Corresponding author. Tel.: +381 11 2453967; fax: +381 11 2447207.

E-mail address: estojicd@vinca.rs (D.Lj. Stojić ).

HYDROGENATION PROPERTIES OF Hf-Ni INTERMETALLICS -

EXPERIMENTAL AND THEORETICAL INVESTIGATION

Dragica Lj.Stojić1*, Katarina D. Ćirić

2, Boţidar Đ. Cekić

2, Vasil J. Koteski

2, Slavica P.

Zec3, Ţarko D. Bogdanov

4

1Department of Physical Chemistry,

2Department of Nuclear and Plasma Physics,

3Department of Material Sciences,

4Department of Atomic Physics, Vinĉa Institute of

Nuclear Sciences, P.O. Box 522, University of Belgrade, 11000 Belgrade, Serbia

The hydrogenation properties of HfNi and Hf2Ni7 intermetallics were investigated at the

constant pressure of 1bar and in the temperature ranges 373-573K for HfNi and 323-

473K for Hf2Ni7. The kinetic parameters, rate constants and activation energies of the

absorption processes were determined. Maximal hydrogen absorption, i.e., number of

hydrogen atoms absorbed per metal atom, H/M, are 1.05 and 0.04 achieved at 373K for

HfNi and Hf2Ni7, respectively. Multiple hydriding/dehydriding was found to influence

the improvement of the kinetic parameters. XRD and SEM methods were used to

investigate the structural and morphological changes of the samples due to hydrogen

absorption. The thermodynamic parameters of hydriding together with the structural

properties of the intermetallics and their hydrides, calculated using the full-potential

linearized augmented plane waves (FP-LAPW) code based on the density functional

theory (DFT), were utilized for the sake of explaining the experimental investigations.

Keywords: HfNi, Hf2Ni

7, hydrogen absorption kinetic, hydriding capacity, activation

energy, FP-LAPW calculations.

1. Introduction

Interaction of hydrogen with metals and intermetallic compounds is widely

studied since it is proven to be important from scientific and technological point of view.

Probably the most investigated application of metal and intermetallic hydrides is in the

field of hydrogen storage, due to growing interest in hydrogen as an energy carrier

compatible with the “sustainable energy“ concept. Optimal hydrogen-storage material is

required to have various properties, such as high hydrogen capacity per unit mass,

reversible hydriding/dehydriding processes that occur at hydrogen partial pressure as

close as possible to the atmospheric pressure, fast kinetics etc. 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 [1]. The storage in numerous metal

hydrides is found to satisfy many of these properties required, and many of them

nowadays have practical application [2,3,4].

The ability of hafnium-transition metal compounds to absorb hydrogen is known

for a long time [5]. Among them, binary systems formed between the hypo- (Ti, Zr, Hf)

and the hyper- d-transition metals (Fe, Co, Ni, Pd, Pt) have been investigated as systems

promising for hydrogen storage, and their thermodynamic characteristics have been

2

determined [6, 7, 8, 9]. Because of the large molecular mass of intermetallic compounds

based on Hf, and thus low gravimetric density of hydrogen in them, they are not

favorable for mobile applications, but their excellent kinetic parameters nominate them

for other applications where hydrogen storage is needed.

The aim of this work is to investigate the hydriding of HfNi and Hf2Ni7 at

atmospheric pressure, since the occurrence of the hydriding processes at hydrogen partial

pressure as close as possible to the atmospheric one is among the requirements for the

optimal hydrogen storage material. Furthermore, we want to elucidate the hydrogen

absorption abilities of compounds from the Hf-Ni phase system by comparing the

structural and thermodynamic properties obtained from first principle calculations

described in this work and reported elsewhere [10, 11, 12].

2. Methods of investigation

2.1 Experimental procedures

Polycrystalline intermetallic compounds, αHfNi and Hf2Ni7, were prepared from

high purity Ni shots (99.999%) and metallic Hf (98.5% Hf and 1.5% Zr) in RF induction

furnace under a pure argon stream. To ensure homogenization melting/solidification

cycles were repeated several times [13, 14]. The single phase CrB-type structures for

αHfNi and orthorhombic structure (C2/m space group) for Hf2Ni7 were confirmed by

X-ray diffractometry.

The samples were subjected to grinding and crushed into powder. The hydrogen

absorption properties, hydrogen absorption capacities and the absorption kinetics, were

investigated using typical volumetric equipment, constructed in our Laboratory. A quartz

tube with weighted amount of the powdered sample, about 0.25 mmol of both HfNi and

Hf2Ni7 was inserted into a thermostated tube furnace. By a set of valves, the sample

container was connected to both a vacuum equipment and a hydrogen bottle (purity

99.9%). Before use, each sample was activated by annealing at 773K for 3h under

vacuum of 1 x 10-3 mbar and cooled to room temperature in hydrogen atmosphere.

Hydrogen was introduced into the system under the initial pressure of 1bar for each

experiment. The hydriding reaction was carried out isothermally, within the temperature

ranges 373-573K (HfNi) and 323-473K (Hf2Ni7). The magnitude of pressure drop due to

the hydrogen absorption was continuously monitored by digital reading device with

precision of 0.01 mbar. 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. The experimental conditions are treated as being isobaric

due to the small relative pressure drop, about 5%, accompanying the achievement of

equilibrium state. After each measurement the samples were degassed at the same

temperature under a vacuum of 1 x 10-3 mbar. The pressure change was measured as a

function of time, and the number of hydrogen atoms absorbed per metal atom, H/M, was

calculated on the basis of equation of state of an ideal gas.

In the temperature ranges of these experiments, HfNi and Hf2Ni7 phases should

not undergo any phase changes [15].

3

X-ray diffractometry (XRD) and Scanning Electron Microscopy (SEM), were used to

investigate the structural and morphological changes influenced by hydrogen absorption.

XRD was performed at room temperature by Siemens D500 powder diffractometer using

Cu Kα radiation (λ = 0.15406 nm, 2θ = 20 - 80o) and scanning speed of 0.02o2θ/s.

Lattice parameters of the identified phases were calculated by Wincell program. SEM

images were performed by JEOL JSM-35 electron microscope (accelerating voltage 25

kV).

2.2 Calculations method

The calculations were done in the framework of the density functional theory

(DFT), using a full-potential augmented plane wave method (FP/APW+lo) [16,17] as

implemented in the WIEN2k computer package [18]. The exchange – correlation

interaction was described within the generalized gradient approximation (GGA) of

Perdew–Burke–Ernzerhof (PBE) [19]. The calculation parameters (Gmax, RMT, lmax) and

procedure were kept the same as in our previous calculations on Hf-Ni [11], in order to

obtain good comparison of different intermetallics. The Brillouin zone sampling has been

done using 210 k-points within the irreducible wedge of the zone.

3. Results and discussion

3.1. Hydrogen absorption kinetics

The hydrogen absorption kinetic was investigated in the temperature ranges 373-

573K for HfNi and 323-473K for Hf2Ni7 at the constant pressure of 1bar.

For HfNi it was observed that, at the all temperatures applied, repeating of the

hydriding process (followed by dehydriding), results in an improvement of the absorption

process kinetics (Fig 1).

Fig. 1 Influence of subsequent hydriding to hydrogen absorption in HfNi powder at 373K

(1)-(4) denote first to fourth absorption process, respectively.

4

It can be seen from Fig. 1 that the hydriding rate is increasing in the each

consecutive cycle. Connected to HfNi absorption capacity, multiple repeating of the

hydriding/dehydriding cycles leads to a decrease of the maximal absorption capacity,

H/M. This can be seen comparing H/M = 1.12 (Fig.1) with H/M = 1.05, where the later

value was obtained after additional 20 hydriding/dehydriding cycles with the final

absorption curve presented in Fig. 5(a). Similar trend have been observed in the other

metal hydrides [20,21]. At the beginning of each absorption process (Fig.1), incubation

periods were observed. At first, the absorption isotherms incubation periods are more

pronounced, but in the subsequent hydriding they are getting shorter and disappear. The

incubation periods were not observed in the case of Hf2Ni7.

In order to understand the reasons for the improvement of absorption kinetic in

HfNi, SEM and XRD measurements were performed. SEM images of starting HfNi

before absorption (a and b) and after hydrogenated through several cycles (c and d) are

shown in Fig. 2 (a-d).

Fig. 2 SEM images of HfNi: (a, b) before absorption and (c, d) after several

absorption/desorption cycles at 373K.

After hydrogenation through several cycles, due to the particles fragmentation,

HfNi particles sizes decrease. The decrease in the HfNi particle size upon cycling

consequently increases the active surface of intermetallic accessible to the hydrogen. The

corresponding results of the improvement in kinetic parameters of the hydrogen

absorption process are shown in Fig.1.

According to the XRD pattern (Fig. 3a), the starting powder was single phase,

identified as α-HfNi (PDF 47-1414) with the orthorhombic structure of CrB-type and the

lattice parameters a = 3.222(13), b = 9.806(36), c = 4.108(14) Å. In the hydrogenated

powder (Fig. 3b), we observed a well crystallized HfNiH3 (PDF 47-1412) with the same

crystal structure as α-HfNi but with larger lattice parameters, a=3.461(2), b=10.416(4),

c=4.265(2) Å. Due to hydrogen incorporation into the crystal lattice of α-HfNi and given

5

that the CrB-type structure of HfNi is retained upon hydrogen absorption [7], this process

results in the expansion of the lattice, as confirmed by our XRD analysis (Fig 3) and

consequently in the additional crashing of particles and exposure of the larger surface

areas of HfNi to the hydrogen gas.

20 30 40 50 60

112

061

151

200

112

002

041130

040

021

b

a

200002

041

130

040

021

HfNiH3

-HfNi

Inte

nsity (

a. u

.)

2 [degree]

Fig. 3 XRD spectrum of HfNi at room temperature: before hydrogen absorption (bottom),

and after hydrogen absorption at 1bar, 373K (top)

Sets of isotherms for the final hydriding process in which the incubation period is

not observed (in the case of HfNi) and for the first hydriding process in the case of

Hf2Ni7 are presented in Fig. 4 (a, b).

Fig. 4 Hydrogen absorption isotherms (at 1bar) as a function of time for HfNi(a) and

Hf2Ni7 (b).

It can be seen from PCT curves and DSC thermal analysis for HfNi that at

pressures close to 1bar formation of hydrides (HfNiH3.0) is expected at temperatures

6

about 430K and under it [5], and in temperature range 330-440 (HfNiH3.0) [6]. Our

measurements show that at the temperature 373K pressure of 1barH2 is above equilibrium

pressure and hydrides are formed (H/M = 1.05), while at higher temperatures here

applied only formation of solid solution is observed.

The analysis of the kinetics was done by applying different models to the obtained

time-dependent absorption curves, as listed in Ref. 22. The basis of all those models that

define different mechanisms is the rate equation for solid-gas reaction:

dα/dt = k f(α)

where α is the reacted fraction at time t, which can be calculated from the starting

pressure, equilibrium pressure and pressure at time t [21], whereas k is the rate constant.

The integral form of the function f(α), g(α), can be used to determine the mechanism of

reaction, since

g(α) = ∫dα/dt = kt

indicate linear time dependence of this function with slope equal to the rate constant. As

it is well known that the reaction mechanism of the absorption process is complicated and

often different at different stages, from all listed kinetic models [22], the g(α) giving the

best linearity was considered as kinetic reaction mechanism of hydrogen absorption at

that stage. In Fig. 5 the isotherms at 373K (up) and kinetic analysis (down) for both

intermetallics, HfNi (a) and Hf2Ni7 (b) are shown.

Fig.5 Hydrogen absorption isotherms (up) and reaction mechanisms of different stages

7

(down) at 373K for HfNi* (a) and Hf2Ni7 (b).*-after additional 20 hydriding/dehydriding

cycles in respect to Fig. 1.

For HfNi the rate limiting step was found to be chemical reaction, which is in

accordance with the results obtained for the isostructural compound ZrNi [20]. The

integral form of the function determining the mechanism of this reaction is g(α)=1-(1-α)r,

(r=1/3). From Fig. 5a (down), it can be seen that there are two stages with different rates

present in the absorption process (rate constants k1 and k2 are given in Table 1), which

may indicate formation of two different hydride phases. However, the same reaction

mechanism is consistent with phase boundary reaction as a rate limiting step (g(α)=1-(1-

α)1-r

, (r=2/3)) [22]. Therefore we could not make a definite conclusion about the reaction

mechanism, but this does not influence the obtained values of rate constants.

For Hf2Ni7 at 373K (as well as at 473K) we found that the limiting step in

hydrogen absorption process is 3-dimension diffusion (3D diffusion), g(α)=[1-(1-α)1/3

]2.

The kinetic parameters of these processes are also given in Table 1. The maximal value

H/M is very low, only 0.04 at 1bar hydrogen pressure.

Table 1. The rate limiting steps and kinetic parameters for hydrogen absorption in HfNi

and Hf2Ni7 at different temperatures

HfNi Hf2Ni7

T(K) g(x) k(s-1

) g(x) k(s-1

)

373

1-(1-α)1-r

,

(r=2/3)

0.00376

[1-(1-α)1/3

]2

0.00161

0.00132

473

[1-(1-α)1/3

]2

0.00249 0.00716

508 0.00560

573 0.00963

As it can be seen from Fig 4 (a), at higher temperatures (473K, 508K and 573K),

the absorption process in HfNi at the beginning is very fast giving over 80% of total

absorption in the first 10 seconds. Therefore, the kinetic analysis was done for the stage

of the absorption process after that period, where for all temperatures applied the 3D

diffusion was found to be the limiting step. Our experimental results are presented in the

form g(α)=[1-(1-α)1/3

]2 as shown in Fig. 6 (a, b, c), while the obtained rate constants are

given in Table 1. The larger rate constant value at 573K (Table 1) for HfNi in comparison

with lower rate constant for Hf2Ni obtained at the same temperature, 0.00038 s-1

[10], is

8

also the another indication that HfNi is a better hydrogen storage material in accordance

with the required

properties

Fig. 6 Hydrogen absorption in HfNi at different temperatures, (a, b, c) - reaction

mechanisms; (d) - Arrhenius plot for 3D diffusion.

Since it is reasonable to compare only the stages of the processes where the

limiting step is the same (in this case 3D diffusion), we were able to estimate the

activation energy corresponding to them. The estimated value of activation energy, (33 ±

7) kJ/mol, was obtained from the Arrhenius plot, Fig. 6 (d).

3.2 Results of ab initio calculations for Hf2Ni7

The Hf2Ni7 intermetallic compound crystallizes in monoclinic symmetry (C2/m

space group), Fig. 7. The calculated and experimental values of the structural parameters

are given in Table 2. The calculated enthalpy of formation at 0K is -45.6 kJ/mol per atom

(Table 3), and is in a good agreement with experimental value of -50.7 kJ/mol per atom

(measured at 1623K) [23].

9

Fig.7 Unit cell of Hf2Ni7; Larger balls (blue) present Hf atoms and smaller (pink) Ni

atoms. The picture was obtained using XCrySDen [20]

Table 2. Crystal cell parameters (a, b, c) and angles (α, β, γ) for Hf2Ni7

calc. exp[25]

a (Å) 4.670 4.657

b (Å) 8.217 8.191

c (Å) 12.119 12.102

α(°) 90 90

β(°) 95.68 95.51

γ (°) 90 90

3.3. The features of hydrogen absorption processes in different Hf-Ni based

intermetallics

Here we compare the hydrogen absorption abilities of HfNi, Hf2Ni and Hf2Ni7, as

obtained by experimental and theoretical investigations in our recent papers 10, 11, 12

and in this work.

The hydriding is faster and the maximal H/M value is achieved at lower

temperature for HfNi (1.05 at 373K) in comparison to Hf2Ni (0.38 at 748K). In Hf2Ni7

the hydriding is also very fast (comparable to HfNi) but the storage capacity is very low

(H/M = 0.04 at 373K).

The obtained kinetic parameters of the hydrogen absorption reactions indicate the

improvement of the properties of Hf-Ni intermetallics with increasing of Ni content. This

is mostly pronounced up to 50 at % of Ni, since the rate constant is an order of magnitude

higher in HfNi as compared to Hf2Ni. Further increasing of the Ni content leads to some

improvements in the absorption kinetics (absence of incubation period), but doesn’t

drastically effect the rate of absorption. This is understandable since the rate of the

absorption process is no longer limited by surface reactions (Hf2Ni7), so we may say that

the catalytic effect of Ni is fulfilled already at 50 at % of Ni in the Hf-Ni systems. As for

the maximal capacity for storing hydrogen, no straightforward conclusions about the

influence of the content of each metal (Hf, Ni) can be made from the obtained results.

10

The increase of Ni content, since Ni is non-hydride forming metal, would be expected to

lead to a decrease of the maximal storage capacity of the intermetallic phase, but this is

not case when going from Hf2Ni to HfNi. However, the influence of the structure of the

intermetallic compound is also very important factor and can be a reason for the very

different hydrogen absorption behavior of the Hf-Ni intermetallics.

On the basis of literature data [7], also confirmed by the investigation in this

work, the hydrogen absorption process leaves the space group unchanged for all the

investigated intermetallics. However, the hydrogen pressure of 1bar along with the

temperature ranges used in this investigation falls within the solid solubility region for

both Hf2Ni and Hf2Ni7 (there is no evidence in literature of hydrides forming in these

compounds even at higher pressures). In contrast, in HfNi at 373K and at lower

temperatures, hydrides are formed even at ambient pressure. In order to explain the

observed differences, the structural, electronic and bonding properties of Hf-Ni

intermetallics calculated using the full-potential linearized augmented plane waves

method (FP-LAPW), were utilized and the relevant parameters are presented in Table 3.

Table 3. Hydrogen absorption capacities (H/M) - obtained experimentally; Unit cell

volumes (V), enthalpies of formation (dHf) and cohesive energies (Ec) - calculated using

FP-LAPW code; dHf for HfNi hydrides refer to enthalpies of formation from pure

intermetallic compound and molecular hydrogen.

Interm

etallic

compo

und

at%

Ni

H/M (T,K) structur

e type

space

group

V

(Å3)

dHf

(kJ/mol

of atoms)

Ec

(eV/atom)

Hf2Ni 33.33 0.38

(748 K)*

CuAl2

(C16)

I4/mcm

216.3* - 31.8 * 8.24*

HfNi 50.00 1.05

(373K)**

CrB Cmcm 132.2* - 53.5 * 7.71*

HfNiH CrB Cmcm 141.9* - 17.3*

HfNiH3 CrB Cmcm 156.8* - 34.6*

Hf2Ni7 77.78 0.04

(373K)**

C2/m 462.76** -45.6** 5.98**

*[6,7,8]

**- this work

It is found that the cohesive energies per atom of intermetallic compound get

smaller as the content of nickel increase, but the enthalpy of formation doesn’t follow this

trend. The superior hydrogen absorption ability of HfNi, compared to Hf2Ni and Hf2Ni7,

is in agreement with the Miedema’s “reverse stability” rule [26], since the enthalpy of

formation of HfNi is the largest among examined intermetallics. However, no significant

difference in thermodynamic parameters is observed and this implies that structural

parameters might be the key factor determining maximal hydrogen storage ability in this

case, and that the intermetallic compounds of CrB structure are advantageous materials

for hydrogen storage.

Therefore, in accordance with all those experimental and theoretical results, HfNi

11

is the best hydrogen storage material among the investigated intermetallics.

4. Conclusion

The hydrogen absorption kinetics has been investigated in the temperature ranges

373- 573K for HfNi and 323-473K for Hf2Ni7 at the constant pressure of 1bar.

For HfNi it has been observed that, at all the applied temperatures, the repeating

of the hydriding process (followed by dehydriding), results in the improvement of the

absorption process kinetics. The improvement of the kinetic parameters of the hydrogen

absorption process can be attributed to the decrease in HfNi particle size upon cycling.

Furthermore, this process results in the expansion of the lattice and consequently in the

additional crashing of particles and exposure of larger surface areas of HfNi to the

hydrogen gas.

For HfNi at 373K, two stages with different rate constants are present in the

absorption process, which is consistent with formation of two different hydride phases.

At higher temperatures, after very fast reaction during the first 10 seconds, it has been

found that for all the temperatures applied, 3D diffusion is the limiting step. The

activation energy for diffusion-controlled absorption was found to be (33±7) kJ/mol. For

Hf2Ni7 at 373K (as well as at 473K) limiting step is 3D diffusion.

The maximal H/M values are 1.05 and 0.04 for HfNi and Hf2Ni7 respectively, at

373K.

DFT calculations have been performed in order to determine the structural,

bonding and thermodynamical properties of Hf2Ni7. The obtained results have been

compared with our earlier investigation of HfNi and Hf2Ni. We have found that HfNi is

the best hydrogen storage material among the investigated compunds from the HfNi

phase system.

Acknowledgement

The Ministry of Science and Technological Development of the Republic of Serbia

provided the financial support for this study through the Project No.171 001. Authors

also thank Dr Vesna Vasić for helpful disscusion on kinetics.

References

[1] Bououdina M, Grant D, Walker G. Review on hydrogen absorbing materials

structure, microstructure and thermodynamic properties. Int J Hydrogen Energy

2006;31:177–82.

[2] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid

hydrogen storage. Int J Hydrogen Energy 2007;32:1121– 40.

[3] Muthukumar P, Groll M. Metal hydride based heating and cooling systems: A review.

Int J Hydrogen Energy 2010;35:3817-31.

[4] Xiangyu Zhao, Liqun Ma. Recent progress in hydrogen storage alloys for nickel/metal

hydride secondary batteries. Int J Hydrogen Energy 2009;34:4788-96.

[5] Van Essen RM, Buschow KHJ. Hydrogen absorption in various zirconium- and

hafnium- based intermetallic compounds. J Less-Common Met 1979;64:277-84.

12

[6] Nemirovskaya IE, Alekseev AM, Lunin VV. Thermodynamics of processes of

hydrogen sorption by hydrides of intermetallic compounds of CrB structural type. J

Alloys Compd 1991;177:1-15.

[7] Mukai D, Miyata H, Aoki K. Hydrogen absorption and desorption properties of Hf

based intermetallic compounds. J Alloys Compd 1999;293-295:417-20.

[8] Kocjan A, McGuiness P.J, Kobe S. Desorption of hydrogen from Ti–Zr–Ni hydrides

using a mass spectrometer. Int J Hydrogen Energy 2010;35:259-65.

[9] Kuriiwa T, Maruyama T, Kamegawa A, Okada M. Effects of V content on hydrogen

storage properties of V-Ti-Cr alloys with high desorption pressure. Int J Hydrogen

Energy 2010;35:9082-7.

[10] Stojić DLj, Kumrić SV, Belošević-Ĉavor JN, Radaković JS, Cekić BDj, Mentus SV.

Hydridic, thermodynamic and kinetic properties of Hf2Ni intermetallic phase. Int J

Hydrogen Energy 2009;34:3764–70.

[11] Radaković J, Ćirić K, Belošević-Ĉavor J, Koteski V. First-principles study of the

thermodynamic properties and electronic structure of compounds from Hf–Ni phase

system. Comp Mat Sci 2010; 49: 55–9.

[12] Ćirić KD, Koteski VJ, Stojić DLj, Radaković JS, Ivanovski VN, HfNi and its

hydrides – First principles calculations. Int J Hydrogen Energy 2010;35;3572–77.

[13] Cekić B, Umićević A, Ivanovski V, Belošević-Ĉavor J, Koteski V, Rongwei H et al.

Thermal evolution of the electric field gradient at 181

Ta in αHNi. Hyperfine Interactions

2010;196(1-3):339-47.

[14] Bsenko L. The Hf-Ni and Zr-Ni systems in the region 65 – 80 at.% Ni. J Less-

Common Met 1979; 63(2):171-9.

[15] Scientific Group Thermodata Europe (SGTE), and Franke P, Neuschütz D. Hf-Ni

(Hafnium - Nickel). In: Franke P, Neuschütz D, editors. Thermodynamic Properties of

Inorganic Materials, Berlin: Springer-Verlag; 2007, IV/19B5.

[16] Madsen GKH, Blaha P, Schwarz K, Sjöstedt E, Nordström L. Efficient linearization

of the augmented plane-wave method. Phys Rev B 2001;64:195134-1-9.

[17] Schwarz K, Blaha P, Madsen GKH. Electronic structure calculations of solids using

the WIEN2k package for material sciences. Comput Phys Commun 2002;147:71-6.

[18] Blaha P, Schwarz K, Madsen GKH, Kvasnicka D, Luitz J. WIEN2k- An Augmented

Plane Wave Plus Local Orbitals Program for Calculating Crystal Properties, User’s

Guide, Vienna:Vienna University of Technology; 2001.

[19] Perdew JP, Burke K, Ernzerhof M. Generalized Gradient Approximation Made

Simple. Phys Rev Lett. 1996;77:3865-8

[20] Simonović BR, Mentus S, Šušić MV. Examination of the kinetics of Zr1.02Ni0.98 alloy

hydriding. J Serb Chem Soc 1999;64:745–52.

[21] Libowitz CG. The solid state chemistry of binary metal hydrides. New York: W.A.

Benjamin;1967.

[22] Wang X, Zhao Y, Wang Y, Houc H, Zeng W. Kinetics of hydrogen

absorption/desorption in TC21 alloy. J Alloys Compd 2010; 490:562–7.

[23] Selhaoui N, Gachon JC, Hertz J. Enthalpies of formation of some solid hafnium

nickel compounds and of the Ni-Rich HfNi liquid by direct reaction calorimetry. Met and

Mat Trans B 1992;23:815-9.

[24] Kokalj A. XCrySDen – a new program for displaying crystalline structures and

electron densities. J Mol Graph Model 1999;17(3–4):176–9.

13

[25] Dattagupta JK, Schubert K. Isotypic relation of zirconium - nickel (Zr2Ni7) and

hafnium - nickel (Hf2 Ni7), Zeitschrift für Metallkunde 1973;64:789-92.

[26] 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.

Figure caption

Fig. 1 - Influence of subsequent hydriding to hydrogen absorption in HfNi powder at

373K. (1)-(4) denotes first to fourth absorption process, respectively.

14

Fig. 2 - SEM images of HfNi: (a, b) before absorption and (c, d) after several

absorption/desorption cycles at 373K.

Fig. 3 - XRD spectrum of HfNi at room temperature: before hydrogen absorption

(bottom), and after hydrogen absorption at 1bar, 373K (top)

Fig. 4 - Hydrogen absorption isotherms (at 1bar) as a function of time for HfNi (a) and

Hf2Ni7 (b).

Fig. 5 - Hydrogen absorption isotherms (up) and reaction mechanisms of different stages

(down) at 373K for HfNi* (a) and Hf2Ni7 (b).*- after additional 20 hydriding/dehydriding

cycles in respect to Fig. 1.

Fig. 6 - Hydrogen absorption in HfNi at different temperatures, (a, b, c) - reaction

mechanisms; (d) - Arrhenius plot for 3D diffusion.

Fig. 7 - Unit cell of Hf2Ni7; Larger balls present Hf atoms and smaller Ni atoms. The

picture was obtained using XCrySDen [24]