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Structural characteristics and hydrogen storage properties of Sm2Co7
Journal of Alloys and Compounds 608 (2014) 14–18
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structural characteristics and hydrogen storage properties of Sm2Co7 Z.J. Cao a,d, L.Z. Ouyang a,d,⇑, H. Wang a,d, J.W. Liu a,d, D.L. Sun b,d, Q.A. Zhang c,d, M. Zhu a,d,⇑ a
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China c School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China d Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 5 March 2014 Received in revised form 11 April 2014 Accepted 15 April 2014 Available online 24 April 2014 Keywords: Hydrogen storage Sm2Co7 Thermodynamics Kinetics
a b s t r a c t The structural characteristics and hydrogen-storage properties of the Ce2Ni7-type compound Sm2Co7 have been investigated for the first time. This alloy transforms to the b phase and then the c phase upon hydrogenation. These two phases have been identified as Sm2Co7H2.9 and Sm2Co7H6.4, and their dehydrogenation enthalpies have been measured as 48.5 and 42.0 kJ/mol H2, respectively. Sm2Co7 shows excellent stability without obvious capacity loss after 50 cycles, and its hydrogenation process follows a three-dimensional-interface-controlled reaction at ambient temperature. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction For potential applications in Ni-MH batteries, it is of fundamental importance to establish the hydrogen storage properties and reaction mechanisms of rare-earth intermetallic alloys [1]. In pioneering work by Zijlstra and Westendorp [2], it was found that SmCo5 could reversibly absorb a large quantity of hydrogen at ambient temperature. Soon thereafter, numerous reports followed of efforts directed towards investigating several series of materials due to their outstanding hydrogen-storage characteristics [3–21]. Ming and Goudy [8] measured the hydriding and dehydriding kinetics of Dy2Co7 hydride and observed that the rate of hydrogen evolution from the c phase was approximately seven times faster than that from the b phase, and that hydrogen evolution from the b phase was approximately eight times faster than that from the a phase. Goudy et al. [15] measured the thermodynamics and kinetics of hydrogen absorption by rare-earth-cobalt (R2Co7 and RCo3) and rare-earth-iron (RFe3) compounds. R2Co7 compounds (R = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) absorbed large quantities of hydrogen (ca. 1.2 wt%) and their dehydriding enthalpies were determined to be in the ranges 36–54 kJ/mol H2 and 36–72 kJ/ mol H2 for the a + b and b + c phases, respectively. Among the family of intermetallics, R2Co7 (R = rare-earth element) systems ⇑ Corresponding authors at: School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China. Tel.: +86 20 87114253; fax: +86 20 87112762. E-mail addresses: [email protected] (L.Z. Ouyang), [email protected] (M. Zhu). http://dx.doi.org/10.1016/j.jallcom.2014.04.106 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
have attracted interest due to their applications as permanent magnets. Apostolov et al. [22] reported the magnetic properties of the compounds R2Co7 (R = Pr, Sm, Ho, Tb), but they only paid attention to the magnetic properties of the hydrides, which had hydrogen contents corresponding to b and c phases. Although a number of studies have been carried out on cobalt-containing intermetallics, it is noteworthy that the hydrogen-storage characteristics of Sm2Co7 still remain unknown. To the best of our knowledge, the precise crystal structures of the a, b, and c phases and their thermodynamic and kinetic properties have not been studied. In this study, we have focused on structural confirmation and changes in the form of Sm2Co7 upon hydrogenation, as well as its hydrogen absorption/desorption properties. The reaction mechanism and the intrinsic rate-limiting step of this alloy have also been identified by fitting the time-dependent transformed fraction with various analytical rate expressions. The results showed that the Sm2Co7 alloy exhibited good hydrogen-storage properties. This work offers a better understanding of the structure and hydrogen storage of Sm2Co7, which in turn provides a foundation for further investigations for on-board applications.
2. Experimental details The Sm2Co7 alloy was prepared by arc-melting of Sm (99.9%) with Co (99.9%) under an atmosphere of high-purity argon (99.999%). The sample was re-melted several times to homogenize it. The obtained ingot was pulverized in air and transferred to a glovebox to minimize further contamination by air and moisture. The glovebox was filled with high-purity argon with a moisture content of less than 3 ppm and an O2 content of less than 5 ppm.
Z.J. Cao et al. / Journal of Alloys and Compounds 608 (2014) 14–18 The crystal structures of these samples were determined by X-ray diffraction (XRD) analysis on a Philips X’pert-MPD instrument employing Cu Ka radiation (k = 0.15406 nm). The lattice constants and the phase contents were calculated from the XRD data by the Rietveld method using RIETAN software. The thermodynamic properties of Sm2Co7 hydrogenation, determined from P–C isotherms and kinetic curves, were evaluated on an Advanced Materials Corporation (AMC) gas reactor. For these measurements, samples of about 1.7 g were utilized, and the reaction cell was inductively heated with an accuracy of ± 2 K.
3. Results and discussion 3.1. Crystal structure of Sm2Co7 Fig. 1(a) shows the Rietveld refinement of the observed XRD pattern for the as-prepared Sm2Co7 alloy. The calculated pattern showed good agreement with the observed pattern with a goodness-of-fit parameter S of 1.91. The refined structure parameters and phase contents of the as-melted alloy are shown in Table. 1. This alloy was composed of the Sm2Co7 phase with a small amount of the SmCo5 phase. The mass fractions of these two alloys were 86% and 14%, respectively. The lattice parameters of Sm2Co7 were determined as a = 0.5048(1) nm and c = 24.319(5) nm, which is consistent with previously reported values (a = 0.5036 nm, c = 24.29 nm) [22]. Rare-earth-cobalt compounds R2Co7 are known to be isomorphous, and crystallize in two polymorphic forms: a low-temperature phase modification with the hexagonal Ce2Ni7 structure and a high temperature phase modification with the rhombohedral Gd2Co7-type structure [23,24]. Fig. 1(b) shows the crystal structure of the hexagonal Ce2Ni7-type Sm2Co7 alloy obtained in this work. It consists of double layers of hexagonal structural blocks of CaCu5-type SmCo5 alternating with double layers of MgCu2-type cubic blocks of SmCo2 along a common hexagonal axis (2SmCo5 + 2SmCo2 ? 2Sm2Co7). It should be noted
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that the minor SmCo5 phase in the as-melted alloy absorbed little hydrogen, hence its influence on the hydrogen-storage properties of Sm2Co7 is neglected hereafter. 3.2. Thermodynamic properties of Sm2Co7 during hydrogenation and dehydrogenation Fig. 2(a) shows the pressure-composition isotherm (PCI) curves for the Sm2Co7–H system at temperatures between 348 K and 453 K. Two plateaus can be clearly discerned in each curve, indicating the existence of two distinguishable phases (b and c) besides the solid-solution a phase. This observation is consistent with the results of Goudy et al. [15], who also observed two plateaus in the PCI curves of similar R2Co7 (R = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) compounds. The first plateau is narrow and sloping, and corresponds to hydrogen contents between 0.33 and 0.43 wt% H2. The second plateau is relatively broad and flat, and corresponds to hydrogen contents between 0.51 and 0.67 wt% H2. Based on the PCI curves, the enthalpy and entropy changes for dehydrogenation of the b and c phases could be calculated according to the Van’t Hoff equation. Pressure plateau data at each temperature were obtained from the mid-point of the plateau region. As shown in Fig. 2(b), the dehydrogenation enthalpies (DH) for the b and c phases were 48.5 and 42.0 kJ/mol H2, respectively, in good agreement with results for other R2Co7 compounds [15]. Ming and Goudy [8] found that two phase transitions in Dy2Co7 –H could be deduced based on the locations of the hydrogen atoms. Hydrogen atoms in the tetrahedral and octahedral sites exhibited different thermodynamic stabilities. Hydrogen atoms in the b phase occupy the more stable octahedral sites, whereas in the c phase they occupy the less stable tetrahedral sites; hence, the desorption temperature for the present b phase was much lower than that for the c phase. 3.3. Hydrogen absorption–desorption mechanisms According to Goudy’s report [15], there are some differences in the compositions of the b and c phases among R2Co7 compounds. For the b and c phases in Sm2Co7, the corresponding hydrides are Sm2Co7H2.9 and Sm2Co7H6.4, respectively, based on calculations of the absorbed hydrogen contents obtained from PCI runs. For hydrogen absorption, the equilibrium pressures of the first and second plateaus at 378 K were close to 0.3 atm and 1.4 atm, respectively. To obtain a single b or c phase, the sample was subjected to hydrogen at a pressure slightly higher than the first or second plateau region. Fig. 3 shows the XRD patterns of samples hydrogenated at different hydrogen pressures at 378 K. It can clearly be seen that with increasing hydrogen pressure, the b phase (Sm2Co7 H2.9), with a structure distinct from that of Sm2Co7, formed at a pressure of 0.4 atm. This b phase then transformed into a hydrogen-rich c phase at a higher hydrogen pressure of 2 atm. As the pressure was increased to 30 atm, the XRD peaks shifted towards lower angles, indicating formation of the c hydride with an increased hydrogen content. Moreover, the curves for the dehydrogenation processes demonstrated that the Sm2Co7 alloy underwent a reverse cycle. From the above structural analysis, the mechanism for hydrogenation and dehydrogenation may be expressed as follows:
Sm2 Co7 þ H2 $ Sm2 Co7 H2:9 ðb phaseÞ Sm2 Co7 H2:9 þ H2 $ Sm2 Co7 H6:4 ðc phaseÞ
Fig. 1. (a) Rietveld refinement of the observed XRD pattern for the as-prepared Sm2Co7 alloy. (b) Crystal structure of this hexagonal Ce2Ni7-type alloy.
Fig. 4 shows the hydrogen desorption capacity of the Sm2Co7 alloy as a function of cycle number at 333 K. It can be seen that this alloy could reach the maximum capacity without any activation process. Moreover, there was little loss in hydrogen-storage
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Z.J. Cao et al. / Journal of Alloys and Compounds 608 (2014) 14–18
Table 1 Structure parameters and phase content of as-prepared sample and cycled sample. Sample
Phases
Space group
a
c
Before hydrogenation
Sm2Co7 SmCo5
P63/mmc P6/mmm
0.5048(1) 0.4979(2)
2.4319(5) 0.3986(1)
86 14
Cycled sample
Sm2Co7 SmCo5
P63/mmc P6/mmm
0.5050(1) 0.5004(1)
2.4397(6) 0.3990(1)
91 9
Fig. 2. (a) PCI curves for the Sm2Co7–H system at temperatures between 348 K and 453 K. (b) Van’t Hoff plots of the b and c phases.
Lattice parameters (nm)
Phase content (wt%)
Fig. 4. Hydrogen desorption capacity as a function of cycle number at 333 K.
Fig. 5. Rietveld refinement XRD pattern of Sm2Co7 after 50 absorption/desorption cycles.
Fig. 3. XRD patterns of samples hydrogenated at different hydrogen pressures at 378 K.
capacity after 50 cycles. Fig. 5 shows the Rietveld refinement pattern of Sm2Co7 after the 50 absorption/desorption cycles. The Bragg peaks were much broader than those before the absorption/desorption cycles, indicating that the Sm2Co7 alloy refined obviously after the hydrogen/desorption reactions. The refined parameters of the cycled Sm2Co7 alloy were also presented in Table 1. Apart from the slightly increased lattice parameters (a = 5.050(1) nm and c = 24.397(6) nm), the Sm2Co7 alloy retained its pristine structure after the cycle measurement, indicating that the Sm2Co7 phase had excellent cycle stability. As shown in Table
1, after the cycle measurement the a-axis remained almost unchanged, while the c-axis was expanded by 0.32%. It is noteworthy that, in contrast to RCo5 alloys, for which the pristine CaCu5 structure tends to degrade to an orthorhombic structure [2], the structure of Sm2Co7 showed no sign of decomposition during the hydrogenation and dehydrogenation cycles. Only the content of the SmCo5 phase decreased from 14 wt% in the pristine material to 9 wt% after the cycle measurement. Xie et al. [25] found that ErNi3 showed a bad cycling performance due to the partial decomposition of ErNi3 into ErNi2 as well as ErNi5 and the increase of host-lattice strain. Iwase et al. [11] found that hydrogen was not evenly distributed in the cells of GdNi5 and Gd2Ni4, which led to different modifications in the Gd2Ni7 lattice when subjected to hydrogen: the GdNi5 cell expanded, whereas the Gd2Ni4 cell shrank. The hydrogen absorption properties of Gd–Ni binary compounds with superlattice structures seemed to be related to the number of GdNi5 cells in the unit cell [26]. Similarly, the good cycle stability of Sm2Co7 may also be related to the hydrogen occupation
Z.J. Cao et al. / Journal of Alloys and Compounds 608 (2014) 14–18
in SmCo2 and SmCo5; therefore, the phase transformation and expansion during hydrogenation have to be further studied by in situ XRD measurements. 3.4. Hydrogen absorption kinetics of Sm2Co7 Fig. 6(a) shows hydrogen absorption kinetic curves for the Sm2 Co7 alloy measured between 293 K and 373 K under an initial pressure of 2 MPa. It can clearly be seen that this alloy displayed very rapid hydrogenation kinetics at ambient temperature, with 80% of the maximum storage capacity being absorbed in less than 5 min, and the hydrogenation rate obviously accelerated with increasing temperature. On going from 293 K to 323 K, the time required to absorb 80% of the maximum hydrogen content decreased from 5 min to 3 min. At temperatures above 343 K, this alloy could absorb more than 90% of the maximum hydrogen content in less than 1 min. To understand the hydrogenation process, the reaction mechanism has to be analyzed. Generally, the mechanism of a solid–gas reaction can be determined by comparing the measured hydriding rate curve with rate equations derived for different reaction processes [27–30]. It was found that the experimental data for hydrogen absorption by Sm2Co7 could be best fitted by the Avrami–Erofeev equation, which is deduced from nucleation and growth:
aðtÞ ¼ 1 expðBtm Þ
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hydrogenation is probably the allotropic transformation from the a phase to the hydride; the kinetics of such transformations could thus be studied through hydrogen absorption by the alloy. The Sm2Co7 alloy achieves a maximum hydrogen storage capacity of about 1.2 wt% at room temperature with excellent hydrogen absorption kinetics after activation. And this promising property may enable it to use in the Ni-MH battery in the future. 4. Conclusions The Sm2Co7 compound was confirmed as adopting a Ce2Ni7type crystal structure. Sm2Co7 is composed of double layers of hexagonal structural blocks of CaCu5-type SmCo5 alternating with double layers of MgCu2-type cubic blocks of SmCo2 along the hexagonal axis. Sm2Co7 is transformed to the b phase and then the c phase upon hydrogenation. These two distinguishable phases were calculated as Sm2Co7H2.9 and Sm2Co7H6.4, and their desorption enthalpies were determined as 48.5 and 42.0 kJ/mol H2, respectively. Sm2Co7 showed excellent de/hydrogenation stability without obvious capacity loss after 50 cycles. This alloy also displayed very rapid hydrogenation kinetics at ambient temperature, and the hydrogenation process followed a three-dimensionalinterface-controlled reaction. These results open a new perspective for potential applications in Ni-MH batteries and/or high-pressure tank materials.
ð1Þ
Here, aðtÞ is the reaction rate, B and m are constants, and t is the reaction time. Fig. 6(b) shows the kinetic experimental data and fitting results for the Sm2Co7 alloy measured at 293 K. From the values of S and r, it is evident that the fitting coincides well with the experimental results, indicating that the hydrogen absorption process of Sm2Co7 obeyed the nucleation and growth mechanism. Although Eq. (1) describes the nucleation and growth of the hydriding reaction, the rate-determining step of the reaction can vary according to different values of m. For instance, the one-dimensional diffusion process and the three-dimensional interface reaction process correspond to m values of 0.62 and 1.07, respectively [27]. The value of m in the present case is around 1.36, which is closer to 1.07. This result indicated that the hydrogenation process of Sm2Co7 was likely to be a three-dimensional-interfacecontrolled reaction. This is consistent with the report by Kuijpers van Mal [3], who showed that the rate of release of hydrogen from SmCo5 hydride was determined by the rate of transformation of the hydride to the dehydrogenation compound, rather than by bulk diffusion of hydrogen. The rate-controlling step of Sm2Co7
Fig. 6. (a) Hydrogen absorption kinetic curves for the Sm2Co7 alloy measured between 293 K and 373 K under an initial pressure of 2 MPa. (b) Kinetic experimental data and its fitting results at 293 K.
Acknowledgements This work was financially supported by the Ministry of Science and Technology of China (No. 2010CB631302), the National Natural Science Foundation of China (Nos. U1201241 and 51271078) and KLGHEI (KLB11003). References [1] L. Huang, Y.F. Liu, R. Li, M.X. Gao, H.G. Pan, Rare Metal Mater. Eng. 41 (2012) 542–547. [2] H. Zijlstra, F.F. Westendorp, Solid State Commun. 7 (1969) 857–859. [3] F.A. Kuijpers, H.H. van Mal, J. Less Common Met. 23 (1971) 395–398. [4] W.E. Wallace, R.F. Karlicek, H. Imamura, J. Phys. Chem. 83 (1979) 1708–1712. [5] L. Belkbir, E. Joly, N. Gerard, Int. J. Hydrogen Energy 6 (1981) 285–294. [6] G.G. Lucas, W.L. Richards, Int. J. Hydrogen Energy 9 (1984) 225–231. [7] C. Deng, P. Shi, S. Zhang, Mater. Chem. Phys. 98 (2006) 514–518. [8] L. Ming, A.J. Goudy, J. Alloys Comp. 283 (1999) 146–150. [9] R.V. Denys, V.A. Yartys, M. Sato, A.B. Riabov, R.G. Delaplane, J. Solid State Chem. 180 (2007) 2566–2576. [10] K. Iwase, K. Sakaki, Y. Nakamura, E. Akiba, Inorg. Chem. 49 (2010) 8763–8768. [11] K. Iwase, K. Mori, A. Hoshikawa, T. Ishigaki, Int. J. Hydrogen Energy 37 (2012) 5122–5127. [12] S.K. Singh, A.K. Singh, K. Ramakrishna, O.N. Srivastava, Int. J. Hydrogen Energy 10 (1985) 523–529. [13] M.N. Mungole, R. Balasubramaniam, Int. J. Hydrogen Energy 23 (1998) 349– 353. [14] M.N. Mungole, R. Balasubramaniam, Int. J. Hydrogen Energy 25 (2000) 55–60. [15] A. Goudy, W.E. Wallace, R.S. Craig, T. Takeshita, in: R. Bau (Ed.), Advances in Chemistry Series, vol. 167, American Chemical Society, Washington, DC, 1978, pp. 312–326. [16] Q.A. Zhang, L.X. Zhang, Q.Q. Wang, J. Alloys Comp. 551 (2013) 376–381. [17] K. Sun, Y.F. Zhu, W. Zhang, L.Q. Li, Rare Metal Mater. Eng. 41 (2012) 1842– 1845. [18] J.J. Zhang, S.M. Han, Y. Li, J.J. Liu, S.Q. Yang, L. Zhang, J.D. Wang, J. Alloys Comp. 581 (2013) 693–698. [19] J.D. Wang, S.M. Han, Y. Li, J.J. Liu, L.D. Che, L. Zhang, J.J. Zhang, J. Alloys Comp. 582 (2014) 552–557. [20] G.B. Xin, Y.Y. Wang, H. Fu, G.L. Li, J. Zheng, X.G. Li, Phys. Chem. Chem. Phys. 16 (2014) 3001–3006. [21] T. Liu, Y.R. Cao, C.G. Qin, W.S. Chou, X.G. Li, J. Power Sources 246 (2014) 277– 282. [22] L.B.A. Apostolov, N. Stanev, T. Mydlafcz, J. Magn. Magn. Mater. 83 (1990) 286– 288. [23] W. Ostertag, J. Less Common Met. 13 (1967) 385–390. [24] R. Fersi, N. Mliki, L. Bessais, R. Guetari, V. Russier, M. Cabie, J. Alloys Comp. 522 (2012) 14–18. [25] S.C. Xie, Z.L. Chen, Y.T. Li, T.Z. Si, D.M. Liu, Q.A. Zhang, J. Alloys Comp. 585 (2014) 650–655.
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[26] K. Iwase, K. Mori, A. Hoshikawa, T. Ishigaki, Int. J. Hydrogen Energy 37 (2012) 15170–15174. [27] R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology, vol. 3, VCH, Weinheim, 1994 (Chapter 13).
[28] M. Zhu, Y. Gao, X.Z. Che, Y.Q. Yang, C.Y. Chung, J. Alloys Comp. 330–332 (2002) 708–713. [29] L.Z. Ouyang, F.X. Qin, M. Zhu, Scripta Mater. 55 (2006) 1075–1078. [30] L.Z. Ouyang, X.S. Yang, H.W. Dong, M. Zhu, Scripta Mater. 61 (2009) 339–342.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structural characteristics and hydrogen storage properties of Sm2Co7 Z.J. Cao a,d, L.Z. Ouyang a,d,⇑, H. Wang a,d, J.W. Liu a,d, D.L. Sun b,d, Q.A. Zhang c,d, M. Zhu a,d,⇑ a
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China c School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China d Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 5 March 2014 Received in revised form 11 April 2014 Accepted 15 April 2014 Available online 24 April 2014 Keywords: Hydrogen storage Sm2Co7 Thermodynamics Kinetics
a b s t r a c t The structural characteristics and hydrogen-storage properties of the Ce2Ni7-type compound Sm2Co7 have been investigated for the first time. This alloy transforms to the b phase and then the c phase upon hydrogenation. These two phases have been identified as Sm2Co7H2.9 and Sm2Co7H6.4, and their dehydrogenation enthalpies have been measured as 48.5 and 42.0 kJ/mol H2, respectively. Sm2Co7 shows excellent stability without obvious capacity loss after 50 cycles, and its hydrogenation process follows a three-dimensional-interface-controlled reaction at ambient temperature. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction For potential applications in Ni-MH batteries, it is of fundamental importance to establish the hydrogen storage properties and reaction mechanisms of rare-earth intermetallic alloys [1]. In pioneering work by Zijlstra and Westendorp [2], it was found that SmCo5 could reversibly absorb a large quantity of hydrogen at ambient temperature. Soon thereafter, numerous reports followed of efforts directed towards investigating several series of materials due to their outstanding hydrogen-storage characteristics [3–21]. Ming and Goudy [8] measured the hydriding and dehydriding kinetics of Dy2Co7 hydride and observed that the rate of hydrogen evolution from the c phase was approximately seven times faster than that from the b phase, and that hydrogen evolution from the b phase was approximately eight times faster than that from the a phase. Goudy et al. [15] measured the thermodynamics and kinetics of hydrogen absorption by rare-earth-cobalt (R2Co7 and RCo3) and rare-earth-iron (RFe3) compounds. R2Co7 compounds (R = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) absorbed large quantities of hydrogen (ca. 1.2 wt%) and their dehydriding enthalpies were determined to be in the ranges 36–54 kJ/mol H2 and 36–72 kJ/ mol H2 for the a + b and b + c phases, respectively. Among the family of intermetallics, R2Co7 (R = rare-earth element) systems ⇑ Corresponding authors at: School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China. Tel.: +86 20 87114253; fax: +86 20 87112762. E-mail addresses: [email protected] (L.Z. Ouyang), [email protected] (M. Zhu). http://dx.doi.org/10.1016/j.jallcom.2014.04.106 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
have attracted interest due to their applications as permanent magnets. Apostolov et al. [22] reported the magnetic properties of the compounds R2Co7 (R = Pr, Sm, Ho, Tb), but they only paid attention to the magnetic properties of the hydrides, which had hydrogen contents corresponding to b and c phases. Although a number of studies have been carried out on cobalt-containing intermetallics, it is noteworthy that the hydrogen-storage characteristics of Sm2Co7 still remain unknown. To the best of our knowledge, the precise crystal structures of the a, b, and c phases and their thermodynamic and kinetic properties have not been studied. In this study, we have focused on structural confirmation and changes in the form of Sm2Co7 upon hydrogenation, as well as its hydrogen absorption/desorption properties. The reaction mechanism and the intrinsic rate-limiting step of this alloy have also been identified by fitting the time-dependent transformed fraction with various analytical rate expressions. The results showed that the Sm2Co7 alloy exhibited good hydrogen-storage properties. This work offers a better understanding of the structure and hydrogen storage of Sm2Co7, which in turn provides a foundation for further investigations for on-board applications.
2. Experimental details The Sm2Co7 alloy was prepared by arc-melting of Sm (99.9%) with Co (99.9%) under an atmosphere of high-purity argon (99.999%). The sample was re-melted several times to homogenize it. The obtained ingot was pulverized in air and transferred to a glovebox to minimize further contamination by air and moisture. The glovebox was filled with high-purity argon with a moisture content of less than 3 ppm and an O2 content of less than 5 ppm.
Z.J. Cao et al. / Journal of Alloys and Compounds 608 (2014) 14–18 The crystal structures of these samples were determined by X-ray diffraction (XRD) analysis on a Philips X’pert-MPD instrument employing Cu Ka radiation (k = 0.15406 nm). The lattice constants and the phase contents were calculated from the XRD data by the Rietveld method using RIETAN software. The thermodynamic properties of Sm2Co7 hydrogenation, determined from P–C isotherms and kinetic curves, were evaluated on an Advanced Materials Corporation (AMC) gas reactor. For these measurements, samples of about 1.7 g were utilized, and the reaction cell was inductively heated with an accuracy of ± 2 K.
3. Results and discussion 3.1. Crystal structure of Sm2Co7 Fig. 1(a) shows the Rietveld refinement of the observed XRD pattern for the as-prepared Sm2Co7 alloy. The calculated pattern showed good agreement with the observed pattern with a goodness-of-fit parameter S of 1.91. The refined structure parameters and phase contents of the as-melted alloy are shown in Table. 1. This alloy was composed of the Sm2Co7 phase with a small amount of the SmCo5 phase. The mass fractions of these two alloys were 86% and 14%, respectively. The lattice parameters of Sm2Co7 were determined as a = 0.5048(1) nm and c = 24.319(5) nm, which is consistent with previously reported values (a = 0.5036 nm, c = 24.29 nm) [22]. Rare-earth-cobalt compounds R2Co7 are known to be isomorphous, and crystallize in two polymorphic forms: a low-temperature phase modification with the hexagonal Ce2Ni7 structure and a high temperature phase modification with the rhombohedral Gd2Co7-type structure [23,24]. Fig. 1(b) shows the crystal structure of the hexagonal Ce2Ni7-type Sm2Co7 alloy obtained in this work. It consists of double layers of hexagonal structural blocks of CaCu5-type SmCo5 alternating with double layers of MgCu2-type cubic blocks of SmCo2 along a common hexagonal axis (2SmCo5 + 2SmCo2 ? 2Sm2Co7). It should be noted
15
that the minor SmCo5 phase in the as-melted alloy absorbed little hydrogen, hence its influence on the hydrogen-storage properties of Sm2Co7 is neglected hereafter. 3.2. Thermodynamic properties of Sm2Co7 during hydrogenation and dehydrogenation Fig. 2(a) shows the pressure-composition isotherm (PCI) curves for the Sm2Co7–H system at temperatures between 348 K and 453 K. Two plateaus can be clearly discerned in each curve, indicating the existence of two distinguishable phases (b and c) besides the solid-solution a phase. This observation is consistent with the results of Goudy et al. [15], who also observed two plateaus in the PCI curves of similar R2Co7 (R = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) compounds. The first plateau is narrow and sloping, and corresponds to hydrogen contents between 0.33 and 0.43 wt% H2. The second plateau is relatively broad and flat, and corresponds to hydrogen contents between 0.51 and 0.67 wt% H2. Based on the PCI curves, the enthalpy and entropy changes for dehydrogenation of the b and c phases could be calculated according to the Van’t Hoff equation. Pressure plateau data at each temperature were obtained from the mid-point of the plateau region. As shown in Fig. 2(b), the dehydrogenation enthalpies (DH) for the b and c phases were 48.5 and 42.0 kJ/mol H2, respectively, in good agreement with results for other R2Co7 compounds [15]. Ming and Goudy [8] found that two phase transitions in Dy2Co7 –H could be deduced based on the locations of the hydrogen atoms. Hydrogen atoms in the tetrahedral and octahedral sites exhibited different thermodynamic stabilities. Hydrogen atoms in the b phase occupy the more stable octahedral sites, whereas in the c phase they occupy the less stable tetrahedral sites; hence, the desorption temperature for the present b phase was much lower than that for the c phase. 3.3. Hydrogen absorption–desorption mechanisms According to Goudy’s report [15], there are some differences in the compositions of the b and c phases among R2Co7 compounds. For the b and c phases in Sm2Co7, the corresponding hydrides are Sm2Co7H2.9 and Sm2Co7H6.4, respectively, based on calculations of the absorbed hydrogen contents obtained from PCI runs. For hydrogen absorption, the equilibrium pressures of the first and second plateaus at 378 K were close to 0.3 atm and 1.4 atm, respectively. To obtain a single b or c phase, the sample was subjected to hydrogen at a pressure slightly higher than the first or second plateau region. Fig. 3 shows the XRD patterns of samples hydrogenated at different hydrogen pressures at 378 K. It can clearly be seen that with increasing hydrogen pressure, the b phase (Sm2Co7 H2.9), with a structure distinct from that of Sm2Co7, formed at a pressure of 0.4 atm. This b phase then transformed into a hydrogen-rich c phase at a higher hydrogen pressure of 2 atm. As the pressure was increased to 30 atm, the XRD peaks shifted towards lower angles, indicating formation of the c hydride with an increased hydrogen content. Moreover, the curves for the dehydrogenation processes demonstrated that the Sm2Co7 alloy underwent a reverse cycle. From the above structural analysis, the mechanism for hydrogenation and dehydrogenation may be expressed as follows:
Sm2 Co7 þ H2 $ Sm2 Co7 H2:9 ðb phaseÞ Sm2 Co7 H2:9 þ H2 $ Sm2 Co7 H6:4 ðc phaseÞ
Fig. 1. (a) Rietveld refinement of the observed XRD pattern for the as-prepared Sm2Co7 alloy. (b) Crystal structure of this hexagonal Ce2Ni7-type alloy.
Fig. 4 shows the hydrogen desorption capacity of the Sm2Co7 alloy as a function of cycle number at 333 K. It can be seen that this alloy could reach the maximum capacity without any activation process. Moreover, there was little loss in hydrogen-storage
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Table 1 Structure parameters and phase content of as-prepared sample and cycled sample. Sample
Phases
Space group
a
c
Before hydrogenation
Sm2Co7 SmCo5
P63/mmc P6/mmm
0.5048(1) 0.4979(2)
2.4319(5) 0.3986(1)
86 14
Cycled sample
Sm2Co7 SmCo5
P63/mmc P6/mmm
0.5050(1) 0.5004(1)
2.4397(6) 0.3990(1)
91 9
Fig. 2. (a) PCI curves for the Sm2Co7–H system at temperatures between 348 K and 453 K. (b) Van’t Hoff plots of the b and c phases.
Lattice parameters (nm)
Phase content (wt%)
Fig. 4. Hydrogen desorption capacity as a function of cycle number at 333 K.
Fig. 5. Rietveld refinement XRD pattern of Sm2Co7 after 50 absorption/desorption cycles.
Fig. 3. XRD patterns of samples hydrogenated at different hydrogen pressures at 378 K.
capacity after 50 cycles. Fig. 5 shows the Rietveld refinement pattern of Sm2Co7 after the 50 absorption/desorption cycles. The Bragg peaks were much broader than those before the absorption/desorption cycles, indicating that the Sm2Co7 alloy refined obviously after the hydrogen/desorption reactions. The refined parameters of the cycled Sm2Co7 alloy were also presented in Table 1. Apart from the slightly increased lattice parameters (a = 5.050(1) nm and c = 24.397(6) nm), the Sm2Co7 alloy retained its pristine structure after the cycle measurement, indicating that the Sm2Co7 phase had excellent cycle stability. As shown in Table
1, after the cycle measurement the a-axis remained almost unchanged, while the c-axis was expanded by 0.32%. It is noteworthy that, in contrast to RCo5 alloys, for which the pristine CaCu5 structure tends to degrade to an orthorhombic structure [2], the structure of Sm2Co7 showed no sign of decomposition during the hydrogenation and dehydrogenation cycles. Only the content of the SmCo5 phase decreased from 14 wt% in the pristine material to 9 wt% after the cycle measurement. Xie et al. [25] found that ErNi3 showed a bad cycling performance due to the partial decomposition of ErNi3 into ErNi2 as well as ErNi5 and the increase of host-lattice strain. Iwase et al. [11] found that hydrogen was not evenly distributed in the cells of GdNi5 and Gd2Ni4, which led to different modifications in the Gd2Ni7 lattice when subjected to hydrogen: the GdNi5 cell expanded, whereas the Gd2Ni4 cell shrank. The hydrogen absorption properties of Gd–Ni binary compounds with superlattice structures seemed to be related to the number of GdNi5 cells in the unit cell [26]. Similarly, the good cycle stability of Sm2Co7 may also be related to the hydrogen occupation
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in SmCo2 and SmCo5; therefore, the phase transformation and expansion during hydrogenation have to be further studied by in situ XRD measurements. 3.4. Hydrogen absorption kinetics of Sm2Co7 Fig. 6(a) shows hydrogen absorption kinetic curves for the Sm2 Co7 alloy measured between 293 K and 373 K under an initial pressure of 2 MPa. It can clearly be seen that this alloy displayed very rapid hydrogenation kinetics at ambient temperature, with 80% of the maximum storage capacity being absorbed in less than 5 min, and the hydrogenation rate obviously accelerated with increasing temperature. On going from 293 K to 323 K, the time required to absorb 80% of the maximum hydrogen content decreased from 5 min to 3 min. At temperatures above 343 K, this alloy could absorb more than 90% of the maximum hydrogen content in less than 1 min. To understand the hydrogenation process, the reaction mechanism has to be analyzed. Generally, the mechanism of a solid–gas reaction can be determined by comparing the measured hydriding rate curve with rate equations derived for different reaction processes [27–30]. It was found that the experimental data for hydrogen absorption by Sm2Co7 could be best fitted by the Avrami–Erofeev equation, which is deduced from nucleation and growth:
aðtÞ ¼ 1 expðBtm Þ
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hydrogenation is probably the allotropic transformation from the a phase to the hydride; the kinetics of such transformations could thus be studied through hydrogen absorption by the alloy. The Sm2Co7 alloy achieves a maximum hydrogen storage capacity of about 1.2 wt% at room temperature with excellent hydrogen absorption kinetics after activation. And this promising property may enable it to use in the Ni-MH battery in the future. 4. Conclusions The Sm2Co7 compound was confirmed as adopting a Ce2Ni7type crystal structure. Sm2Co7 is composed of double layers of hexagonal structural blocks of CaCu5-type SmCo5 alternating with double layers of MgCu2-type cubic blocks of SmCo2 along the hexagonal axis. Sm2Co7 is transformed to the b phase and then the c phase upon hydrogenation. These two distinguishable phases were calculated as Sm2Co7H2.9 and Sm2Co7H6.4, and their desorption enthalpies were determined as 48.5 and 42.0 kJ/mol H2, respectively. Sm2Co7 showed excellent de/hydrogenation stability without obvious capacity loss after 50 cycles. This alloy also displayed very rapid hydrogenation kinetics at ambient temperature, and the hydrogenation process followed a three-dimensionalinterface-controlled reaction. These results open a new perspective for potential applications in Ni-MH batteries and/or high-pressure tank materials.
ð1Þ
Here, aðtÞ is the reaction rate, B and m are constants, and t is the reaction time. Fig. 6(b) shows the kinetic experimental data and fitting results for the Sm2Co7 alloy measured at 293 K. From the values of S and r, it is evident that the fitting coincides well with the experimental results, indicating that the hydrogen absorption process of Sm2Co7 obeyed the nucleation and growth mechanism. Although Eq. (1) describes the nucleation and growth of the hydriding reaction, the rate-determining step of the reaction can vary according to different values of m. For instance, the one-dimensional diffusion process and the three-dimensional interface reaction process correspond to m values of 0.62 and 1.07, respectively [27]. The value of m in the present case is around 1.36, which is closer to 1.07. This result indicated that the hydrogenation process of Sm2Co7 was likely to be a three-dimensional-interfacecontrolled reaction. This is consistent with the report by Kuijpers van Mal [3], who showed that the rate of release of hydrogen from SmCo5 hydride was determined by the rate of transformation of the hydride to the dehydrogenation compound, rather than by bulk diffusion of hydrogen. The rate-controlling step of Sm2Co7
Fig. 6. (a) Hydrogen absorption kinetic curves for the Sm2Co7 alloy measured between 293 K and 373 K under an initial pressure of 2 MPa. (b) Kinetic experimental data and its fitting results at 293 K.
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