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Control of hydrogen storage properties of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with microstructural parameters
Journal of Alloys and Compounds 570 (2013) 114–118
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Control of hydrogen storage properties of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with microstructural parameters Hanjung Kwon, Jiwoong Kim, Jeong-Hyun Yoo, Sung-Wook Cho ⇑ Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 February 2013 Received in revised form 12 March 2013 Accepted 12 March 2013 Available online 26 March 2013 Keywords: Hydrogen storage materials Rare earth compounds Pressure–composition isotherms Plateau pressure X-ray diffraction
a b s t r a c t Atomic radius factor—a factor related to the composition in (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys, was deduced and used to prepare low-cost alloys. The plateau pressure and hydrogen storage capacity were changed with respect to the crystal lattice volume of the (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5, which could be controlled by changing the ratio of the elements in the alloy. Therefore, the plateau pressure and hydrogen storage capacity were adjusted by using the atomic radius factor, defined as the summation of the multiplication of the atomic radii and molar ratios of each element in the alloy. Finally, low-cost alloys (Ndfree/Ce-rich) were designed and prepared using the atomic radius factor. By simultaneously increasing the amounts of Ce and Mn in the alloy, the atomic radius factor and the crystal lattice volume of the alloy could be made similar to those of a commercial alloy. Consequently, it is possible to control the hydrogen storage properties and to prepare low-cost alloys with hydrogen storage properties similar to those of commercial alloy using the atomic radius factor. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ever since the hydrogen storage properties of LaNi5 alloy were first discovered by Vucht et al. [1], LaNi5 alloy has been studied as a hydrogen storage alloy because of its high volumetric storage density, easy activation, and moderate kinetic properties. However, the LaNi5 alloy has several disadvantages, including low hydrogen capacity, a significant decrease in storage capacity over the cyclic absorption/desorption process, a relatively low plateau pressure, and high cost [2]. Therefore, it has limited application. Many researchers [3–11] have carried out research to improve its properties. Various properties would be considered for use of LaNi5 alloy because the required properties were different by its usage in LaNi5 alloy. If LaNi5 alloy is used for battery material, the properties like corrosion resistance, pulverizing property, hydrogen storage capacity, plateau pressure, and cycle lifetime are critical and the properties depend on element properties in the alloy. For example, it was found that the partial respective replacement of Ni in LaNi5 by small amounts of Al resulted in a prominent increase in the cycle lifetime without causing much decrease in capacity [12]. And previous studies [13–17] showed that Mn partial substitution for Ni can increase the cell volume, decrease the hydrogen plateau pressure, shorten the activation cycle and improve the high rate discharge ability. ⇑ Corresponding author. Tel.: +82 42 868 3632; fax: +82 42 868 3415. E-mail address: [email protected] (S.-W. Cho). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.124
Therefore, substituting La and Ni with other elements in the LaNi5 alloy has been considered an alternative for overcoming its limitations and the effect of the substituted elements on the hydrogen storage properties of the resulting alloy has also been investigated. Van Mal et al. stated that the plateau pressure of the alloy changed when elements such as Y, Er, Gd, or Nd were substituted for La and that both the hydrogen storage capacity and the plateau pressure varied when Pd, Ag, Cu, Co, Fe, or Cr were substituted for Ni [3]. Marshall et al. found that the change in the plateau pressure of the LaNi5 alloy was caused by changes in the crystal lattice volume; they also investigated the relationship between the crystal lattice volume and plateau pressure [4]. They found that the plateau pressure increased as the crystal lattice volume of the alloy decreased. Thus, many researchers have tried to control the hydrogen storage properties of these alloys by using the relationship between the hydrogen storage properties and the crystal lattice volume [5–11]. In these studies, the crystal lattice volume was increased either by adding elements with a large atomic radii or by ball-milling process. As a result, the hydrogen storage capacity was increased while the plateau pressure was decreased. This is because the use of elements with large atomic radii made available more space for hydrogen absorption and the strain induced by ballmilling increased the crystal lattice volume of the alloy. In this work, a factor related to the composition in (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys, was deduced and used to prepare low-cost alloys. First, the effect of the amount of Ce (the most abundant rare-earth element) on the crystal lattice volume of the
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LaNi5 alloy partly substituted by Ce was investigated by a firstprinciples calculation. Then, the factor related to the composition for controlling the crystal lattice volume and hydrogen storage properties, were experimentally deduced. Finally, the low-cost alloys (with large amount of Ce, without Nd) with similar properties to those of a commercial alloy were designed and prepared using the factor. The properties, which were discussed in this paper, were limited to plateau pressure and hydrogen storage capacity among various properties of hydrogen storage material.
X-ray diffraction patterns from the alloys were acquired using Cu Ka radiation from an X-ray diffractometer (SmartLab, Rigaku, Japan) operating in the Bragg– Brentano geometry with counting time of 5 s and steps of 0.01° in 2h. In particular, a Si sample (SRM640D, NIST) was used as an internal standard to remove the instrumental error. The compositions of the ingots were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. The pressure– composition (P–C) isotherms of the samples were measured in a Sievert’s type apparatus up to 5 MPa pressure.
4. Results and discussion 4.1. Effect of the amount of Ce on the crystal lattice volume of (La,Ce)Ni5 alloy
2. Calculation method The Vienna ab initio simulation package (VASP) was used to calculate the crystal lattice volume of the (La,Ce)Ni5 alloys. Exchange– correlation effects were analyzed in the framework of the generalized gradient approximation (GGA) proposed by Perdew and Wang [18]. Integration in the Brillouin zone was performed using the Monkhorst Pack: 7 7 7 k points for the 2 2 2 super-cell (83 atoms) model as shown in Fig. 1. To improve the accuracy of the results, we employed a high-energy cutoff energy of 600 eV with an energy convergence of 0.01 eV/Å, and the first-order Methfessel–Paxton method was used for the Fermi-surface smearing to obtain accurate forces. To obtain accurate equilibrium volume, model relaxations were conducted at least three times for each model. 3. Experimental procedure (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys were prepared into button-shaped ingots, each of 20–30 g, by arc melting under Ar atmosphere. To make alloys with a homogeneous composition, the ingots were turned over and remelted 5 times. The purity and size of the raw materials (Ni,Co,Mn,Al and Misch-metals (La,Ce,Nd,Pr or La,Ce,Pr)) used in this experiment were 99.5% and 3–20 mm, respectively. The ratios of metals in the starting materials were varied for preparing the alloys and had not only different amounts of Ce but also different crystal lattice volumes, as shown in Table 1.
Table 2 shows the lattice parameters and crystal lattice volumes of the (La,Ce)Ni5 alloys obtained from the first-principles calculation. As the content of Ce in the alloys increased, the lattice parameters (a-) decreased. As a result, the crystal lattice volumes of the LaNi5 alloys substituted with Ce are smaller than those of the LaNi5 alloy. This is because the atomic radius of Ce (185 pm) is smaller than that of La (195 pm). Therefore, (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloy with a larger amount of Ce would have a smaller crystal lattice volume. 4.2. Deduction of the factor for designing the composition Fig. 2 shows the XRD results for the alloys with the compositions shown in Table 1. The main peak of the alloy moves to a higher angle as the Ce content increases. This means that the crystal lattice volume of the alloy with a higher amount of Ce is smaller than that of the alloy with a lower amount of Ce, as confirmed in the first-principles calculation. This change in the crystal lattice volume influences the hydrogen storage properties of the resulting alloys. When the hydrogen storage properties of the alloys were estimated, it was found that the plateau pressure of the alloys with smaller crystal lattice volumes was higher than that of alloys with higher crystal lattice volumes (Fig. 3). Also, the hydrogen storage Table 2 Lattice parameters and crystal lattice volumes calculated by first-principles calculation. Lattice parameter (Å)
LaNi5 (La0.875,Ce0.125)Ni5 (La0.75,Ce0.25)Ni5 CeNi5
a
c
5.002 4.990 4.974 4.883
3.976 3.981 3.986 4.003
Crystal lattice volume (Å3)
86.154 85.856 85.406 82.675
Fig. 1. Schematic diagrams of four supercell models used in this calculation: (a) LaNi5 (b) (La0.875,Ce0.125)Ni5 (c) (La0.75,Ce0.25)Ni5 and (d) CeNi5.
Table 1 Ratios of the metals used for preparation (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys (wt.%).
#1 #2 #3 #4 #5 #6 #7 #8
La
Ce
Nd
Pr
Ni
Co
Mn
Al
25.3 19.8 19.7 17.2 13.0 11.0 5.2 4.8
4.5 11.5 11.7 14.2 18.7 20.7 25.8 27.3
1.4 – – – – – – –
0.9 0.9 0.7 0.8 0.5 0.5 0.2 0.2
54.2 54.9 54.2 54.9 54.2 54.9 54.9 54.8
8.1 6.2 8.1 6.2 8.1 6.2 8.2 6.2
3.8 5.1 3.8 5.1 3.8 5.1 3.8 5.1
1.8 1.6 1.8 1.6 1.8 1.6 1.8 1.6 Fig. 2. XRD profiles of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys prepared by arc melting.
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Fig. 4. Plateau pressure vs. atomic radius factor of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys prepared by arc melting. Fig. 3. P–C isotherms of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys prepared by arc melting (estimated at 293 K).
capacity of alloys with smaller lattice volumes decreased because the space for hydrogen in the alloys decreased when the crystal lattice volume decreased [3]. The hydrogen storage properties can thus be varied by changing the crystal lattice volume, which in turn can be changed with the change in atomic radii of the elements in the alloy. Therefore, the hydrogen storage properties can be controlled using a factor that is correlated with the atomic radii of the elements in the alloy, denoted as the atomic radius factor. And it is thought that the hydrogen storage properties of the alloy can be predicted by the factor. In this paper, the atomic radius factor is defined by the atomic radii and the molar fractions of the elements in the alloys as follows:
Table 4 Ratios of the metals used for (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with the designed compositions (wt.%).
Commercial A B
La
Ce
Nd
Pr
Ni
Co
Mn
Al
25.6 25.3 23.4
4.5 5.9 7.7
1.3 – –
0.7 0.9 1.1
54.2 54.2 54.9
8.1 8.1 6.2
3.8 3.8 5.1
1.8 1.8 1.6
worthy that in spite of the larger amounts of Ce in samples #6 and #8, the atomic radius factors of these alloys are larger than those of alloys #5 and #7 because the amount of Mn in alloys #6 and #8, which has the largest atomic radius (140 pm) among Ni, Co, Mn, and Al, is higher than the amount in alloys #5 and #7 [19].
Atomic radius factor ¼ ða atomic radius of LaÞ þ ðb atomic radius of CeÞ þ ðc
4.3. Preparation of low-cost alloys using the atomic radius factor
atomic radius of NdÞ þ ðd atomic radius of PrÞ þ ðx atomic radius of NiÞ þ ðy atomic radius of CoÞ þ ðz atomic radius of MnÞ þ ðw atomic radius of AlÞ in ðLaa ; Ceb ; Ndc ; Prd Þ ðNix ; Coy ; Mnz ; Alw Þ The molar ratios of the elements in the alloys were confirmed by the results from ICP-AES analysis and the atomic radius factors of the alloys were calculated using the results as represented in Table 3. The plateau pressures of the alloys decreased with respect to the atomic radius factor of the alloy (Fig. 4). It is especially note-
Using the relationship between the plateau pressure and the atomic radius factor shown in Fig. 4, alloys having different compositions but hydrogen storage properties similar to those of a commercial alloy (30-lm average particle size) were designed and prepared. Nd was eliminated, and Ce was made more abundant in the designed compositions than in the commercial composition, as represented in Table 4. These designed compositions are lowcost because Nd is the most expensive component while Ce is the most abundant rare-earth element. These compositions (Ndfree/Ce-rich) were designed such that the atomic radius factors of the compositions would be similar to those of the commercial composition by simultaneously adding Ce (the element with smaller atomic radius than La) and Mn (the largest element among Ni, Co, Mn, and Al). The atomic radius factors of the designed alloys (A
Table 3 Molar ratios of metals and atomic radius factor of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys analyzed by ICP-ASE.
#1 #2 #3 #4 #5 #6 #7 #8
La
Ce
Nd
Pr
Ni
Co
Mn
Al
Atomic radius factor
0.775 0.606 0.606 0.524 0.395 0.332 0.190 0.141
0.142 0.371 0.377 0.457 0.597 0.659 0.810 0.859
0.040 – – – – – – –
0.043 0.023 0.017 0.018 0.007 0.008 – –
3.924 3.961 3.911 3.990 3.989 3.978 3.905 3.976
0.542 0.424 0.551 0.435 0.553 0.430 0.548 0.424
0.267 0.374 0.270 0.338 0.179 0.355 0.273 0.361
0.268 0.240 0.267 0.237 0.278 0.237 0.275 0.239
866.404 865.533 864.738 864.564 862.066 862.726 860.518 860.826
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H. Kwon et al. / Journal of Alloys and Compounds 570 (2013) 114–118 Table 5 Molar ratios of metals of the designed (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys analyzed by ICP-ASE.
Commercial A B
La
Ce
Nd
Pr
Ni
Co
Mn
Al
Atomic radius factor
0.791 0.784 0.723
0.139 0.192 0.248
0.042 – –
0.028 0.024 0.029
3.858 3.896 3.987
0.574 0.537 0.417
0.289 0.300 0.363
0.279 0.266 0.233
866.565 866.716 866.998
and B) and the commercial alloy were similar according to the ICPAES results, as shown in Table 5. Therefore, it can be predicted that the hydrogen storage properties of the designed alloys are analogous to those of the commercial alloy. When the XRD results, shown in Fig. 5 and Table 6, were checked, it was confirmed that the crystal lattice volumes of the designed alloys were approximately equal to that of the commercial alloy. The P–C isotherms in Fig. 6 shows that all the alloys, including the commercial alloy, had similar hydrogen storage properties. Consequently, it can be said that the atomic radius factor is very useful for not only controlling the hydrogen storage properties but also designing the low-cost alloys with the hydrogen storage properties similar to those of commercial alloy.
5. Conclusions
Fig. 5. XRD profiles of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with the designed compositions.
Table 6 Lattice parameters and crystal lattice volumes of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys calculated from XRD patterns.a
Commercial A B a
the
designed
Lattice parameter (a)
Lattice parameter (c)
Crystal lattice volume
5.0250 (9) 5.024 (1) 5.019 (1)
4.045 (2) 4.041 (2) 4.0440 (5)
88.437 88.339 88.211
Standard deviations in parentheses.
A factor related to composition in (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys was deduced and low-cost alloys were prepared using the factor. The results are summarized as follows. When the amount of Ce in the alloy increased, the crystal lattice volume decreased because the atomic radius of Ce (185 pm) is smaller than that of La (195 pm). The decrease in the crystal lattice volume of the alloy induced a higher plateau pressure and a smaller hydrogen storage capacity. Therefore, the hydrogen storage properties such as the plateau pressure and the hydrogen storage capacity could be changed using a factor correlated with the atomic radii and molar ratios of the elements in the alloy, the atomic radius factor because the crystal lattice volume was related with the atomic radii and molar ratios of the elements in the alloy. Finally, using the atomic radius factor, the hydrogen storage properties were controlled and the low-cost alloys (Nd-free/Ce-rich) with hydrogen storage properties similar to those of commercial alloy were designed and prepared.
Acknowledgement This work was supported by a grant-in-aid awarded by the Joint Research Project of ISTK (Korea Research Council for Industrial Science and Technology).
References
Fig. 6. P–C isotherms of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with the designed compositions (estimated at 293 K).
[1] J.V. Vucht, F.A. Kuijpers, H. Bruning, Reversible room-temperature absorption of large quantities of hydrogen by intermetallic compounds, Philips. Res. Rep. 25 (1970) 133–140. [2] B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Metal hydride materials for solid hydrogen storage: A review, Int. J. Hydrogen Energy 32 (2007) 1121–1140. [3] H.H. Van Mal, K.H.J. Buschow, A.R. Miedema, Hydrogen absorption in LaNi5 and related compounds: experimental observations and their explanation, J. LessCommon Met. 35 (1974) 65–76. [4] H.M. Marshall, M.G. Dieter, E.D. Austin, LaNi5xAlx is a versatile alloy system for metal hydride applications, Nature 269 (1977) 45–47. [5] W. Hu, D.M. Kim, S.W. Jeon, J.Y. Lee, Effect of annealing treatment on electrochemical properties of Mm-based hydrogen storage alloys for Ni/MH batteries, J. Alloys. comp. 270 (1998) 255–264. [6] Y. Liu, H. Pan, M. Gao, Y. Zhu, Y. Lei, Q. Wang, The effect of Mn substitution for Ni on the structural and electrochemical properties of La0.7Mg0.3Ni2.55xCo0.45Mnx hydrogen storage electrode alloys, Int. J. Hydrogen Energy 29 (2004) 297–305.
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[7] A. Singh, B.K. Singh, D.J. Davidson, O.N. Srivastava, Studies on improvement of hydrogen storage capacity of AB5 type:MmNi4.6Fe0.4 alloy, Int. J. Hydrogen Energy 29 (2004) 1151–1156. [8] Y. Liu, H. Pan, M. Gao, Y. Zhu, Y. L, Q. Wang, Electrochemical studies on La0.7Mg0.3Ni3.4xCo0.6 metal hydride electrode alloys, Mater. Chem. Phys. 84 (2004) 171–181. [9] Y. Liu, H. Pan, M. Gao, R. Li, Y. Lei, Effect of Co content on the structural and electrochemical properties of the La0.7Mg0.3Ni3.4xMn0.1Cox hydride alloys I. The structure and hydrogen storage, J. Alloys. Comp. 376 (2004) 296–303. [10] S.K. Pandey, A. Srivastava, O.N. Srivastava, Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe, Int. J. Hydrogen Energy 32 (2007) 2461–2465. [11] K. Giza, W. Iwasieczko, V.V. Pavlyuk, H. Bala, H. Drulis, L. Adamczyk, Hydrogen absorption and corrosion resistance of LaNi4.8Al0.2 and LaNi4.8Al0.1Li0.1 alloys, J. Alloys. Comp. 429 (2007) 352–356.
[12] Y. Nakamura, H. Nakamura, S. Fujitani, I. Yonezu, J. Alloys. comp. 210 (1994) 299–303. [13] T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, A. Kato, H. Ishikawa, J. LessCommon Met. 161 (1990) 193–202. ´ ojcik, A. Czerwinski, J. Skowronski, M. Kopczyk, M. Beltowska[14] J. Kleperis, G. W Brzezinska, J. Solid State Electrochem. 5 (2001) 229–249. [15] J.K. Chang, D.S. Shong, W.T. Tsai, J. Power Sources 103 (2002) 280–285. [16] C. Lartigue, A. Percheron-Guegan, J.C. Achard, J. Less-Common Met. 75 (1980) 23–29. [17] A. Percheron-Guegan, C. Lartigue, J.C. Achard, J. Less-Common Met. 109 (1985) 287–309. [18] J. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy, Phys. Rev. B 45 (1992) 13244–13249. [19] J.C. Slater, Atomic radii in crystals, J. Chem. Phys. 41 (10) (1964) 3199–3205.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Control of hydrogen storage properties of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with microstructural parameters Hanjung Kwon, Jiwoong Kim, Jeong-Hyun Yoo, Sung-Wook Cho ⇑ Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 February 2013 Received in revised form 12 March 2013 Accepted 12 March 2013 Available online 26 March 2013 Keywords: Hydrogen storage materials Rare earth compounds Pressure–composition isotherms Plateau pressure X-ray diffraction
a b s t r a c t Atomic radius factor—a factor related to the composition in (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys, was deduced and used to prepare low-cost alloys. The plateau pressure and hydrogen storage capacity were changed with respect to the crystal lattice volume of the (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5, which could be controlled by changing the ratio of the elements in the alloy. Therefore, the plateau pressure and hydrogen storage capacity were adjusted by using the atomic radius factor, defined as the summation of the multiplication of the atomic radii and molar ratios of each element in the alloy. Finally, low-cost alloys (Ndfree/Ce-rich) were designed and prepared using the atomic radius factor. By simultaneously increasing the amounts of Ce and Mn in the alloy, the atomic radius factor and the crystal lattice volume of the alloy could be made similar to those of a commercial alloy. Consequently, it is possible to control the hydrogen storage properties and to prepare low-cost alloys with hydrogen storage properties similar to those of commercial alloy using the atomic radius factor. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ever since the hydrogen storage properties of LaNi5 alloy were first discovered by Vucht et al. [1], LaNi5 alloy has been studied as a hydrogen storage alloy because of its high volumetric storage density, easy activation, and moderate kinetic properties. However, the LaNi5 alloy has several disadvantages, including low hydrogen capacity, a significant decrease in storage capacity over the cyclic absorption/desorption process, a relatively low plateau pressure, and high cost [2]. Therefore, it has limited application. Many researchers [3–11] have carried out research to improve its properties. Various properties would be considered for use of LaNi5 alloy because the required properties were different by its usage in LaNi5 alloy. If LaNi5 alloy is used for battery material, the properties like corrosion resistance, pulverizing property, hydrogen storage capacity, plateau pressure, and cycle lifetime are critical and the properties depend on element properties in the alloy. For example, it was found that the partial respective replacement of Ni in LaNi5 by small amounts of Al resulted in a prominent increase in the cycle lifetime without causing much decrease in capacity [12]. And previous studies [13–17] showed that Mn partial substitution for Ni can increase the cell volume, decrease the hydrogen plateau pressure, shorten the activation cycle and improve the high rate discharge ability. ⇑ Corresponding author. Tel.: +82 42 868 3632; fax: +82 42 868 3415. E-mail address: [email protected] (S.-W. Cho). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.124
Therefore, substituting La and Ni with other elements in the LaNi5 alloy has been considered an alternative for overcoming its limitations and the effect of the substituted elements on the hydrogen storage properties of the resulting alloy has also been investigated. Van Mal et al. stated that the plateau pressure of the alloy changed when elements such as Y, Er, Gd, or Nd were substituted for La and that both the hydrogen storage capacity and the plateau pressure varied when Pd, Ag, Cu, Co, Fe, or Cr were substituted for Ni [3]. Marshall et al. found that the change in the plateau pressure of the LaNi5 alloy was caused by changes in the crystal lattice volume; they also investigated the relationship between the crystal lattice volume and plateau pressure [4]. They found that the plateau pressure increased as the crystal lattice volume of the alloy decreased. Thus, many researchers have tried to control the hydrogen storage properties of these alloys by using the relationship between the hydrogen storage properties and the crystal lattice volume [5–11]. In these studies, the crystal lattice volume was increased either by adding elements with a large atomic radii or by ball-milling process. As a result, the hydrogen storage capacity was increased while the plateau pressure was decreased. This is because the use of elements with large atomic radii made available more space for hydrogen absorption and the strain induced by ballmilling increased the crystal lattice volume of the alloy. In this work, a factor related to the composition in (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys, was deduced and used to prepare low-cost alloys. First, the effect of the amount of Ce (the most abundant rare-earth element) on the crystal lattice volume of the
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H. Kwon et al. / Journal of Alloys and Compounds 570 (2013) 114–118
LaNi5 alloy partly substituted by Ce was investigated by a firstprinciples calculation. Then, the factor related to the composition for controlling the crystal lattice volume and hydrogen storage properties, were experimentally deduced. Finally, the low-cost alloys (with large amount of Ce, without Nd) with similar properties to those of a commercial alloy were designed and prepared using the factor. The properties, which were discussed in this paper, were limited to plateau pressure and hydrogen storage capacity among various properties of hydrogen storage material.
X-ray diffraction patterns from the alloys were acquired using Cu Ka radiation from an X-ray diffractometer (SmartLab, Rigaku, Japan) operating in the Bragg– Brentano geometry with counting time of 5 s and steps of 0.01° in 2h. In particular, a Si sample (SRM640D, NIST) was used as an internal standard to remove the instrumental error. The compositions of the ingots were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. The pressure– composition (P–C) isotherms of the samples were measured in a Sievert’s type apparatus up to 5 MPa pressure.
4. Results and discussion 4.1. Effect of the amount of Ce on the crystal lattice volume of (La,Ce)Ni5 alloy
2. Calculation method The Vienna ab initio simulation package (VASP) was used to calculate the crystal lattice volume of the (La,Ce)Ni5 alloys. Exchange– correlation effects were analyzed in the framework of the generalized gradient approximation (GGA) proposed by Perdew and Wang [18]. Integration in the Brillouin zone was performed using the Monkhorst Pack: 7 7 7 k points for the 2 2 2 super-cell (83 atoms) model as shown in Fig. 1. To improve the accuracy of the results, we employed a high-energy cutoff energy of 600 eV with an energy convergence of 0.01 eV/Å, and the first-order Methfessel–Paxton method was used for the Fermi-surface smearing to obtain accurate forces. To obtain accurate equilibrium volume, model relaxations were conducted at least three times for each model. 3. Experimental procedure (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys were prepared into button-shaped ingots, each of 20–30 g, by arc melting under Ar atmosphere. To make alloys with a homogeneous composition, the ingots were turned over and remelted 5 times. The purity and size of the raw materials (Ni,Co,Mn,Al and Misch-metals (La,Ce,Nd,Pr or La,Ce,Pr)) used in this experiment were 99.5% and 3–20 mm, respectively. The ratios of metals in the starting materials were varied for preparing the alloys and had not only different amounts of Ce but also different crystal lattice volumes, as shown in Table 1.
Table 2 shows the lattice parameters and crystal lattice volumes of the (La,Ce)Ni5 alloys obtained from the first-principles calculation. As the content of Ce in the alloys increased, the lattice parameters (a-) decreased. As a result, the crystal lattice volumes of the LaNi5 alloys substituted with Ce are smaller than those of the LaNi5 alloy. This is because the atomic radius of Ce (185 pm) is smaller than that of La (195 pm). Therefore, (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloy with a larger amount of Ce would have a smaller crystal lattice volume. 4.2. Deduction of the factor for designing the composition Fig. 2 shows the XRD results for the alloys with the compositions shown in Table 1. The main peak of the alloy moves to a higher angle as the Ce content increases. This means that the crystal lattice volume of the alloy with a higher amount of Ce is smaller than that of the alloy with a lower amount of Ce, as confirmed in the first-principles calculation. This change in the crystal lattice volume influences the hydrogen storage properties of the resulting alloys. When the hydrogen storage properties of the alloys were estimated, it was found that the plateau pressure of the alloys with smaller crystal lattice volumes was higher than that of alloys with higher crystal lattice volumes (Fig. 3). Also, the hydrogen storage Table 2 Lattice parameters and crystal lattice volumes calculated by first-principles calculation. Lattice parameter (Å)
LaNi5 (La0.875,Ce0.125)Ni5 (La0.75,Ce0.25)Ni5 CeNi5
a
c
5.002 4.990 4.974 4.883
3.976 3.981 3.986 4.003
Crystal lattice volume (Å3)
86.154 85.856 85.406 82.675
Fig. 1. Schematic diagrams of four supercell models used in this calculation: (a) LaNi5 (b) (La0.875,Ce0.125)Ni5 (c) (La0.75,Ce0.25)Ni5 and (d) CeNi5.
Table 1 Ratios of the metals used for preparation (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys (wt.%).
#1 #2 #3 #4 #5 #6 #7 #8
La
Ce
Nd
Pr
Ni
Co
Mn
Al
25.3 19.8 19.7 17.2 13.0 11.0 5.2 4.8
4.5 11.5 11.7 14.2 18.7 20.7 25.8 27.3
1.4 – – – – – – –
0.9 0.9 0.7 0.8 0.5 0.5 0.2 0.2
54.2 54.9 54.2 54.9 54.2 54.9 54.9 54.8
8.1 6.2 8.1 6.2 8.1 6.2 8.2 6.2
3.8 5.1 3.8 5.1 3.8 5.1 3.8 5.1
1.8 1.6 1.8 1.6 1.8 1.6 1.8 1.6 Fig. 2. XRD profiles of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys prepared by arc melting.
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Fig. 4. Plateau pressure vs. atomic radius factor of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys prepared by arc melting. Fig. 3. P–C isotherms of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys prepared by arc melting (estimated at 293 K).
capacity of alloys with smaller lattice volumes decreased because the space for hydrogen in the alloys decreased when the crystal lattice volume decreased [3]. The hydrogen storage properties can thus be varied by changing the crystal lattice volume, which in turn can be changed with the change in atomic radii of the elements in the alloy. Therefore, the hydrogen storage properties can be controlled using a factor that is correlated with the atomic radii of the elements in the alloy, denoted as the atomic radius factor. And it is thought that the hydrogen storage properties of the alloy can be predicted by the factor. In this paper, the atomic radius factor is defined by the atomic radii and the molar fractions of the elements in the alloys as follows:
Table 4 Ratios of the metals used for (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with the designed compositions (wt.%).
Commercial A B
La
Ce
Nd
Pr
Ni
Co
Mn
Al
25.6 25.3 23.4
4.5 5.9 7.7
1.3 – –
0.7 0.9 1.1
54.2 54.2 54.9
8.1 8.1 6.2
3.8 3.8 5.1
1.8 1.8 1.6
worthy that in spite of the larger amounts of Ce in samples #6 and #8, the atomic radius factors of these alloys are larger than those of alloys #5 and #7 because the amount of Mn in alloys #6 and #8, which has the largest atomic radius (140 pm) among Ni, Co, Mn, and Al, is higher than the amount in alloys #5 and #7 [19].
Atomic radius factor ¼ ða atomic radius of LaÞ þ ðb atomic radius of CeÞ þ ðc
4.3. Preparation of low-cost alloys using the atomic radius factor
atomic radius of NdÞ þ ðd atomic radius of PrÞ þ ðx atomic radius of NiÞ þ ðy atomic radius of CoÞ þ ðz atomic radius of MnÞ þ ðw atomic radius of AlÞ in ðLaa ; Ceb ; Ndc ; Prd Þ ðNix ; Coy ; Mnz ; Alw Þ The molar ratios of the elements in the alloys were confirmed by the results from ICP-AES analysis and the atomic radius factors of the alloys were calculated using the results as represented in Table 3. The plateau pressures of the alloys decreased with respect to the atomic radius factor of the alloy (Fig. 4). It is especially note-
Using the relationship between the plateau pressure and the atomic radius factor shown in Fig. 4, alloys having different compositions but hydrogen storage properties similar to those of a commercial alloy (30-lm average particle size) were designed and prepared. Nd was eliminated, and Ce was made more abundant in the designed compositions than in the commercial composition, as represented in Table 4. These designed compositions are lowcost because Nd is the most expensive component while Ce is the most abundant rare-earth element. These compositions (Ndfree/Ce-rich) were designed such that the atomic radius factors of the compositions would be similar to those of the commercial composition by simultaneously adding Ce (the element with smaller atomic radius than La) and Mn (the largest element among Ni, Co, Mn, and Al). The atomic radius factors of the designed alloys (A
Table 3 Molar ratios of metals and atomic radius factor of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys analyzed by ICP-ASE.
#1 #2 #3 #4 #5 #6 #7 #8
La
Ce
Nd
Pr
Ni
Co
Mn
Al
Atomic radius factor
0.775 0.606 0.606 0.524 0.395 0.332 0.190 0.141
0.142 0.371 0.377 0.457 0.597 0.659 0.810 0.859
0.040 – – – – – – –
0.043 0.023 0.017 0.018 0.007 0.008 – –
3.924 3.961 3.911 3.990 3.989 3.978 3.905 3.976
0.542 0.424 0.551 0.435 0.553 0.430 0.548 0.424
0.267 0.374 0.270 0.338 0.179 0.355 0.273 0.361
0.268 0.240 0.267 0.237 0.278 0.237 0.275 0.239
866.404 865.533 864.738 864.564 862.066 862.726 860.518 860.826
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H. Kwon et al. / Journal of Alloys and Compounds 570 (2013) 114–118 Table 5 Molar ratios of metals of the designed (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys analyzed by ICP-ASE.
Commercial A B
La
Ce
Nd
Pr
Ni
Co
Mn
Al
Atomic radius factor
0.791 0.784 0.723
0.139 0.192 0.248
0.042 – –
0.028 0.024 0.029
3.858 3.896 3.987
0.574 0.537 0.417
0.289 0.300 0.363
0.279 0.266 0.233
866.565 866.716 866.998
and B) and the commercial alloy were similar according to the ICPAES results, as shown in Table 5. Therefore, it can be predicted that the hydrogen storage properties of the designed alloys are analogous to those of the commercial alloy. When the XRD results, shown in Fig. 5 and Table 6, were checked, it was confirmed that the crystal lattice volumes of the designed alloys were approximately equal to that of the commercial alloy. The P–C isotherms in Fig. 6 shows that all the alloys, including the commercial alloy, had similar hydrogen storage properties. Consequently, it can be said that the atomic radius factor is very useful for not only controlling the hydrogen storage properties but also designing the low-cost alloys with the hydrogen storage properties similar to those of commercial alloy.
5. Conclusions
Fig. 5. XRD profiles of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with the designed compositions.
Table 6 Lattice parameters and crystal lattice volumes of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys calculated from XRD patterns.a
Commercial A B a
the
designed
Lattice parameter (a)
Lattice parameter (c)
Crystal lattice volume
5.0250 (9) 5.024 (1) 5.019 (1)
4.045 (2) 4.041 (2) 4.0440 (5)
88.437 88.339 88.211
Standard deviations in parentheses.
A factor related to composition in (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys was deduced and low-cost alloys were prepared using the factor. The results are summarized as follows. When the amount of Ce in the alloy increased, the crystal lattice volume decreased because the atomic radius of Ce (185 pm) is smaller than that of La (195 pm). The decrease in the crystal lattice volume of the alloy induced a higher plateau pressure and a smaller hydrogen storage capacity. Therefore, the hydrogen storage properties such as the plateau pressure and the hydrogen storage capacity could be changed using a factor correlated with the atomic radii and molar ratios of the elements in the alloy, the atomic radius factor because the crystal lattice volume was related with the atomic radii and molar ratios of the elements in the alloy. Finally, using the atomic radius factor, the hydrogen storage properties were controlled and the low-cost alloys (Nd-free/Ce-rich) with hydrogen storage properties similar to those of commercial alloy were designed and prepared.
Acknowledgement This work was supported by a grant-in-aid awarded by the Joint Research Project of ISTK (Korea Research Council for Industrial Science and Technology).
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Fig. 6. P–C isotherms of (La,Ce,Nd,Pr)(Ni,Co,Mn,Al)5 alloys with the designed compositions (estimated at 293 K).
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