Post on 29-Jan-2023
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.
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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]