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Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe
International Journal of Hydrogen Energy 32 (2007) 2461 – 2465 www.elsevier.com/locate/ijhydene
Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe Sunil Kumar Pandey, Anchal Srivastava, O.N. Srivastava ∗ Department of Physics, Banaras Hindu University, Varanasi 221005, India Received 2 August 2006; accepted 9 December 2006 Available online 25 January 2007
Abstract This paper deals with improvement in the hydrogenation behavior of LaNi5 type alloys brought in through material tailoring by substitution of elements having higher electron attractive power and bigger sizes than that of Ni. Although substitutions by both Co and Fe were investigated, optimum results were obtained only through Fe. The higher electron attractive power of Fe, Co (3d6 , 3d7 as against 3d8 of Ni) is expected to lead to possibility of higher number of hydrogen atoms in the unit cell, thus resulting in higher storage capacity. Fe substitution has been ˚ is higher than that of Ni (1.67 A) ˚ by found to be more affective in the enhancement of storage capacity. The size of the Fe atom (1.72 A) about 6.13%. This may lead to larger size of interstitial voids. Thus, there may be higher number of interstitial voids occupied by hydrogen for the Fe substituted version as compared to Ni alone. Keeping these aspects in view, we have investigated Fe substituted version of LaNi5 , i.e. the material La(Ni1−x Fex )5 . It has been found that all the phases of La(Ni1−x Fex )5 exhibit better hydrogenation characteristics than the parent material LaNi5 . Thus, the storage capacity of La(Ni1−x Fex )5 for x = 0, 0.05, 0.10, 0.20, 0.25, 0.30 are ∼1.5, 1.93, 1.96, 2.20, 1.78 and 1.60 wt%, respectively. The material La(Ni0.80 Fe0.20 )5 has been found to exhibit the highest storage capacity ∼2.20 wt%. The possible reasons for the enhancement of storage capacity of Fe substituted LaNi5 have been described and discussed. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; XRD; SEM; PCT curve; LaNi5
1. Introduction Intermetallic compounds (IMCs) are known to store large quantity of hydrogen (H/m>1) [1,2]. Improvements of the storage capacity of these are required for practical application of these compounds as energy storage materials. IMCs suffer from relatively poor weight capacity in comparison to the storage requisites by the expected standard of DOE(US) ∼6 wt% and WENET (Japan) is 3 wt% [3]. Therefore, several researches are being carried out in order to improve the storage capacity of these hydrogen storage materials. The improvement of crucial hydrogenation parameter, i.e. the storage capacity can be achieved in two ways. In one of them altogether new materials are to be discovered. The graphitic nanotubes/nanofibres, sodium alnates, lithium amides and several other new upcoming materials e.g. clathrates come under this category [4]. The ∗ Corresponding author. Tel.: +91 542 2368468; fax: +91 542 2307307.
E-mail address: [email protected] (O.N. Srivastava).
second approach corresponds to efforts of improvement of storage capacity of the known materials through material tailoring. Out of several types of hydrogen storage materials, one wellknown category is the AB5 type. The prototype of this class, i.e. LaNi5 is an important hydrogen storage material because of its good storage capacity (∼1.5 wt%) and manageable plateau pressure of ∼1 atm, ease of activation and tolerance towards impurities with the reacting hydrogen gas [5,6]. One prominent way through which material tailoring has been carried out to enhance the storage capacity for the three well-known categories of storage material (AB5 e.g. LaNi5 ; AB2 e.g. ZrFe2 ; A2 B e.g. Mg2 Ni; and AB, FeTi) is the substitution of suitable elements on both A and B sites [7,8]. No well-defined rules exist in regard to the selection of atoms to be substituted. However, there are some suggestive criteria e.g. higher electron attractive power, electronegativity, tailoring of interstitial positions to relax the Switendick criterion (minimum H–H distance), relative size of the atom, which is being substituted. Mal et al. have investigated, substitutions of La by Nd,
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.12.003
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Gd,Y, Er, Th and Zr [9–11]. In each case, a reduction in stability, i.e. a higher plateau pressure was observed. Van Vucht et al. have examined the influence of Ce on LaNi5 . For these specific substitutions some increase in storage capacity (from ∼1.50 to ∼1.55 wt%) was also noticed. Several substitutions at Ni sites have also been studied. Several investigations at improving the hydrogenation behavior particularly the capacity of LaNi5 have involved substitution of Ni by Al, Mn, Cr, Fe and Co for Ni [12–16]. In these studies the stability of hydride has been found to change without causing much change in capacity [17]. However, no systematic study of increase of storage capacity as a result of substitution through viable elements seems to have been made so far. If the storage capacity of intermetallic hydrides can be raised from its present estimate of ∼1.5 wt% to 3 wt%, it will make the hydrides, which are safest and volumetrically already feasible mode of hydrogen containment, a viable hydrogen storage option. It may be mentioned that one of the criterion for increasing the hydrogen storage capacity through replacement of Ni in the prototype material LaNi5 corresponds to find a more electron attractive element substituting for Ni [18]. Another criterion will be to find an atom having larger size than Ni so as to obtain larger interstitial void size. For satisfying the first criterion both Fe(3d6 ) and Co(3d7 ) will be suitable candidates since they have higher electron attractive power than Ni(3d8 ). Another criterion for obtaining higher storage capacity may be substitution of Ni with atoms, capable of changing the size of the interstitial voids [19]. This may induce occupation of higher number of interstitial voids and hence higher hydrogen storage capacity. ˚ respecHere again Co and Fe having sizes of 1.67 and 1.72 A, tively, are viable candidates. Keeping the aforesaid criterion in view, we synthesized the intermetallics La(Ni1−x Cox /Fex )5 . Investigations spread over one dozen materials revealed that Fe is always a better substitute than Co for obtaining higher storage capacity. We will therefore, describe here only the substitution of Fe for Ni. Detailed investigations of La(Ni1−x Fex )5 for x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30 revealed that the material La(Ni0.80 Fe0.20 )5 exhibits the highest storage capacity ∼2.20 wt%. It is about 47% higher than the capacity of the parent alloy LaNi5 (∼1.5 wt%). A noticeable characteristic of this new storage material is that its plateau pressure of about 1 atm is nearly the same as that of the parent material LaNi5 . Yet another advantageous feature of the present new material is that, it exhibits high hydrogen kinetics of ∼19 cc/g/ min which is about 46% higher than the kinetics for the parent material (13 cc/g/ min).
melting process, water was circulated in the outer jacket around the silica tube to suppress the contamination of the alloy by the tube materials (silica). The alloy ingots so prepared were melted repeatedly (3–4 times) to achieve homogeneity. The alloy samples were removed from the silica tube, powdered and subjected to X-ray diffraction (XRD) characterization for phase identification. A Philips X-ray powder diffractometer PW-1710 equipped with a graphite monochromator and employing CuK ˚ was used for the structural characteriradiation ( = 1.5418 A) zation of the as-synthesized and hydrogenated alloys. The surface/microstructural features have been examined by scanning electron microscopy employing a Philips XL-20 Series SEM (30 kV) employing secondary electron imaging. The hydrogenation behavior was investigated by monitoring the P-C isotherms and desorption kinetics. For hydrogenation first vacuum of 10−5 torr was created in the steel reactor containing alloys La(Ni1−x Fex )5 , x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30. This was then followed by induction of ∼60 atm. H2 gas for hydrogenation of the as-synthesized intermetallics La(Nix Fe1−x )5 for x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30. The amount of hydrogen desorbed was monitored by the volume displacement method using a Sievert’s type apparatus, developed in our laboratory [20]. A similar technique was utilized to monitor the amount of hydrogen desorbed as a function of time to obtain kinetic curves. 3. Results and discussions 3.1. Hydrogenation behavior The hydrogenation characteristics of the alloys were evaluated through the measurement of P-C isotherms and kinetics. Desorption isotherms corresponding to the alloy LaNi5 and La(Ni1−x Fex )5 are shown in Fig. 1. The maximum storage capacity was found to be 1.93, 1.96, 2.20, 1.78, 1.60 wt%,
2. Experimental details The hydrogen storage alloy La(Ni1−x Fex )5 has been synthesized by melting stoichiometric mixtures of individual elements. High purity elements including La (99.9%), Ni, Fe, and Co (99.99%) were taken in the correct stoichiometric proportions and pressed into pellet form (1 × 0.5 cm2 ). The pellets were placed into silica tube. Under continuous flow of an argon gas into the silica tube, metallic mixture pellets were melted using a radio-frequency induction furnace (18 kW). During the
Fig. 1. Representative pressure-composition desorption isotherms of the La(Ni1−x Fex )5 alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30).
S.K. Pandey et al. / International Journal of Hydrogen Energy 32 (2007) 2461 – 2465
Fig. 2. Dehydriding kinetics of the La(Ni1−x Fex )5 alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30).
Table 1 Storage capacity and desorption kinetics of the La(Ni1−x Fex )5 hydrogen storage alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30) S.N.
1. 2. 3. 4. 5. 6.
Alloys
LaNi5 La(Ni0.95 Fe0.05 )5 La(Ni0.90 Fe0.10 )5 La(Ni0.80 Fe0.20 )5 La(Ni0.75 Fe0.25 )5 La(Ni0.70 Fe0.30 )5
Storage capacity (wt%)
Desorption kinetics (cc/g/min)
1.5 1.93 1.96 2.20 1.78 1.60
13 15 16 19 13 10
respectively. It may be pointed out that the storage capacity of 2.20 wt% is the highest reported value for the LaNi5 and its substituted system. The rate of flow of hydrogen during desorption (desorption kinetics) of the material are shown in Fig. 2. The storage capacity and kinetics for all the materials studied here are given in Table 1. 3.2. Structural/microstructural characterization: using XRD, SEM techniques In order to unravel the curious hydrogenation behavior La(Ni1−x Fex )5 , x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30, structural (XRD) were carried out. Analysis of XRD patterns confirmed the formation of CaCu5 type hexagonal structures, for all the composition studied. The XRD patterns of optimum alloy as-synthesized and after hydrogenation through several cycles are shown in Fig. 3(a) and (b). The a and c lattice parameters for all the variants La(Ni1−x Fex )5 , x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30 were calculated from the observed d values through a least square fitting computer programme provided by S. Sivashankaran (Crystal Growth Centre, Anna University). The unit cell volumes were calculated from
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Fig. 3. X-ray diffractograms of the (a) as-synthesized and (b) dehydrogenated La(Ni0.80 Fe0.20 )5 alloy.
Table 2 Lattice parameters and unit cell volume of the La(Ni1−x Fex )5 hydrogen storage alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30) S.N.
1. 2. 3. 4. 5. 6.
Alloys
LaNi5 La(Ni0.95 Fe0.05 )5 La(Ni0.90 Fe0.10 )5 La(Ni0.80 Fe0.20 )5 La(Ni0.75 Fe0.25 )5 La(Ni0.70 Fe0.30 )5
Fe contents
Lattice parameter CaCu5 type (AB5 )
Unit cell volume
x
˚ a (A)
˚ c (A)
˚ 3) (A
0 0.05 0.10 0.20 0.25 0.30
4.9762 5.0201 5.0481 5.0484 4.9839 4.9798
4.0019 3.9792 4.0134 4.0259 4.0091 4.0026
85.671 86.8453 88.5722 88.8568 86.821 85.961
the observed lattice parameters. The lattice parameters and the unit cell volumes for various variants are shown in Table 2. After hydrogenation through several cycles, the particle size gets reduced. Fig. 3(b) shows the XRD peak broadening for a sample, which has undergone five hydrogenation/dehydrogenation cycles The particle size as obtained from FWHM after taking into account instrumental broadening effect was determined as ∼36 nm compared to the initial particle size of ∼60 nm. This implies a decrease in particle size from 60 to 36 nm. The microstructural characterization of the as-synthesized and hydrogenated/dehydrogenated La(Ni1−x Fex )5 was carried through scanning electron microscope (XL-20). Fig. 4(a) and (b) show the SEM micrographs of the as-synthesized and hydrogenated samples, respectively. It may be pointed out that some cracks developed after hydrogenation through several cycles [21] which can easily be seen in Fig. 4(b). As can be seen from Table 2, the unit cell volume for AB5 type phase La(Ni1−x Fex )5 increases with decreasing value of x. Substitution of Fe at the Ni site beyond x=0.20 (e.g. x=0.10) leads to the decrement of the unit cell volume which is shown
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Fig. 4. Scanning electron micrographs of La(Ni0.80 Fe0.20 )5 alloy (a) as-synthesized and (b) after dehydrogenation.
Fig. 5. Variation of unit cell volume with variation of x in La(Ni1−x Fex )5 alloy.
in Table 2. Fig. 5 exhibits the variation of unit cell volume with Fe concentration (x). Table 2 and Fig. 5 clearly show that the ˚ is lattice parameters and hence unit cell volume (88.8568 A) maximum for the material La(Ni0.80 Fe0.20 )5 . The increase in unit cell volume on Fe substitution is an interesting result. This may have bearing on storage capacity of the La(Ni1−x Fex )5 . The increase in unit cell volume will result in increase of interstitial size. Thus, some interstitials, which would not be getting occupied by hydrogen atoms in the native phase LaNi5 , may get occupied in the substituted versions. As can be seen the maximum increase in the unit cell volume is for the variant La(Nai0.80 Fe0.20 )5 . Therefore, this variant is expected to have the maximum number of additional interstitials occupied by hydrogen. It is known that the LaNi5 intermetallic has total number of 37 interstitial positions. On hydrogenation generally only 6 of these are occupied by H atoms [22,23]. The structure of the unit cell is such that next to the 6 interstitial voids, which normally get occupied, 3 other interstitials are the next sites, which can be occupied [22,23]. If these 3 interstitials also get occupied making the total number of interstitials occupies as 9. It is known that the dominant reason for non-occupation of more than 6 interstitial sites is the increased coulomb repulsion of the H–H atoms (lowest permissible distance between H–H atom; Switendick Criterion). In the present investigation as already found out the unit cell volume and hence interstitial hole size increases. As is known the storage capacity is sensitive to the electron attractive power of the substituting atom. The higher electron attractive power of Fe may increase the storage capacity. In fact Fe has four unpaired ‘d’ electrons as against two for Ni. Thus, the combined effect of (a) substitution of Fe which is more electron attractive than Ni and will bring extra hydrogen atoms and (b) accommodation of the extra hydrogen atoms in interstitials with higher hole size, will lead to higher storage capacity. This is in keeping with experimental result where La(Ni0.80 Fe0.20 )5 exhibits high storage capacity 2.20 wt%. This will lead to relaxation of the minimum H–H distance criterion. In view of this, higher number of voids will get occupied. As outlined earlier next to 6, 3 more interstitials can get occupied. If these 3 sites get populated by H atoms making the total H atoms as 9 in the unit cell, the storage capacity will get enhanced to ∼2.25 wt%, an increase of about ∼50% over the normal storage capacity of ∼1.5 wt%. In the present case the enhancement in storage capacity is about 47% increase over the normal storage capacity. This is very close to be expected enhancement in storage capacity. It is one of the highest storage capacity reported for the AB5 type storage material. It may be pointed that in one of our earlier studies of AB5 type material we have found enhanced capacity up to ∼2.04 wt%. However, these results were on AB5 type material, which was Mm based. Also the enhancement results due to ball milling producing strained particles. It can thus be said that in the present La(Ni0.80 Fe0.20 )5 intermetallic phase, 9 instead of the use 6 interstitial sites get populated by hydrogen atoms. The results of the present investigation clearly suggest that if more than 9 interstitial sites can get populated the storage capacity can get enhanced to values higher than ∼2.25 wt%. This is feasible since there are 37 interstitial sites
S.K. Pandey et al. / International Journal of Hydrogen Energy 32 (2007) 2461 – 2465
available for LaNi5 structure. Material tailoring to achieve this will be worth attempting. It may be pointed out that relaxation of minimum H–H distance criterion are being envisaged [24]. We are carrying out further investigation in this direction and results will be forthcoming. 4. Conclusion We have synthesized AB5 -type materials with formula La(Ni1−x Fex )5 (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30). The assynthesized alloys are hexagonal (CaCu5 type, AB5 ) for all the compositions studied. A maximum storage capacity of ∼2.20 wt% at room temperature and faster kinetics 19 cc/g/min have been observed for the La(Ni0.80 Fe0.20 )5 capacity has been put forward. Acknowledgments The authors will like to acknowledge Professor A.R. Verma (Delhi, India), Professor T.N. Veziroglu (President, IAHE Florida, USA) and Professor M. Groll (Stuttgart, Germany) for helpful discussions. Financial assistance from Ministry of Non-conventional Energy Sources, New Delhi (India) is gratefully acknowledged. References [1] Van Vucht JHN, Kuijpers FA, Bruning HCAM. Philips Res Rep 1970;25:133. [2] Hotten PHL, Einerhand REF, Daams JLC. J Alloy Compd 1995;231:604. [3] Msika E, Latroche M, Cuevas F, Percheron-Guegan A. J Mat Sci Eng 2004;108:91. [4] Takayuki I, Shigehito I, Nobuko H, Hironobu F. J Alloy Compd 2004;365:271.
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[5] Barbir F, Veziroglu TN, Plass Jr. HJ. Int J Hydrogen Energy 1990;15(10):739. [6] Huston FL, Sandrock GD. J Less-Common Met 1980;74:435. [7] Willems JJG. Philips J Res 1984;39:11. [8] Lei Y, Jiang J, Sun D, Wand Q, Wu J. J Alloy Compd 1995;31:533. [9] Bowman RC, Fultz Jr. B. MRS Bull 2002;27(9):688. [10] Joubert JM, Latroche M, Percheron-Guegan A. MRS Bull 2002;27(9):694. [11] Van Mal HH, Buschow KHJ, Miedima AR. J Less-Common Met 1976;49:463. [12] Adzic GD, Johnson JR, Mukerjee S, McBoeen J, Reilly JJ. J Alloy Compd 1997;253/254:579. [13] Ogawa H, Ikowa M, Kawano H, Matrsumoto I. J Power Sources 1988;12:3939. [14] Hu WK, Kim DM, Jean SW, Lee JY. J Alloy Compd 1998;270:255. [15] Iwakura C, Fukudo K, Senoh H, Inoue H, Matsuoka M, Yamamoto Y. Electrochem Acta 1998;43:2041. [16] Geng M, Han J, Feng F, Northhood DO. Int J Hydrogen Energy 2000;25:203. [17] Sakai T, Miyamura H, Orguro K, Kato A, Kuriyama N, Ishikawa H. Denkikagaku 1989;57:12; Sakai T, Miyamura H, Orguro K, Kato A, Kuriyama N, Ishikawa H. Electrochem Soc 1990;137:795. [18] Singh SK, Singh AK, Ramakrishna K, Srivastava ON. Int J Hydrogen Energy 1985;10:523. [19] Singh A, Singh BK, Davidson DJ, Srivastava ON. Int J Hydrogen Energy 2004;29(11):1151. [20] Davidson DJ. Thesis, Banaras Hindu University, Varanasi, 221005, India; 2002. p. 108. [21] Singh RK, Gupta BK, Lototsky MV, Srivastava ON. J Alloy Compd 2004;373:208. [22] Ramakrisna K, Singh SK, Singh AK, Srivastava ON. In: Dahiya RP, editor. Progress in hydrogen energy. Reidel Corrects; 1987. p. 81. [23] Furrer A, Fischer P, Halg W, Schlapbach L. In: Anderson AP, Macland AJ, editors. Hydrides for energy storage. Oxford: Pergamon Press; 1977. p. 73. [24] Ravindran P, Vajeeston P, Vidya R, Kjekshus A, Fjellvag H. Phys Rev Lett 2002;89(10):106403.
Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe Sunil Kumar Pandey, Anchal Srivastava, O.N. Srivastava ∗ Department of Physics, Banaras Hindu University, Varanasi 221005, India Received 2 August 2006; accepted 9 December 2006 Available online 25 January 2007
Abstract This paper deals with improvement in the hydrogenation behavior of LaNi5 type alloys brought in through material tailoring by substitution of elements having higher electron attractive power and bigger sizes than that of Ni. Although substitutions by both Co and Fe were investigated, optimum results were obtained only through Fe. The higher electron attractive power of Fe, Co (3d6 , 3d7 as against 3d8 of Ni) is expected to lead to possibility of higher number of hydrogen atoms in the unit cell, thus resulting in higher storage capacity. Fe substitution has been ˚ is higher than that of Ni (1.67 A) ˚ by found to be more affective in the enhancement of storage capacity. The size of the Fe atom (1.72 A) about 6.13%. This may lead to larger size of interstitial voids. Thus, there may be higher number of interstitial voids occupied by hydrogen for the Fe substituted version as compared to Ni alone. Keeping these aspects in view, we have investigated Fe substituted version of LaNi5 , i.e. the material La(Ni1−x Fex )5 . It has been found that all the phases of La(Ni1−x Fex )5 exhibit better hydrogenation characteristics than the parent material LaNi5 . Thus, the storage capacity of La(Ni1−x Fex )5 for x = 0, 0.05, 0.10, 0.20, 0.25, 0.30 are ∼1.5, 1.93, 1.96, 2.20, 1.78 and 1.60 wt%, respectively. The material La(Ni0.80 Fe0.20 )5 has been found to exhibit the highest storage capacity ∼2.20 wt%. The possible reasons for the enhancement of storage capacity of Fe substituted LaNi5 have been described and discussed. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; XRD; SEM; PCT curve; LaNi5
1. Introduction Intermetallic compounds (IMCs) are known to store large quantity of hydrogen (H/m>1) [1,2]. Improvements of the storage capacity of these are required for practical application of these compounds as energy storage materials. IMCs suffer from relatively poor weight capacity in comparison to the storage requisites by the expected standard of DOE(US) ∼6 wt% and WENET (Japan) is 3 wt% [3]. Therefore, several researches are being carried out in order to improve the storage capacity of these hydrogen storage materials. The improvement of crucial hydrogenation parameter, i.e. the storage capacity can be achieved in two ways. In one of them altogether new materials are to be discovered. The graphitic nanotubes/nanofibres, sodium alnates, lithium amides and several other new upcoming materials e.g. clathrates come under this category [4]. The ∗ Corresponding author. Tel.: +91 542 2368468; fax: +91 542 2307307.
E-mail address: [email protected] (O.N. Srivastava).
second approach corresponds to efforts of improvement of storage capacity of the known materials through material tailoring. Out of several types of hydrogen storage materials, one wellknown category is the AB5 type. The prototype of this class, i.e. LaNi5 is an important hydrogen storage material because of its good storage capacity (∼1.5 wt%) and manageable plateau pressure of ∼1 atm, ease of activation and tolerance towards impurities with the reacting hydrogen gas [5,6]. One prominent way through which material tailoring has been carried out to enhance the storage capacity for the three well-known categories of storage material (AB5 e.g. LaNi5 ; AB2 e.g. ZrFe2 ; A2 B e.g. Mg2 Ni; and AB, FeTi) is the substitution of suitable elements on both A and B sites [7,8]. No well-defined rules exist in regard to the selection of atoms to be substituted. However, there are some suggestive criteria e.g. higher electron attractive power, electronegativity, tailoring of interstitial positions to relax the Switendick criterion (minimum H–H distance), relative size of the atom, which is being substituted. Mal et al. have investigated, substitutions of La by Nd,
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.12.003
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S.K. Pandey et al. / International Journal of Hydrogen Energy 32 (2007) 2461 – 2465
Gd,Y, Er, Th and Zr [9–11]. In each case, a reduction in stability, i.e. a higher plateau pressure was observed. Van Vucht et al. have examined the influence of Ce on LaNi5 . For these specific substitutions some increase in storage capacity (from ∼1.50 to ∼1.55 wt%) was also noticed. Several substitutions at Ni sites have also been studied. Several investigations at improving the hydrogenation behavior particularly the capacity of LaNi5 have involved substitution of Ni by Al, Mn, Cr, Fe and Co for Ni [12–16]. In these studies the stability of hydride has been found to change without causing much change in capacity [17]. However, no systematic study of increase of storage capacity as a result of substitution through viable elements seems to have been made so far. If the storage capacity of intermetallic hydrides can be raised from its present estimate of ∼1.5 wt% to 3 wt%, it will make the hydrides, which are safest and volumetrically already feasible mode of hydrogen containment, a viable hydrogen storage option. It may be mentioned that one of the criterion for increasing the hydrogen storage capacity through replacement of Ni in the prototype material LaNi5 corresponds to find a more electron attractive element substituting for Ni [18]. Another criterion will be to find an atom having larger size than Ni so as to obtain larger interstitial void size. For satisfying the first criterion both Fe(3d6 ) and Co(3d7 ) will be suitable candidates since they have higher electron attractive power than Ni(3d8 ). Another criterion for obtaining higher storage capacity may be substitution of Ni with atoms, capable of changing the size of the interstitial voids [19]. This may induce occupation of higher number of interstitial voids and hence higher hydrogen storage capacity. ˚ respecHere again Co and Fe having sizes of 1.67 and 1.72 A, tively, are viable candidates. Keeping the aforesaid criterion in view, we synthesized the intermetallics La(Ni1−x Cox /Fex )5 . Investigations spread over one dozen materials revealed that Fe is always a better substitute than Co for obtaining higher storage capacity. We will therefore, describe here only the substitution of Fe for Ni. Detailed investigations of La(Ni1−x Fex )5 for x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30 revealed that the material La(Ni0.80 Fe0.20 )5 exhibits the highest storage capacity ∼2.20 wt%. It is about 47% higher than the capacity of the parent alloy LaNi5 (∼1.5 wt%). A noticeable characteristic of this new storage material is that its plateau pressure of about 1 atm is nearly the same as that of the parent material LaNi5 . Yet another advantageous feature of the present new material is that, it exhibits high hydrogen kinetics of ∼19 cc/g/ min which is about 46% higher than the kinetics for the parent material (13 cc/g/ min).
melting process, water was circulated in the outer jacket around the silica tube to suppress the contamination of the alloy by the tube materials (silica). The alloy ingots so prepared were melted repeatedly (3–4 times) to achieve homogeneity. The alloy samples were removed from the silica tube, powdered and subjected to X-ray diffraction (XRD) characterization for phase identification. A Philips X-ray powder diffractometer PW-1710 equipped with a graphite monochromator and employing CuK ˚ was used for the structural characteriradiation ( = 1.5418 A) zation of the as-synthesized and hydrogenated alloys. The surface/microstructural features have been examined by scanning electron microscopy employing a Philips XL-20 Series SEM (30 kV) employing secondary electron imaging. The hydrogenation behavior was investigated by monitoring the P-C isotherms and desorption kinetics. For hydrogenation first vacuum of 10−5 torr was created in the steel reactor containing alloys La(Ni1−x Fex )5 , x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30. This was then followed by induction of ∼60 atm. H2 gas for hydrogenation of the as-synthesized intermetallics La(Nix Fe1−x )5 for x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30. The amount of hydrogen desorbed was monitored by the volume displacement method using a Sievert’s type apparatus, developed in our laboratory [20]. A similar technique was utilized to monitor the amount of hydrogen desorbed as a function of time to obtain kinetic curves. 3. Results and discussions 3.1. Hydrogenation behavior The hydrogenation characteristics of the alloys were evaluated through the measurement of P-C isotherms and kinetics. Desorption isotherms corresponding to the alloy LaNi5 and La(Ni1−x Fex )5 are shown in Fig. 1. The maximum storage capacity was found to be 1.93, 1.96, 2.20, 1.78, 1.60 wt%,
2. Experimental details The hydrogen storage alloy La(Ni1−x Fex )5 has been synthesized by melting stoichiometric mixtures of individual elements. High purity elements including La (99.9%), Ni, Fe, and Co (99.99%) were taken in the correct stoichiometric proportions and pressed into pellet form (1 × 0.5 cm2 ). The pellets were placed into silica tube. Under continuous flow of an argon gas into the silica tube, metallic mixture pellets were melted using a radio-frequency induction furnace (18 kW). During the
Fig. 1. Representative pressure-composition desorption isotherms of the La(Ni1−x Fex )5 alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30).
S.K. Pandey et al. / International Journal of Hydrogen Energy 32 (2007) 2461 – 2465
Fig. 2. Dehydriding kinetics of the La(Ni1−x Fex )5 alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30).
Table 1 Storage capacity and desorption kinetics of the La(Ni1−x Fex )5 hydrogen storage alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30) S.N.
1. 2. 3. 4. 5. 6.
Alloys
LaNi5 La(Ni0.95 Fe0.05 )5 La(Ni0.90 Fe0.10 )5 La(Ni0.80 Fe0.20 )5 La(Ni0.75 Fe0.25 )5 La(Ni0.70 Fe0.30 )5
Storage capacity (wt%)
Desorption kinetics (cc/g/min)
1.5 1.93 1.96 2.20 1.78 1.60
13 15 16 19 13 10
respectively. It may be pointed out that the storage capacity of 2.20 wt% is the highest reported value for the LaNi5 and its substituted system. The rate of flow of hydrogen during desorption (desorption kinetics) of the material are shown in Fig. 2. The storage capacity and kinetics for all the materials studied here are given in Table 1. 3.2. Structural/microstructural characterization: using XRD, SEM techniques In order to unravel the curious hydrogenation behavior La(Ni1−x Fex )5 , x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30, structural (XRD) were carried out. Analysis of XRD patterns confirmed the formation of CaCu5 type hexagonal structures, for all the composition studied. The XRD patterns of optimum alloy as-synthesized and after hydrogenation through several cycles are shown in Fig. 3(a) and (b). The a and c lattice parameters for all the variants La(Ni1−x Fex )5 , x = 0, 0.05, 0.10, 0.20, 0.25 and 0.30 were calculated from the observed d values through a least square fitting computer programme provided by S. Sivashankaran (Crystal Growth Centre, Anna University). The unit cell volumes were calculated from
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Fig. 3. X-ray diffractograms of the (a) as-synthesized and (b) dehydrogenated La(Ni0.80 Fe0.20 )5 alloy.
Table 2 Lattice parameters and unit cell volume of the La(Ni1−x Fex )5 hydrogen storage alloys (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30) S.N.
1. 2. 3. 4. 5. 6.
Alloys
LaNi5 La(Ni0.95 Fe0.05 )5 La(Ni0.90 Fe0.10 )5 La(Ni0.80 Fe0.20 )5 La(Ni0.75 Fe0.25 )5 La(Ni0.70 Fe0.30 )5
Fe contents
Lattice parameter CaCu5 type (AB5 )
Unit cell volume
x
˚ a (A)
˚ c (A)
˚ 3) (A
0 0.05 0.10 0.20 0.25 0.30
4.9762 5.0201 5.0481 5.0484 4.9839 4.9798
4.0019 3.9792 4.0134 4.0259 4.0091 4.0026
85.671 86.8453 88.5722 88.8568 86.821 85.961
the observed lattice parameters. The lattice parameters and the unit cell volumes for various variants are shown in Table 2. After hydrogenation through several cycles, the particle size gets reduced. Fig. 3(b) shows the XRD peak broadening for a sample, which has undergone five hydrogenation/dehydrogenation cycles The particle size as obtained from FWHM after taking into account instrumental broadening effect was determined as ∼36 nm compared to the initial particle size of ∼60 nm. This implies a decrease in particle size from 60 to 36 nm. The microstructural characterization of the as-synthesized and hydrogenated/dehydrogenated La(Ni1−x Fex )5 was carried through scanning electron microscope (XL-20). Fig. 4(a) and (b) show the SEM micrographs of the as-synthesized and hydrogenated samples, respectively. It may be pointed out that some cracks developed after hydrogenation through several cycles [21] which can easily be seen in Fig. 4(b). As can be seen from Table 2, the unit cell volume for AB5 type phase La(Ni1−x Fex )5 increases with decreasing value of x. Substitution of Fe at the Ni site beyond x=0.20 (e.g. x=0.10) leads to the decrement of the unit cell volume which is shown
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Fig. 4. Scanning electron micrographs of La(Ni0.80 Fe0.20 )5 alloy (a) as-synthesized and (b) after dehydrogenation.
Fig. 5. Variation of unit cell volume with variation of x in La(Ni1−x Fex )5 alloy.
in Table 2. Fig. 5 exhibits the variation of unit cell volume with Fe concentration (x). Table 2 and Fig. 5 clearly show that the ˚ is lattice parameters and hence unit cell volume (88.8568 A) maximum for the material La(Ni0.80 Fe0.20 )5 . The increase in unit cell volume on Fe substitution is an interesting result. This may have bearing on storage capacity of the La(Ni1−x Fex )5 . The increase in unit cell volume will result in increase of interstitial size. Thus, some interstitials, which would not be getting occupied by hydrogen atoms in the native phase LaNi5 , may get occupied in the substituted versions. As can be seen the maximum increase in the unit cell volume is for the variant La(Nai0.80 Fe0.20 )5 . Therefore, this variant is expected to have the maximum number of additional interstitials occupied by hydrogen. It is known that the LaNi5 intermetallic has total number of 37 interstitial positions. On hydrogenation generally only 6 of these are occupied by H atoms [22,23]. The structure of the unit cell is such that next to the 6 interstitial voids, which normally get occupied, 3 other interstitials are the next sites, which can be occupied [22,23]. If these 3 interstitials also get occupied making the total number of interstitials occupies as 9. It is known that the dominant reason for non-occupation of more than 6 interstitial sites is the increased coulomb repulsion of the H–H atoms (lowest permissible distance between H–H atom; Switendick Criterion). In the present investigation as already found out the unit cell volume and hence interstitial hole size increases. As is known the storage capacity is sensitive to the electron attractive power of the substituting atom. The higher electron attractive power of Fe may increase the storage capacity. In fact Fe has four unpaired ‘d’ electrons as against two for Ni. Thus, the combined effect of (a) substitution of Fe which is more electron attractive than Ni and will bring extra hydrogen atoms and (b) accommodation of the extra hydrogen atoms in interstitials with higher hole size, will lead to higher storage capacity. This is in keeping with experimental result where La(Ni0.80 Fe0.20 )5 exhibits high storage capacity 2.20 wt%. This will lead to relaxation of the minimum H–H distance criterion. In view of this, higher number of voids will get occupied. As outlined earlier next to 6, 3 more interstitials can get occupied. If these 3 sites get populated by H atoms making the total H atoms as 9 in the unit cell, the storage capacity will get enhanced to ∼2.25 wt%, an increase of about ∼50% over the normal storage capacity of ∼1.5 wt%. In the present case the enhancement in storage capacity is about 47% increase over the normal storage capacity. This is very close to be expected enhancement in storage capacity. It is one of the highest storage capacity reported for the AB5 type storage material. It may be pointed that in one of our earlier studies of AB5 type material we have found enhanced capacity up to ∼2.04 wt%. However, these results were on AB5 type material, which was Mm based. Also the enhancement results due to ball milling producing strained particles. It can thus be said that in the present La(Ni0.80 Fe0.20 )5 intermetallic phase, 9 instead of the use 6 interstitial sites get populated by hydrogen atoms. The results of the present investigation clearly suggest that if more than 9 interstitial sites can get populated the storage capacity can get enhanced to values higher than ∼2.25 wt%. This is feasible since there are 37 interstitial sites
S.K. Pandey et al. / International Journal of Hydrogen Energy 32 (2007) 2461 – 2465
available for LaNi5 structure. Material tailoring to achieve this will be worth attempting. It may be pointed out that relaxation of minimum H–H distance criterion are being envisaged [24]. We are carrying out further investigation in this direction and results will be forthcoming. 4. Conclusion We have synthesized AB5 -type materials with formula La(Ni1−x Fex )5 (x = 0, 0.05, 0.10, 0.20, 0.25, 0.30). The assynthesized alloys are hexagonal (CaCu5 type, AB5 ) for all the compositions studied. A maximum storage capacity of ∼2.20 wt% at room temperature and faster kinetics 19 cc/g/min have been observed for the La(Ni0.80 Fe0.20 )5 capacity has been put forward. Acknowledgments The authors will like to acknowledge Professor A.R. Verma (Delhi, India), Professor T.N. Veziroglu (President, IAHE Florida, USA) and Professor M. Groll (Stuttgart, Germany) for helpful discussions. Financial assistance from Ministry of Non-conventional Energy Sources, New Delhi (India) is gratefully acknowledged. References [1] Van Vucht JHN, Kuijpers FA, Bruning HCAM. Philips Res Rep 1970;25:133. [2] Hotten PHL, Einerhand REF, Daams JLC. J Alloy Compd 1995;231:604. [3] Msika E, Latroche M, Cuevas F, Percheron-Guegan A. J Mat Sci Eng 2004;108:91. [4] Takayuki I, Shigehito I, Nobuko H, Hironobu F. J Alloy Compd 2004;365:271.
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