Hydridic, thermodynamic and kinetic properties of Hf2Ni intermetallic phase

international journal of hydrogen energy 34 (2009) 3764–3770

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Hydridic, thermodynamic and kinetic properties of Hf2Ni intermetallic phase c  Dragica Lj. Stojic´a,*, Sandra V. Kumric´b, Jelena N. Belosˇevic´-Cavor , Jana S. Radakovic´c, idar Dj. Cekic´c, Slavko V. Mentusd Boz a

Department of Physical Chemistry, Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia Department of Material Science, Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia c Department of Nuclear and Plasma Physics, Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia d Faculty of Physical Chemistry, University of Belgrade, 11001 Belgrade, Serbia b

article info

abstract

Article history:

The intermetallic compound Hf2Ni was the subject of investigations by several methods.

Received 25 December 2008

The relative stability was checked by calculating its enthalpy and cohesive properties,

Received in revised form

using the augmented plane wave plus local orbitals (APW þ lo) method of ab initio calcu-

20 February 2009

lations. The kinetics of hydrogen absorption in this compound was investigated in the

Accepted 21 February 2009

temperature range from 573 to 823 K, under the constant hydrogen pressure of 1 bar. The

Available online 2 April 2009

obtained rate constants, k (s1), and hydriding capacities (H/M ) are as follows: 0.00038 (0.69), 0.00131 (0.95), 0.00246 (1.13) and 0.0042 (0.92) for temperatures 573, 673, 723 and

Keywords:

823 K, respectively. The changes in crystal structure and morphology caused by multiple

Hf2Ni

hydriding/dehydriding cycles were followed by XRD and SEM.

Hydriding isotherm

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Hydriding capacity

reserved.

XRD SEM Activation energy (APW þ lo) method

1.

Introduction

Nowadays, hydrogen storage is an important part of hydrogen energetic. The storage in metal hydrides was widely investigated and brought to widespread practical use [1–3]. In this sense, structural, thermodynamical and kinetical aspects of hydride formation, as well as the mechanism of hydrogen uptake were the topics of extensive consideration [4]. Metal and intermetallics intended to serve as hydrogen storage media should provide the hydriding/dehydriding procedure to be reversible and to occur at the hydrogen partial

pressure as close as possible to the atmospheric one. It is also important that the hydride forming metals and intermetallics are able to keep their hydriding capacity after a great number of hydriding/dehydriding cycles. Nowadays, intermetallics based on lanthanum and nickel satisfy many of these requirements [5]. Some of the most common drawbacks of the currently used metal hydride materials are that (a) hydrogen-metal ratios are too low; (b) metals involved are too costly; (c) the absorption or release of hydrogen is slow and sensitive to the poisoning phenomena and (d) significant hysteresis is

* Corresponding author. Tel.: þ381 11 2453967; fax: þ381 11 2447207. E-mail address: [email protected] (D.Lj. Stojic´). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.02.055

international journal of hydrogen energy 34 (2009) 3764–3770

exhibited [6]. During the repetition of hydriding/dehydriding cycles, alloy decomposition usually occurs, accomplished by the loss in hydriding capacity [7]. The decomposition may be decelerated by addition of suitable nonhydrideable metal [8]. The ability of hafnium-transition metal compounds to absorb hydrogen is known long time ago [2,9–11]. Polycrystalline compounds, Hf2Fe, Hf2Co and Hf2Ni are Pauli paramagnetics. Binary systems formed between hypo- (Ti, Zr, Hf) and hyper-d-transition metals (Fe, Co, Ni, Pd, Pt), have been investigated as systems interesting for hydrogen storage in hydrogen–metal atom ratios H/M  1 at rather low temperatures (T / 300  C) and high pressures (>1 kPa) [12]. For transition metal compounds, Miedema [13] reported that the interaction energy is controlled by a mechanism similar to the Miedema’s ‘‘rule of reversed stability’’, i.e., the larger the binding energy of the compounds is, the stronger is the repulsion between the alloying atom and H. In the other words, the heat of hydrogen absorption by the intermetallic should be increased with decreasing its standard enthalpy of formation. Thus it was suggested that intermetallic compounds with calculated heats of hydride formation (metal–hydrogen systems with dissociation pressures of about 1 bar, around the room temperature) in the following range: 40 kJ/mol H  DH  15 kJ/mol H, are of the practical interest as the potential hydrogen storage alloys [14]. In the sense of practical use, having in mind relative abundances in Earth’s crust, it seems unlikely that Hf-based intermetallics may concur to La based ones in hydrogen storage practice. Nevertheless, comparison of their hydrogen storage capabilities may contribute to the understanding of hydriding behavior of various hydride forming systems. In this paper we performed first principle calculations of enthalpy of formation and cohesive energy of bulk Hf2Ni using WIEN 2k code [15]. In addition to the paper by Mukai et al. [11], in which H/M ratio in HfNi as a function of hydrogen pressure was reported, we investigated the hydriding of Hf2Ni, in order to determine the kinetic parameters of this process in function of the number of hydriding/dehydriding cycles.

2.

Calculations procedures

MolDraw program [16], was used to draw the unit cell of Hf2Ni. The unit cell parameters, enthalpy of formation and cohesive energy were calculated using the APW þ lo method. In this method, the unit cell is divided into non-overlapping spheres, called muffin-tin spheres (MT) centered at the atomic nucleus and the interstitial region (IT). Basis sets utilized in the interstitial region are plane waves, while inside the muffin-tin spheres it is a linear combination of radial functions multiplied by a spherical harmonics. The sphere radii, Rmt, were set ˚ and 1.137 A ˚ for Hf and Ni, respectively. The plane to 1.217 A wave cutoff, RmtKmax, was set to 8.5, and lmax up to 10. These two parameters describe basis sets used. Electron correlation and exchange energy were treated using generalized gradient approximation (GGA) with the scheme of Perdew–Burke–Ernzerhof [17]. For the Hf2Ni ground state energy calculations, kpoint mesh used to sample the entire Brillouin-zone was 4000, yielding 288 points in the irreducible wedge. The convergence with respect to the size of the basis set and k-point sampling

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was carefully checked on a series of test cases, and the error in total energy was estimated to be less than 20 meV/atom. A series of calculations was done changing the volume within 5% around the experimental value and calculating the total energy as its function. Theoretical values for volume and total energies were extracted from Murnaghan’s equation of state. Finally c/a ratio was optimized by changing it within 3% of the experimental value, while keeping the optimized volume constant. As far as the elemental metals, Hf and Ni, are concerned, their total energy was calculated in their most stable ground state modifications (Hf-hcp and Ni-fcc). Ni calculations included spin-polarization. k-point mesh used in Hf ground state energy calculations gave 427 inequivalent points, while for metallic Ni this figure was 288. Their Rmt was kept identical as in the Hf2Ni calculations. It was also necessary to calculate the atomic energies of Hf and Ni in order to obtain the value for Hf2Ni cohesive energy. For that purpose, a supercell was constructed of 30 a.u. side length, with a single atom placed at its origin. Only one k-point at G (0,0,0) was used and the calculations included spinpolarization. The cohesive energy of Hf2Ni was calculated according to the equation: Ni 2 Ni Ec ¼ EHf þ 2EHf tot atom þ Eatom ;

(1)

while the enthalpy of formation of this intermetallic compound was calculated using the following equation: Ni 2 Ni 2 Ni ¼ EHf  2EHf DHHf f tot metal þ Emetal :

3.

(2)

Experimental procedures

The intermetallic phase Hf2Ni was prepared from Ni (5 N) and Hf (2 N) in RF induction furnace under a pure argon stream. To ensure homogenization, melting/solidification cycles were repeated several times. The phase Hf2Ni, directed by metals mole ratio, is formed peritectically according to the reaction: L þ HfNi 4 Hf2Ni at 1200  C, where L is liquid phase, as may be predict from phase Hf–Ni phase diagram [18]. The single phase, BCT CuAl2 (C16)-type structure, was confirmed by Xray diffractometry. The lattice constants at room temperature were proven to agree to these published elsewhere: ˚ and c ¼ 5.271(3)A ˚ [19]. a ¼ 6.479(3) A After grinding the sample, hydriding and dehydriding were carried out in typical volumetric equipment [20]. A quartz tube with weighted amount of the powdered sample (about 150 mg) was inserted into a thermostated tube furnace. By a set of valves, the sample container was connected to both vacuum equipment and a hydrogen reservoir. Before use, each sample was pretreated by annealing at 873 K (600  C) for 2 h under vacuum of 1  103 mbar. In the present work, the hydrogen absorption and desorption experiments were carried out in a constant volume system. Then the samples were subjected to the multiple hydriding/dehydriding cycles. Hydrogen (99.9%) was introduced into the system under the initial pressure of 1 bar for each experiment. The hydriding reaction was carried out isothermally, within the temperature range 573–823 K (300–550  C). The pressure decrease during hydride formation was continuously measured by means of a closed mercury manometer. After hydriding, dehydriding

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international journal of hydrogen energy 34 (2009) 3764–3770

Table 2 – Calculated values of cohesive energy Ec and 2 Ni enthalpy of formation DHHf (eV/atom) for Hf2Ni. f DHfHf 2 Ni ðeV=atomÞ Ec (eV/atom)

Theory

Semi-empiricala

0.33 8.22

0.47

a [25].

Fig. 1 – The model of unit cell of Hf2Ni obtained by MolDraw program.

was executed under the same conditions. Starting always with the hydrogen pressure equal to the atmospheric one, the quantity of absorbed hydrogen as a function of time was calculated on the basis of pressure drop. Since the relative pressure drop accompanying the achievement of equilibrium state never exceeded 5%, one can treat the experimental conditions as being isobaric. The pressure change was measured as a function of time, and the number of hydrogen atoms absorbed per 1 mol of intermetallic (i.e. H/M ratio) was calculated on the basis of equation of state of an ideal gas. According to the phase diagram [18], in the temperature range of this experiment, Hf2Ni phase should not undergo any phase change [21]. X-ray diffractometry (XRD) of powdered samples was performed by Brucker D8 Advanced, using Cu Ka radiation (l ¼ 0.15406 nm, 2q ¼ 10–80 ). XRD diffractograms were recorded at room temperature with the step of 0.05 . For SEM imaging JSM-35 electron microscope was used. The tilt angle was between 0 and 30 .

4.

Results and discussion

Fig. 2 – The hydrogen absorption isotherms of Hf2Ni at various temperatures: - 300 8C, C 400 8C, : 450 8C, and ; 550 8C.

Calculated values are in fair agreement with the experimental ones. Calculated volume of unit cell of Hf2Ni underestimates the experimental one by 2%, which causes corresponding lattice parameters to differ up to 0.7%.

4.1. The calculations of structural and thermodynamic parameters of Hf2Ni intermetallic compound The model of unit cell, obtained by MolDraw program [16], is presented in Fig. 1. The unit cell consists of four Hf2Ni molecules [22]. The structural parameters of unit cells determined on the basis of ab initio calculations as well as their experimental values [23,24] for Hf2Ni, Hf and Ni, are listed in Table 1.

Table 1 – Calculated unit cell dimensions and volume for fcc Ni, hcp Hf and Hf2Ni. Hf2Ni

˚) a (A ˚) c (A c/a ˚ 3) V (A

Hf

Ni

Theory

XRDa

Theory

XRDa

Theory

XRDa

6.432 5.228 0.813 108.169

6.479 5.271 0.813 110.632

3.203 5.072 1.583 22.563

3.195 5.051 1.581 22.32

3.525 – – 10.953

3.524 – – 10.940

a [23,24].

Fig. 3 – The dependence of hydrogen absorption capacity (H/M ) on temperature.

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Fig. 4 – The dependence ln½a=ðaLxÞ vs. time at different temperatures, - 300 8C, C 400 8C, : 450 8C, and ; 550 8C.

Calculated ratio c/a shows excellent agreement with the experimental one. As far as the constituents Hf and Ni are concerned, calculated volumes overestimated experimental ones by approximately 1% and 0.1%, respectively, while lattice parameters are also larger than the corresponding experimental values by 0.3% and 0.02%.

Fig. 6 – The isotherms of subsequent hydriding of Hf2Ni alloy at 450 8C (753 K): - 1st cycle, C 2nd cycle, : 3rd cycle.

Calculated enthalpy of formation and cohesive energy of this intermetallic compound, compared to previously reported semi-empirical values [25], are given in Table 2. The negative value of enthalpy of formation indicates that this compound is thermodynamically stable. Apart from a small underestimation, our calculations reproduce the

Table 4 – The rate constants of hydriding and the hydriding capacity for the first, the second and the third cycle for Hf2Ni at temperature of 723 K. Cycle 3

1

k  10 (s ) H/M max

1st

2nd

3rd

2.46 1.13

3.25 1.03

7.55 0.94

Fig. 5 – The dependence of rate constants vs. reciprocal temperature of hydriding of Hf2Ni alloy.

Table 3 – First hydriding rate constants at different temperatures for Hf2Ni alloy. T, K k  103 (s1)

573 0.38

673 1.31

723 2.46

823 4.20

Fig. 7 – XRD spectrum of Hf2Ni intermetallic at room temperature: before hydrogen absorption (bottom), and after 20 cycles of hydriding/dehydriding at 673 K.

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semi-empirical enthalpy of Hf2Ni. According to the obtained result and Hong’s findings [14] this intermetallic compound is of practical interest as the potential hydrogen storage material. However, if we compare the calculated results with the previously reported ones for Hf2Fe [22], we can conclude that, in accordance with the Miedema’s rule [13], Hf2Ni has smaller potential for hydrogen absorption, which was experimentally verified [26].

the given temperature, the hydriding/dehydriding cycles of the same sample were repeated at least three times. It could be seen that, at higher temperatures, hydrogen absorption capacity increases with temperature increasing from 300 to 450  C and then the decreasing was observed for 550  C (Fig. 3). It is the consequence of the equilibrium shift in reaction Hf2Ni þ (x/2) H2 4 Hf2NiHx to the left at higher temperatures. The experimental data were found to follow the equation

4.2.

dx ¼ k1 pH1=2 ða  xÞ; 2 dt

Hydrogen absorption kinetics

The hydrogen absorption was investigated at several temperatures in the range 573–823 K (300–550  C). Sets of isotherms for the first hydriding process at temperatures 573, 673, 723 and 823 K, were obtained and presented in Fig. 2. At

(3)

where a is the equilibrium (maximum) amount of H/M and x is H/M for time t. Keeping hydrogen pressure always close to 1 bar, the condition k ¼ k1 pH1=2 was fairly fulfilled. Under this 2 condition, the integration of equation (3) gives:

Fig. 8 – SEM images of Hf2Ni: (a) before absorption, (b) after 20 cycles of absorption/desorption processes at 673 K; amplifications: 600 (I), 1000 (II) and 2000 (III).

international journal of hydrogen energy 34 (2009) 3764–3770

1 a k ¼ ln : t ax

(4)

Fig. 4 shows the experimental data presented in the form of dependence ln ða=a  xÞ vs. t. Obviously, the experimental data fairly obey the equation (4). The slopes of these plots are rate constants k at corresponding temperatures, which were presented in Table 3. The logarithms of the rate constants, ln k, are presented as a function of reciprocal temperature in Fig. 5. From this plot of Arrhenius type the apparent activation energy for hydriding process, 38.44 kJ/mol, and the pre-exponential factor of 1.286 s1 were obtained. The values found for this intermetallic are one magnitude order lower in comparison to those found for ZrNi [7], amounting between 0.004 and 0.012 for temperatures between 423 and 523  C. Till now, only the fist hydriding was considered. Fig. 6 presents the results of three subsequent hydriding of the same sample at 450  C (723 K). The analysis of these results gave the following values of the rate constants and the maximum hydriding capacities obtained (Table 4). On the basis of the results presented in Table 4, one may conclude that the multiple hydriding of the same sample leads to the increase of the rate of hydrogen absorption and simultaneously to the decrease of hydriding capacity. This is obviously due to a crushing of the compound particles and increase of the surface area accessible to hydrogen because of expansion of alloy particles during the hydride formation [7,27]. For the second hydriding process, the activation energy was determined to amount to 14.55 kJ/mol.

4.3.

XRD and SEM characterisation

To follow the structure and morphology changes of the intermetallic sample during multiple hydriding, the XRD spectra and SEM micrographs before and after hydriding experiments were taken. The XRD spectrum of Hf2Ni powder before and after 20 cycles of hydrogen absorption/desorption cycles at 673 K is presented in Fig. 7. It could be concluded that, as usually for simple hydride forming intermetallics [3] partial decomposition of the intermetallic occurs during cycling, however lattice parameters of the remaining Hf2Ni were maintained. The SEM micrographs of Hf2Ni powder, before and after 20 hydrogen absorption–desorption cycles, obtained with different amplifications: 600 (I), 1000 (II) and 2000 (III) are presented in Fig. 8. It can be seen that after 20 absorption– desorption cycles the average particle diameter (7 mm) was apparently not affected. However, the numerous cracks appear along the particles, which are particularly visible under the amplification 2000, as a known consequence of repetition hydriding/dehydriding cycles [20].

5.

Conclusion

On the basis of calculations performed for Hf2Ni, the unit cell ˚ and c ¼ 5.228 A ˚ , enthalpy of formation parameters a ¼ 6.432 A

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0.33 eV/atom and cohesive energy 8.22 eV/atom were determined. For the first hydriding of powder of mean particle diameter about 7 mm, the rate constant of hydriding (s1) and maximum (H/M) ratio: 0.00038 (0.69), 0.00131 (0.95), 0.00246 (1.13) and 0.0042 (0.92) were found at temperatures 573, 673, 723 and 823 K, respectively. This is for one magnitude order lower than in the case of ZrNi intermetallic compound. Similarly to many other hydriding intermetallics, multiple hydriding/dehydriding procedures accelerate hydriding, but diminish the hydriding capacity.

Acknowledgement The Ministry of Science and Technological Development of the Republic of Serbia provided the financial support for this study through the Projects No. 141 022 and 142 047.

references

[1] Schlapbach L. Hydrogen in intermetallic compounds I. Berlin/Heidelberg PA: Springer-Verlag; 1988. p. 3. [2] Klyamkin SN, Semenenko KN. Pseudobinary intermetallic compounds in Hf2M0 –Hf2M00 (M0 , M0 ¼ Mn, Fe, Ni, Cu) systems and their interaction with hydrogen at high pressures. J Alloys Compd 1999;293–295:426–8. [3] Willems JJG, Buschow KHJ. From permanent magnets to rechargeable hydride electrodes. J Less Common Met 1987; 129:14. [4] Eartl G. Interaction of Hydrogen with Metal Surfaces. Z Phys Chem Neue Fol 1989;164:1115. [5] Iwakura C, Fukuda K, Senoh H, Inoue H, Matsuoka M, Yamamoto Y. Electrochemical characterization of MmNi4.0xMn0.75Al0.25Cox electrodes as a function of cobalt content. Electrochim Acta 1998;43:2041–6. [6] Esayed AY. Hysteresis and thermodynamic characterization of Nb1xCrx (x ¼ 0.03, 0.05, 0.1). Int J Hydrogen Energy 2000; 25(4):363–8. [7] Simonovic´ BR, Mentus S, Sˇusˇic´ MV. Examination of the kinetics of Zr1.02Ni0.98 alloy hydriding. J Serb Chem Soc 1999; 64:745–52. [8] Joubert JM, Latroche M, Percheron-Guegan A. Hydrogen absorption properties of several intermetallic compounds of the Zr–Ni system. J Alloys Compd 1995;231(1–2):494–7. [9] Van Essen RM, Buschow KHJ. Hydrogen absorption in various zirconium- and hafnium-based intermetallic compounds. J Less Common Met 1979;64:277–84. [10] Baudry A, Boyer P, Chikdene A, Harris SW, Cox FS. Muon study of the intermetallic hydrides Zr2NiHx. Hyperfine Interact 1991;64:657–63. [11] Mukai D, Miyata H, Aoki K. Hydrogen absorption and desorption properties of Hf-based intermetallic compounds. J Alloys Compd 1999;293–295:417–20. [12] Koteski V, Ivanovic´ N, Cekic´ B, Milosˇevic´ Z. FP-LAPW study of Hf2Ni: structure, electronic properties, and electric field gradients. J Phys Soc Jpn 2004;73(8):2158–63. [13] Miedema AR. The electronegativity parameter for transition metals: heat of formation and charge transfer in alloys. J Less Common Met 1973;32(1):117–36. [14] Hong K. Method for preparing materials for hydrogen storage and for hydride electrode applications. US Patent 5006328.

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international journal of hydrogen energy 34 (2009) 3764–3770

[15] Blaha P, Schwarz K, Madsen GKH, Kvasnicka D, Luitz J. WIEN 2k, an augmented plane wave plus local orbitals program for calculating crystal properties. Vienna, Austria: Vienna University of Technology; 2001. [16] Kokalj A. XCrySDen – a new program for displaying crystalline structures and electron densities. J Mol Graph Model 1999;17(3–4):176–9. [17] Perdew JP, Burke S, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77(18): 3865–8. [18] Masalsky TB. Binary alloys phase diagrams. Material Park OH: ASM International; 1990.  [19] Cekic´ B, Koicˇki S, Umic´evic´ A, Belosˇevic´-Cavor J, Koteski V, Ivanovski V. TDPAC and XRPD measurements of the polycrystalline Hf2Ni phase. J Optoelectron Adv Mater 2008; 10(4):794–7. [20] Simonovic´ BR, Mentus S, Susˇic´ MV. Kinetics of tantalum hydriding: the effect of palladization. Int J Hydrogen Energy 2000;25:1069–73.  [21] Koteski V, Belosˇevic´-Cavor J, Cekic´ B, Stojic´ D, Simic´ N, Umic´evic´ A, et al. Bonding and stability of the intermetallic

[22] [23]

[24]

[25]

[26]

[27]

compounds of hafnium with Ti2Ni structure. J Alloys Compd 2007;442(1–2):252–4. Keyun Z, Jin Z. Optimization and calculation of the Hf–Ni phase diagram. J Less Common Met 1990;166:21–7. Manasijevic´ M, Koicˇki S, Cekic´ B, Ivanovic´ N. The nuclear quadrupole interaction of 181Ta in the intermetallic compound Hf2Ni. J Phys Condens Matter 1994; 6:9781–7. Foster AS, Lopez Gejo F, Shluger AL, Nieminen RM. Vacancy and interstitial defects in hafnia. Phys Rev B 2002;65(17): 174117. Neissen AK, De Boer FR, Boom R, De Chaˆtel PF, Mattens WCM, Miedema AR. Model predictions for the enthalpy of formation of transition metal alloys II. CALPHAD 1983;7(1):51–70. Stojic´ DLj, Kumeric´ SV, Grozdic´ TD, Koteski VJ, Cekic´ BDj. Hydrogen production and storage – investigation of Hf-based intermetallics. J Power Sources doi:10.1016/j.jpowsour.2009. 01.064. Libowitz GG. The solid state chemistry of binary metal hydrides. New York: Benjamin; 1967. p. 71.