgeNieR hydrogen storage alloys have mainly focused on the exploration of a suitable rare earth element R under the circumstance of ensuring sufficient Mg content [14e16]. Generally, the amorphous or nanostructured MgeNieR alloys prepared by melt-spinning have faster hydrogen absorptionedesorption kinetics than the as-cast alloys [17e20]. The improvement of absorption kinetics can be ascribed to the nano-sized particles of rare earth metal hydrides and Mg2Ni embedded in Mg matrix after activation; and the enhancement of desorption kinetics is attributed to the nano-sized particles of rare earth metal hydrides and Mg2NiH4 embedded in MgH2 matrix after hydrogenation [9,10,21,22]. Recently a study on the in situ synchrotron X-ray diffraction of hydrogenated Mg80Ni10Y10 during its vacuum thermal decomposition indicated that the dehydrogenation of YH3 to YH2 was much slower than the dehydrogenation
* Corresponding author. Tel.: þ86 555 2311570; fax: þ86 555 2471263. E-mail address: [email protected] (Q.A. Zhang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.038
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transformations of MgH2 to Mg and Mg2NiH4 to Mg2Ni; and accordingly metallic Mg, intermetallic Mg2Ni, hydrides YH2 and YH3 coexisted in the sample after hydrogen desorption at 473 K for 1 h [23]. This raises the question as to whether different rare earth metal hydrides have the same catalytic effect on the hydrogen absorptionedesorption in MgeNieR alloys. So far, the rare earth metal hydrides have been studied extensively [24e30]. Generally, trihydrides RH3 can be formed in all ReH2 systems under suitable temperature and pressure conditions. It is noteworthy that the trigonal NdH3 decomposes into cubic NdH2.61 (substoichiometric NdH3) under vacuum starting at 493 K and NdH2.61 can exist under the hydrogen pressure below 5 MPa at 800 K [24e27]. On the other hand, however, the hexagonal SmH3 is only stable in a very narrow range and decomposes into SmH2 starting at 373 K [27,28]. This means that NdH2.61 and SmH2 are stable during the hydrogen absorptionedesorption process under a hydrogen pressure no higher than 3 MPa at 423e623 K. Given that LaH2 and LaH3 can transform into each other during hydrogen absorptionedesorption below 723 K [29,30], the comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R ¼ La, Nd and Sm) alloys have been carried out in this work for clarifying the question above and further providing us new information on the understanding of the catalytic mechanism of rare earth metal hydrides in Mg-based alloys.
2.
The melt-spun Mg10NiR alloys obtained were mechanically crushed into 300 mesh powder samples in a glove box under dry argon atmosphere. The hydrogen absorption/desorption kinetics for the powder samples was measured using a Sieverts-type apparatus at 423 and 523 K, respectively. Prior to the formal measurements, the powder samples were fully activated by repeatedly hydridingdehydriding at 623 K for three times. In each activation cycle, the samples were first hydrogenated under a hydrogen pressure of 3 MPa for 1 h and then evacuated for 1 h. To evaluate the phase structures of the melt-spun, hydrogenated, activated and rehydrogenated samples, powder X-ray diffraction (XRD) measurements were carried out on a Rigaku D/Max 2500 VL/PC diffractometer with Cu Ka radiation at 50 kV and 200 mA. The XRD samples were loaded
Experimental details
The Mg10NiR (R ¼ La, Nd and Sm) alloy ingots were prepared by induction melting of appropriate amounts of mixtures of pure Mg, rare earth metals and Ni under an argon atmosphere (about 0.1 MPa). The as-cast alloys were re-melted and quenched by melt-spinning with a constant rotating copper roller surface velocity of 40 m/s. The continuously long ribbons with width of about 3 mm and thickness of 30e50 mm were obtained. About 3 wt.% of rare earth metals and 18 wt.% of Mg were excessively added to compensate for the losses of rare earth metals and Mg during melting and melt-spinning.
Fig. 1 e XRD patterns of melt-spun (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm alloys.
Fig. 2 e Rietveld refinements of the observed XRD patterns for the (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm samples hydrogenated under an initial hydrogen pressure of 3 MPa at 623 K for 4 h.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 0 9 e1 0 7 1 4
and sealed in a special home-made holder that can keep the samples under argon atmosphere in the course of measurement. The XRD profiles were analyzed with the Rietveld refinement program RIETAN-2000 [31].
3.
Results and discussion
3.1.
Hydrogenation behavior
Fig. 1 shows the XRD patterns of the melt-spun Mg10NiLa, Mg10NiNd and Mg10NiSm alloys. It can be seen that the meltspun Mg10NiNd and Mg10NiSm alloys are in amorphous state. However, the melt-spun Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17. This means that the Mg10NiLa alloy has a poor glass-forming ability compared with Mg10NiNd and Mg10NiSm alloys. Such a result complies with the reported result that the glass-forming ability of Mg60Cu25R10 alloys strongly depended on the atomic size of rare earth metals [32]. Fig. 2a presents the Rietveld refinement of the observed XRD pattern for the Mg10NiLa sample hydrogenated under an initial hydrogen pressure of 3 MPa at 623 K for 4 h. It can be seen that the hydrogenated sample consists of MgH2, LaH3 and cubic Mg2NiH4 accompanied by a small amount of Mg2NiH0.3, indicating that the crystalline La2Mg17 can be hydrogenated into MgH2 and LaH3 while the mainly amorphous phase is hydrogenated into MgH2, LaH3 and Mg2NiH4. The presence of the small amount of Mg2NiH0.3 (interstitial solid solution of hydrogen in Mg2Ni) means that the first hydrogenation of the melt-spun sample is incomplete. Similarly, the amorphous Mg10NiNd alloy is also hydrogenated into MgH2, NdH2.61 and Mg2NiH4 (together with minor Mg2NiH0.3, as shown in Fig. 2b). It should be noted that a trigonal NdH3 is usually formed under a very high hydrogen pressure [25,26]
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and unstable at the present hydrogen pressure of 3 MPa [27]. As a result, the substoichiometric NdH2.61 with a cubic structure is formed instead of the trigonal NdH3. Fig. 2c shows the Rietveld refinement of the observed XRD pattern for the hydrogenated Mg10NiSm sample. It can be seen that the amorphous Mg10NiSm alloy is hydrogenated into MgH2, SmH2 and Mg2NiH4 (also accompanied by minor Mg2NiH0.3) under the present experiment condition. Obviously, the present hydrogenation pressure (3 MPa) is insufficient for the formation of SmH3 [27]. This agrees with the fact that SmH3 is hardly obtained during the hydrogenation process under a hydrogen pressure of 10 MPa [28].
3.2.
Activation characteristics
The activation curves of the melt-spun Mg10NiLa, Mg10NiNd and Mg10NiSm alloys at 623 K are compared in Fig. 3. Evidently, all the alloys have a good activation property, namely two hydridingdehydriding cycles at 623 K are sufficient to activate the melt-spun alloys. Nevertheless, it should be noted that the first hydrogen absorption curves of the alloys have small slopes. This implies that the all the first hydrogenation reactions are kinetically slow, even incomplete as shown in Fig. 2. Interestingly, the subsequent hydrogen desorption and absorption processes show fast kinetics. Such a result is ascribed to the rare earth metal hydrides and Mg2NiH4 produced during the hydrogenation process, which will be further discussed below. Fig. 4 shows the Rietveld refinements of the observed XRD patterns for the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples by three hydridingdehydriding cycles at 623 K. The structural parameters and phase abundance of the activated Mg10NiR (R ¼ La, Nd and Sm) samples refined from the XRD patterns are listed in Table 1. It can be seen that the activated Mg10NiLa sample consists of Mg, Mg2Ni and LaH2, indicating
Fig. 3 e Activation curves of melt-spun (a, b) Mg10NiLa (c, d) Mg10NiNd and (e, f) Mg10NiSm alloys at 623 K.
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that LaH3 is dehydrogenated into LaH2 while MgH2 and Mg2NiH4 are fully dehydrogenated in the hydrogen desorption process. However, NdH2.61 and SmH2 remain in the activated Mg10NiNd and Mg10NiSm samples, respectively; even MgH2 and Mg2NiH4 can be fully dehydrogenated in both samples. This means that the NdH2.61 and SmH2 produced in the first hydrogenation processes of the amorphous Mg10NiNd and Mg10NiSm alloys do not desorb or absorb hydrogen anymore in the subsequent dehydrogenation or rehydrogenation processes. Hence, these rare earth metal hydrides remained in the activated samples can play an important role in subsequent hydrogen absorption and desorption processes.
3.3.
Hydrogen absorption/desorption kinetics
Fig. 5a presents the kinetic curves of hydrogen absorption of the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples under an initial hydrogen pressure of 3 MPa at 423 K. It can be seen that all the samples have excellent hydrogen absorption kinetics. Similarly, the hydrogen desorption kinetics against
Fig. 4 e Rietveld refinements of the observed XRD patterns for the activated (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm samples by repeatedly hydridingLdehydriding at 623 K for three times.
Table 1 e Structural parameters and phase abundance of the activated Mg10NiR (R [ La, Nd and Sm) samples. Sample
Mg10NiLa
Phase Space Lattice parameters Abundance group (wt.%) ˚) ˚) a (A c (A
Mg LaH2 Mg2Ni Mg10Ni10Nd Mg NdH2.61 Mg2Ni Mg10Ni10Sm Mg SmH2 Mg2Ni
P63/mmc Fm3m P6222 P63/mmc Fm3m P6222 P63/mmc Fm3m P6222
3.2068 5.6203 5.2152 3.2087 5.4323 5.2188 3.2081 5.3537 5.2108
(1) (9) (5) (9) (2) (6) (8) (6) (5)
5.2050 (8) 13.253 (9) 5.2100 (1) 13.278 (3) 5.2100 (7) 13.256 (3)
45 31 24 47 35 18 43 36 21
Fig. 5 e Time dependence of hydrogen content of (a) The activated samples for absorption under an initial hydrogen pressure of 3 MPa at 423 K and (b) The fully hydrogenated samples for desorption against an initial state of vacuum at 523 K.
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an initial state of vacuum at 523 K is also enhanced remarkably (see Fig. 5b), which is comparable with that for the ballmilled Mg with a catalyst [33]. Such a kinetic property is much better than that for Mg90Ni10 alloy and equivalent to that for Mg80Ni10Y10 alloy [23]. This indicates that the contribution to the enhancement in the hydrogen absorptionedesorption kinetics is actually from both the rare earth metal hydrides and Mg2NiH0.3 (or Mg2NiH4) [10,23]. Fig. 6 shows the Rietveld refinements of the observed XRD patterns for the Mg10NiLa, Mg10NiNd and Mg10NiSm samples rehydrogenated at 423 K for 1 h. Clearly, MgH2 is accompanied with a rare earth metal hydride and Mg2NiH4 (coexistence of monoclinic and cubic Mg2NiH4) in each sample. From the kinetic curves of hydrogen absorptionedesorption in Fig. 5, it is unassailable that a rare earth metal hydride and Mg2Ni hydride in each sample synergistically provide a catalytic effect on the hydrogen absorptionedesorption reactions in the MgH2 system, no matter what the rare earth metal
hydride is. Although the exact catalytic mechanism has not been fully understood, in situ synchrotron X-ray diffraction studies of hydrogen absorptionedesorption behaviors provide us a scientific basis for the catalytic effect [34,35]. It was reported that the transition of Mg / MgH2 was accompanied by MmH2 / MmH3 after Mg2Ni / Mg2NiH0.3 in the hydrogen absorption process of MgeMmeNi alloy but the transition of MgH2 / Mg occurred prior to other dehydrogenation transitions in the hydrogen desorption process [34]. This means that all hydride phases including Mg2NiH0.3, MmH2, MmH3 and Mg2NiH4 have a catalytic effect. Our present result further shows that the catalytic effect is not influenced by different rare earth metal hydrides. Furthermore, the Mg10NiNd and Mg10NiSm samples have almost the same kinetic properties of hydrogen absorptionedesorption (see Fig. 5) though the NdH2.61 and SmH2 do not absorb or desorb hydrogen anymore in the hydrogen absorptionedesorption processes. Note that the kinetic property of hydrogen desorption of the Mg10NiLa sample at 523 K is slightly worse than those of the Mg10NiNd and Mg10NiSm samples. However, the worse kinetics is not related to the type of lanthanum hydrides (LaH2 or LaH3) even though the transformations between LaH2 and LaH3 occur during hydrogen absorption and desorption. Indeed, the catalytic effect of rare earth metal hydrides on the hydrogen absorptionedesorption kinetics of the MgH2 system is related to the particle size of rare earth metal hydrides [10]. As shown in Fig. 1, the melt-spun Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17. The LaH3 or LaH2 particles produced by hydrogenation of the crystalline La2Mg17 should have a larger size than the NdH2.61 and SmH2 particles obtained by hydrogenation of the amorphous Mg10NiNd and Mg10NiSm alloys [10,12]. To obtain the sizes of rare earth metal hydrides in the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples, Rietveld refinements on the XRD profiles were performed and the refined parameters in the Lorentzian part of a pseudo-Voigt function were used as described elsewhere [10]. It is found that the average particle sizes of the rare earth metal hydrides in the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples are 37, 20 and 18 nm, respectively. As a result, the hydrogen desorption rate of the Mg10NiLa sample is slightly slower than those of the Mg10NiNd and Mg10NiSm samples at 523 K.
4.
Fig. 6 e Rietveld refinements of the observed XRD patterns for the (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm samples rehydrogenated at 423 K for 1 h.
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Conclusions
The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg10NiR (R ¼ La, Nd and Sm) alloys have been studied comparatively. It is found that the melt-spun Mg10NiNd and Mg10NiSm alloys are in amorphous state but the Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17. The Mg10NiR alloys can be hydrogenated into MgH2, Mg2NiH4 and RHx (LaH3, NdH2.61 and SmH2, respectively) under an initial hydrogen pressure of 3 MPa at 623 K. During the subsequent dehydrogenation or rehydrogenation processes, NdH2.61 and SmH2 do not desorb or absorb hydrogen anymore but the transformations between LaH3 and LaH2 occur reversibly. Together with Mg2NiH4 (or Mg2NiH0.3), all the rare earth metal hydrides have
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a good catalytic effect on the hydrogen absorptionedesorption reactions in the MgH2 system. However, the particle size of rare earth metal hydrides influences the hydrogen storage kinetics of MgeNieR alloys.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 50971001).
references
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[16] Wu Y, Lototskyy MV, Solberg JK, Yartys VA. Effect of microstructure on the phase composition and hydrogen absorption-desorption behaviour of melt-spun Mge20Nie8Mn alloys. Int J Hydrogen Energy 2012;37:1495e508. [17] Lass EA. Hydrogen storage measurements in novel Mg-based nanostructured alloys produced via rapid solidification and devitrification. Int J Hydrogen Energy 2011;36:10787e96. [18] Kalinichenka S, Rontzsch L, Riedl T, Gemming T, Weibgarber T, Kieback B. Microstructure and hydrogen storage properties of melt-spun MgeCueNieY alloys. Int J Hydrogen Energy 2011;36:1592e600. [19] Lin HJ, Ouyang LZ, Wang H, Liu JW, Zhu M. Phase transition and hydrogen storage properties of melt-spun Mg3LaNi0.1 alloy. Int J Hydrogen Energy 2012;37:1145e50. [20] Poletaev AA, Denys RV, Maehlen JP, Solberg JK, Tarasov BP, Yartys VA. Nanostructured rapidly solidified LaMg11Ni alloy: microstructure, crystal structure and hydrogenation properties. Int J Hydrogen Energy 2012;37:3548e57. [21] Kalinichenka S, Rontzsch L, Kieback B. Structural and hydrogen storage properties of melt-spun MgeNieY alloys. Int J Hydrogen Energy 2009;34:7749e55. [22] Tanaka K, Miwa T, Sasaki K, Kuroda K. TEM studies of nanostructure in melt-spun MgeNieLa alloy manifesting enhanced hydrogen desorbing kinetics. J Alloys Compd 2009; 478:308e16. [23] Kalinichenka S, Rontzsch L, Baehtz C, Kieback B. Hydrogen desorption kinetics of melt-spun and hydrogenated Mg90Ni10 and Mg80Ni10Y10 using in situ synchrotron, X-ray diffraction and thermogravimetry. J Alloys Compd 2010;496:608e13. [24] Ishikawa K, Ogasawara N, Aoki K. Pressure dependence of hydrogen-induced amorphization in C14 Laves phase NdMn2. J Alloys Compd 2005;404e406:599e603. [25] Renaudin G, Fischer P, Yvon K. Neodymium trihydride, NdH3, with tysonite type structure. J Alloys Compd 2000;313:L10e4. [26] Bashkin IO, Ponyatovskii EG, Kost ME. Phase Transformation in hydrides rare-earth metals under pressure: I Structural Transition in NdH2.82. Phys Status Solidi (b) 1977;83:517e20. [27] Yartys VA, Gutfleisch O, Panasyuk VV, Harris IR. Desorption characteristics of rare earth (R) hydrides (R ¼ Y, Ce, Pr, Nd. Sm, Gd and Tb) in relation to the HDDR behaviour of ReFe-based compounds. J Alloys Compd 1997;243/254:128e33. [28] Wang L, Conder K, Kaldis E. Thermodynamic studies in the system SmH2eSmH3. Solid State Ionics 1990;43:103e11. [29] Denys RV, Poletaev AA, Solberg JK, Tarasov BP, Yartys VA. LaMg11 with a giant unit cell synthesized by hydrogen metallurgy: crystal structure and hydrogenation behavior. Acta Mater 2010;58:2510e9. [30] Messer CE, Hung GW. Dissociation pressures in the system LaH2eSmH3, 250e450 deg . J Phys Chem 1968;72:3958e62. [31] Izumi F, Ikeda T. Rietveld-analysis program RIETAN-98 and its applications to zeolites. Mater Sci Forum 2000;321e324:198e203. [32] Xi XK, Zhao DQ, Pan MX, Wang WH. On the criteria of bulk metallic glass formation in MgCu-based alloys. Intermetallics 2005;13:638e41. [33] Barkhordarian G, Klassen T, ormann R. Effect of Nb2O5 content on hydrogen reaction kinetics of Mg. J Alloys Compd 2004;364:242e6. [34] Denys RV, Riabov AB, Maehlen JP, Lototsky MV, Solberg JK, Yartys VA. In situ synchrotron X-ray diffraction studies of hydrogen desorption and absorption properties of Mg and MgeMmeNi after reaction ball milling in hydrogen. Acta Mater 2009;57:3989e4000. [35] Denys RV, Poletaev AA, Maehlen JP, Solberg JK, Tarasov BP, Yartys VA. Nanostructured rapidly solidified LaMg11Ni alloy. II. In situ synchrotron X-ray diffraction studies of hydrogen absorption edesorption. Int J Hydrogen Energy 2012;37: 5710e22.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R [ La, Nd and Sm) alloys Q.A. Zhang*, C.J. Jiang, D.D. Liu School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, PR China
article info
abstract
Article history:
The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg10NiR
Received 23 January 2012
(R ¼ La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg10NiNd
Received in revised form
and Mg10NiSm alloys are in amorphous state but the Mg10NiLa alloy is composed of an
22 March 2012
amorphous phase and minor crystalline La2Mg17 after melt-spinning. The alloys can be
Accepted 6 April 2012
hydrogenated into MgH2, Mg2NiH4 and a rare earth metal hydride RHx. The rare earth metal
Available online 16 May 2012
hydride and Mg2NiH4 synergistically provide a catalytic effect on the hydrogen absorption edesorption reactions in the MgH2 system. The hydrogen storage kinetics is not influ-
Keywords:
enced by different rare earth metal hydrides but by the particle size of the rare earth metal
Magnesium alloy
hydrides.
Rare earth metal hydride
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Hydrogen storage
1.
Introduction
Magnesium hydride has been extensively studied because it can store 7.6 wt.% hydrogen [1]. However, the slow kinetics and high temperature of hydrogen desorption limit the practical applications of MgH2. To improve the hydrogen absorptionedesorption kinetics, ball-milling is usually used to prepare Mg-based hydrogen storage materials [2e8]. Nevertheless, the ball-milled Mg-based materials easily suffer from serious oxidation during preparation due to high reactivity. Hence, another method for accelerating the hydrogen absorptionedesorption rate of MgeH2 system is to alloy Mg with some catalytic elements such as Ni or/and rare earth elements [9e11]. An early investigation indicated that the hydrogen absorptionedesorption kinetics of MgeNieR (R ¼ rare earth metals) alloys can be improved with increasing the Ni and R contents [12]. Considered the requirement of hydrogen storage capacity, however, the Ni and R contents
reserved.
should be kept in a reasonable range [13]. Therefore, the recent studies of MgeNieR hydrogen storage alloys have mainly focused on the exploration of a suitable rare earth element R under the circumstance of ensuring sufficient Mg content [14e16]. Generally, the amorphous or nanostructured MgeNieR alloys prepared by melt-spinning have faster hydrogen absorptionedesorption kinetics than the as-cast alloys [17e20]. The improvement of absorption kinetics can be ascribed to the nano-sized particles of rare earth metal hydrides and Mg2Ni embedded in Mg matrix after activation; and the enhancement of desorption kinetics is attributed to the nano-sized particles of rare earth metal hydrides and Mg2NiH4 embedded in MgH2 matrix after hydrogenation [9,10,21,22]. Recently a study on the in situ synchrotron X-ray diffraction of hydrogenated Mg80Ni10Y10 during its vacuum thermal decomposition indicated that the dehydrogenation of YH3 to YH2 was much slower than the dehydrogenation
* Corresponding author. Tel.: þ86 555 2311570; fax: þ86 555 2471263. E-mail address: [email protected] (Q.A. Zhang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.038
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 0 9 e1 0 7 1 4
transformations of MgH2 to Mg and Mg2NiH4 to Mg2Ni; and accordingly metallic Mg, intermetallic Mg2Ni, hydrides YH2 and YH3 coexisted in the sample after hydrogen desorption at 473 K for 1 h [23]. This raises the question as to whether different rare earth metal hydrides have the same catalytic effect on the hydrogen absorptionedesorption in MgeNieR alloys. So far, the rare earth metal hydrides have been studied extensively [24e30]. Generally, trihydrides RH3 can be formed in all ReH2 systems under suitable temperature and pressure conditions. It is noteworthy that the trigonal NdH3 decomposes into cubic NdH2.61 (substoichiometric NdH3) under vacuum starting at 493 K and NdH2.61 can exist under the hydrogen pressure below 5 MPa at 800 K [24e27]. On the other hand, however, the hexagonal SmH3 is only stable in a very narrow range and decomposes into SmH2 starting at 373 K [27,28]. This means that NdH2.61 and SmH2 are stable during the hydrogen absorptionedesorption process under a hydrogen pressure no higher than 3 MPa at 423e623 K. Given that LaH2 and LaH3 can transform into each other during hydrogen absorptionedesorption below 723 K [29,30], the comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R ¼ La, Nd and Sm) alloys have been carried out in this work for clarifying the question above and further providing us new information on the understanding of the catalytic mechanism of rare earth metal hydrides in Mg-based alloys.
2.
The melt-spun Mg10NiR alloys obtained were mechanically crushed into 300 mesh powder samples in a glove box under dry argon atmosphere. The hydrogen absorption/desorption kinetics for the powder samples was measured using a Sieverts-type apparatus at 423 and 523 K, respectively. Prior to the formal measurements, the powder samples were fully activated by repeatedly hydridingdehydriding at 623 K for three times. In each activation cycle, the samples were first hydrogenated under a hydrogen pressure of 3 MPa for 1 h and then evacuated for 1 h. To evaluate the phase structures of the melt-spun, hydrogenated, activated and rehydrogenated samples, powder X-ray diffraction (XRD) measurements were carried out on a Rigaku D/Max 2500 VL/PC diffractometer with Cu Ka radiation at 50 kV and 200 mA. The XRD samples were loaded
Experimental details
The Mg10NiR (R ¼ La, Nd and Sm) alloy ingots were prepared by induction melting of appropriate amounts of mixtures of pure Mg, rare earth metals and Ni under an argon atmosphere (about 0.1 MPa). The as-cast alloys were re-melted and quenched by melt-spinning with a constant rotating copper roller surface velocity of 40 m/s. The continuously long ribbons with width of about 3 mm and thickness of 30e50 mm were obtained. About 3 wt.% of rare earth metals and 18 wt.% of Mg were excessively added to compensate for the losses of rare earth metals and Mg during melting and melt-spinning.
Fig. 1 e XRD patterns of melt-spun (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm alloys.
Fig. 2 e Rietveld refinements of the observed XRD patterns for the (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm samples hydrogenated under an initial hydrogen pressure of 3 MPa at 623 K for 4 h.
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and sealed in a special home-made holder that can keep the samples under argon atmosphere in the course of measurement. The XRD profiles were analyzed with the Rietveld refinement program RIETAN-2000 [31].
3.
Results and discussion
3.1.
Hydrogenation behavior
Fig. 1 shows the XRD patterns of the melt-spun Mg10NiLa, Mg10NiNd and Mg10NiSm alloys. It can be seen that the meltspun Mg10NiNd and Mg10NiSm alloys are in amorphous state. However, the melt-spun Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17. This means that the Mg10NiLa alloy has a poor glass-forming ability compared with Mg10NiNd and Mg10NiSm alloys. Such a result complies with the reported result that the glass-forming ability of Mg60Cu25R10 alloys strongly depended on the atomic size of rare earth metals [32]. Fig. 2a presents the Rietveld refinement of the observed XRD pattern for the Mg10NiLa sample hydrogenated under an initial hydrogen pressure of 3 MPa at 623 K for 4 h. It can be seen that the hydrogenated sample consists of MgH2, LaH3 and cubic Mg2NiH4 accompanied by a small amount of Mg2NiH0.3, indicating that the crystalline La2Mg17 can be hydrogenated into MgH2 and LaH3 while the mainly amorphous phase is hydrogenated into MgH2, LaH3 and Mg2NiH4. The presence of the small amount of Mg2NiH0.3 (interstitial solid solution of hydrogen in Mg2Ni) means that the first hydrogenation of the melt-spun sample is incomplete. Similarly, the amorphous Mg10NiNd alloy is also hydrogenated into MgH2, NdH2.61 and Mg2NiH4 (together with minor Mg2NiH0.3, as shown in Fig. 2b). It should be noted that a trigonal NdH3 is usually formed under a very high hydrogen pressure [25,26]
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and unstable at the present hydrogen pressure of 3 MPa [27]. As a result, the substoichiometric NdH2.61 with a cubic structure is formed instead of the trigonal NdH3. Fig. 2c shows the Rietveld refinement of the observed XRD pattern for the hydrogenated Mg10NiSm sample. It can be seen that the amorphous Mg10NiSm alloy is hydrogenated into MgH2, SmH2 and Mg2NiH4 (also accompanied by minor Mg2NiH0.3) under the present experiment condition. Obviously, the present hydrogenation pressure (3 MPa) is insufficient for the formation of SmH3 [27]. This agrees with the fact that SmH3 is hardly obtained during the hydrogenation process under a hydrogen pressure of 10 MPa [28].
3.2.
Activation characteristics
The activation curves of the melt-spun Mg10NiLa, Mg10NiNd and Mg10NiSm alloys at 623 K are compared in Fig. 3. Evidently, all the alloys have a good activation property, namely two hydridingdehydriding cycles at 623 K are sufficient to activate the melt-spun alloys. Nevertheless, it should be noted that the first hydrogen absorption curves of the alloys have small slopes. This implies that the all the first hydrogenation reactions are kinetically slow, even incomplete as shown in Fig. 2. Interestingly, the subsequent hydrogen desorption and absorption processes show fast kinetics. Such a result is ascribed to the rare earth metal hydrides and Mg2NiH4 produced during the hydrogenation process, which will be further discussed below. Fig. 4 shows the Rietveld refinements of the observed XRD patterns for the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples by three hydridingdehydriding cycles at 623 K. The structural parameters and phase abundance of the activated Mg10NiR (R ¼ La, Nd and Sm) samples refined from the XRD patterns are listed in Table 1. It can be seen that the activated Mg10NiLa sample consists of Mg, Mg2Ni and LaH2, indicating
Fig. 3 e Activation curves of melt-spun (a, b) Mg10NiLa (c, d) Mg10NiNd and (e, f) Mg10NiSm alloys at 623 K.
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that LaH3 is dehydrogenated into LaH2 while MgH2 and Mg2NiH4 are fully dehydrogenated in the hydrogen desorption process. However, NdH2.61 and SmH2 remain in the activated Mg10NiNd and Mg10NiSm samples, respectively; even MgH2 and Mg2NiH4 can be fully dehydrogenated in both samples. This means that the NdH2.61 and SmH2 produced in the first hydrogenation processes of the amorphous Mg10NiNd and Mg10NiSm alloys do not desorb or absorb hydrogen anymore in the subsequent dehydrogenation or rehydrogenation processes. Hence, these rare earth metal hydrides remained in the activated samples can play an important role in subsequent hydrogen absorption and desorption processes.
3.3.
Hydrogen absorption/desorption kinetics
Fig. 5a presents the kinetic curves of hydrogen absorption of the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples under an initial hydrogen pressure of 3 MPa at 423 K. It can be seen that all the samples have excellent hydrogen absorption kinetics. Similarly, the hydrogen desorption kinetics against
Fig. 4 e Rietveld refinements of the observed XRD patterns for the activated (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm samples by repeatedly hydridingLdehydriding at 623 K for three times.
Table 1 e Structural parameters and phase abundance of the activated Mg10NiR (R [ La, Nd and Sm) samples. Sample
Mg10NiLa
Phase Space Lattice parameters Abundance group (wt.%) ˚) ˚) a (A c (A
Mg LaH2 Mg2Ni Mg10Ni10Nd Mg NdH2.61 Mg2Ni Mg10Ni10Sm Mg SmH2 Mg2Ni
P63/mmc Fm3m P6222 P63/mmc Fm3m P6222 P63/mmc Fm3m P6222
3.2068 5.6203 5.2152 3.2087 5.4323 5.2188 3.2081 5.3537 5.2108
(1) (9) (5) (9) (2) (6) (8) (6) (5)
5.2050 (8) 13.253 (9) 5.2100 (1) 13.278 (3) 5.2100 (7) 13.256 (3)
45 31 24 47 35 18 43 36 21
Fig. 5 e Time dependence of hydrogen content of (a) The activated samples for absorption under an initial hydrogen pressure of 3 MPa at 423 K and (b) The fully hydrogenated samples for desorption against an initial state of vacuum at 523 K.
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an initial state of vacuum at 523 K is also enhanced remarkably (see Fig. 5b), which is comparable with that for the ballmilled Mg with a catalyst [33]. Such a kinetic property is much better than that for Mg90Ni10 alloy and equivalent to that for Mg80Ni10Y10 alloy [23]. This indicates that the contribution to the enhancement in the hydrogen absorptionedesorption kinetics is actually from both the rare earth metal hydrides and Mg2NiH0.3 (or Mg2NiH4) [10,23]. Fig. 6 shows the Rietveld refinements of the observed XRD patterns for the Mg10NiLa, Mg10NiNd and Mg10NiSm samples rehydrogenated at 423 K for 1 h. Clearly, MgH2 is accompanied with a rare earth metal hydride and Mg2NiH4 (coexistence of monoclinic and cubic Mg2NiH4) in each sample. From the kinetic curves of hydrogen absorptionedesorption in Fig. 5, it is unassailable that a rare earth metal hydride and Mg2Ni hydride in each sample synergistically provide a catalytic effect on the hydrogen absorptionedesorption reactions in the MgH2 system, no matter what the rare earth metal
hydride is. Although the exact catalytic mechanism has not been fully understood, in situ synchrotron X-ray diffraction studies of hydrogen absorptionedesorption behaviors provide us a scientific basis for the catalytic effect [34,35]. It was reported that the transition of Mg / MgH2 was accompanied by MmH2 / MmH3 after Mg2Ni / Mg2NiH0.3 in the hydrogen absorption process of MgeMmeNi alloy but the transition of MgH2 / Mg occurred prior to other dehydrogenation transitions in the hydrogen desorption process [34]. This means that all hydride phases including Mg2NiH0.3, MmH2, MmH3 and Mg2NiH4 have a catalytic effect. Our present result further shows that the catalytic effect is not influenced by different rare earth metal hydrides. Furthermore, the Mg10NiNd and Mg10NiSm samples have almost the same kinetic properties of hydrogen absorptionedesorption (see Fig. 5) though the NdH2.61 and SmH2 do not absorb or desorb hydrogen anymore in the hydrogen absorptionedesorption processes. Note that the kinetic property of hydrogen desorption of the Mg10NiLa sample at 523 K is slightly worse than those of the Mg10NiNd and Mg10NiSm samples. However, the worse kinetics is not related to the type of lanthanum hydrides (LaH2 or LaH3) even though the transformations between LaH2 and LaH3 occur during hydrogen absorption and desorption. Indeed, the catalytic effect of rare earth metal hydrides on the hydrogen absorptionedesorption kinetics of the MgH2 system is related to the particle size of rare earth metal hydrides [10]. As shown in Fig. 1, the melt-spun Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17. The LaH3 or LaH2 particles produced by hydrogenation of the crystalline La2Mg17 should have a larger size than the NdH2.61 and SmH2 particles obtained by hydrogenation of the amorphous Mg10NiNd and Mg10NiSm alloys [10,12]. To obtain the sizes of rare earth metal hydrides in the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples, Rietveld refinements on the XRD profiles were performed and the refined parameters in the Lorentzian part of a pseudo-Voigt function were used as described elsewhere [10]. It is found that the average particle sizes of the rare earth metal hydrides in the activated Mg10NiLa, Mg10NiNd and Mg10NiSm samples are 37, 20 and 18 nm, respectively. As a result, the hydrogen desorption rate of the Mg10NiLa sample is slightly slower than those of the Mg10NiNd and Mg10NiSm samples at 523 K.
4.
Fig. 6 e Rietveld refinements of the observed XRD patterns for the (a) Mg10NiLa, (b) Mg10NiNd and (c) Mg10NiSm samples rehydrogenated at 423 K for 1 h.
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Conclusions
The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg10NiR (R ¼ La, Nd and Sm) alloys have been studied comparatively. It is found that the melt-spun Mg10NiNd and Mg10NiSm alloys are in amorphous state but the Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17. The Mg10NiR alloys can be hydrogenated into MgH2, Mg2NiH4 and RHx (LaH3, NdH2.61 and SmH2, respectively) under an initial hydrogen pressure of 3 MPa at 623 K. During the subsequent dehydrogenation or rehydrogenation processes, NdH2.61 and SmH2 do not desorb or absorb hydrogen anymore but the transformations between LaH3 and LaH2 occur reversibly. Together with Mg2NiH4 (or Mg2NiH0.3), all the rare earth metal hydrides have
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a good catalytic effect on the hydrogen absorptionedesorption reactions in the MgH2 system. However, the particle size of rare earth metal hydrides influences the hydrogen storage kinetics of MgeNieR alloys.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 50971001).
references
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