Electrochemical hydriding of amorphous and nanocrystalline TiNi-based alloys

Article abstract of DOI:10.1016/j.jallcom.2006.09.071

Amorphous and nanocrystalline TiNi_1-xM_x (M = Co, Fe, Sn; x = 0 and 0.2) alloys were synthesized by mechanical alloying. Powder particles with an average diameter of about 5 μm were produced. X-ray diffraction analysis showed that aft

Full text of DOI:10.1016/j.jallcom.2006.09.071

Journal of Alloys and Compounds 441 (2007) 197–201
Electrochemical hydriding of amorphous and
nanocrystalline TiNi-based alloys
∗
Boris Drenchev, Tony Spassov
Department of Chemistry, University of Sofia “St.Kl.Ohridski”, 1 J. Bourchier Str., 1164 Sofia, Bulgaria
Received 13 July 2006; received in revised form 19 September 2006; accepted 19 September 2006
Available online 17 October 2006
Abstract
Amorphous and nanocrystalline TiNi1−x
x
M (M = Co, Fe, Sn; x = 0 and 0.2) alloys were synthesized by mechanical alloying. Powder particles
with an average diameter of about 5 ␮m were produced. X-ray diffraction analysis showed that after 10.5 h of milling the initial mixture of metal
powders transformed into amorphous or fine nanocrystalline material. DSC analysis displayed high thermal stability of the as-milled amorphous
alloys. Electrochemical hydrogen charge/discharge measurements of the as-milled as well as annealed alloys were carried out at galvanostatic
conditions. It was found that among the alloys studied TiNi0.8Fe0.2 revealed the highest discharge capacity of about 65 mAh/g in the as-milled state
◦
and 80 mAh/g after annealing at 500 C.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Ti alloys; Mechanical alloying; Amorphous materials; Ni-MH
1
. Introduction
improving the hydrogen absorption rate, due to the reduction of
the particle and grain size [4,8,9]. The influence of Ni content
on the electrochemical properties of ball-milled nanocrystalline
TiFe1−xNix (x = 0, 0.25, 0.5, 0.75, 1) alloys has been investigated
and a maximum discharge capacity has been found at x = 0.75
(155 mAh/g) [4,10].
In the present work, the influence of the alloying ele-
ments (Co, Fe, Sn) and microstructure on the electrochemical
hydrogen capacity and cycle life of TiNi-type alloys has been
studied.
Titanium-based alloys are promising materials for hydrogen
storage [1–4]. AB-type TiFe and TiNi alloys react reversibly
with hydrogen and form ternary hydrides. Some of these mate-
rials have shown excellent properties as reversible negative
electrode for Ni-MH batteries. TiNi-based alloys reveal notable
electrochemical hydrogen capacity (245 mAh/g [5,6]), low
specific weight and good oxidation resistance, but the
charge/discharge kinetics are slow. TiFe alloys, despite of
their high hydrogen capacity at room temperature (up to
2
H/f.u.), have also limited application in batteries due to poor
2. Experimental part
absorption/desorption kinetics and complicated activation [4].
Replacement of Fe/Ni or Ti by other transition metals is one
of the approaches used to improve the activation of this type
of alloys [4,7]. Cuevas et al. [7] have shown that marten-
sitic (Ti0.64Zr0.36)Ni exhibits much higher reversible capacity
Powders of Ni (99.5%), Ti (99.7%), Co (>99%), Fe (>99%) and Sn (99.8%)
in appropriate amounts were mixed together in the reactors of high-energy plan-
etary mill to produce TiNi, TiNi0.8Co0.2, TiNi0.8Fe0.2 and TiNi0.8Sn0.2. Alloying
started after 5 h of milling at ball to powder mass ratio of 9:1 under argon atmo-
sphere (n-heptane) and ended after 10.5 h. After each 1 h of continuous milling
30 min relaxation time was applied. The alloys powders were studied by X-ray
(
330 mAh/g) than the austenitic phase (85 mAh/g), but the cycle
diffraction (XRD) using Cu K␣ radiation, scanning electron microscope (SEM,
JEOL-5510), differential scanning calorimeter (DSC, Perkin-Elmer) and gal-
vanostate “KRIONA-KR515”.
life of the last is longer.
Ball milling is a suitable method to produce amorphous and
nanocrystalline Ti-based alloys and is another effective way for
The as-milled and annealed materials were used to prepare metal-hydride
electrodes by mixing 100 mg alloy with 30 mg teflonysed carbon black
2
(
VULKAN 72 with 10 wt.% PTFE). The mixture was pressed at 1240 kg/cm .
∗
Corresponding author.
E-mail address: tspassov@chem.uni-sofia.bg (T. Spassov).
An electrode with a shape of pellet with a diameter of about 10 mm and thick-
ness of 1 mm was prepared. The electrode was charged and discharged in a
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.09.071

1
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B. Drenchev, T. Spassov / Journal of Alloys and Compounds 441 (2007) 197–201
Fig. 1. SEM micrographs of the as-milled TiNi alloys.

B. Drenchev, T. Spassov / Journal of Alloys and Compounds 441 (2007) 197–201
three-electrode cell in 30 wt.% water solution of KOH at room temperature. The
199
reference electrode was Hg/HgO and the counter electrode was a nickel net. The
charge and discharge current was 100 and 50 mA/g, respectively.
3
. Results and discussions
In the present study four TiNi-based alloys (TiNi,
TiNi0.8Co0.2, TiNi0.8Fe0.2 and TiNi0.8Sn0.2) were prepared by
mechanical alloying (MA). SEM analysis displayed particles
size in the range of 1–15 ␮m with an average particle size of
about5 ␮m, Fig. 1. Agglomeratesofsmallerparticles(<2–3 ␮m)
were also detected. At higher magnifications a lamellar particle
structure and a presence of cleavages was observed (Fig. 1).
Noticeable difference in the particles size distribution and mor-
phology of the different alloys was not detected.
X-ray diffraction patterns of the as-milled alloys are pre-
sented in Fig. 2. The alloy with Sn shows one broad diffraction
Fig. 3. Charge and discharge curves of TiNi0.8Fe0.2 alloy (second cycle).
◦
◦
peak around 41.5 (2θ) with a shoulder at about 36 , whereas
◦
◦
the other three alloys, except the peaks at 36 and 42.5 , have an
◦
additional less intensive broad peak at 61 . On the base of the
XRD results it can be concluded that the alloying has been real-
ized successfully for all compositions studied. Although, due
to the very large width of the diffraction peaks, it is difficult to
assign them to those of exact phases they are all positioned in
the range of Bragg angles (2θ) of the cubic and monoclinic TiNi
phases. The microstructure of the as-milled materials is amor-
phous or extremely fine nanocrystalline. It was found that the
addition of Sn favors the grain size refinement of the TiNi alloys
during ball milling.
The electrodes prepared from the as-milled alloys were
charged for 1 h at current density of 100 mA/g and discharged
at 50 mA/g to cut-off potential of 650 mV (EHg/HgO − EMH)
for TiNi, TiNi0.8Co0.2 and TiNi0.8Fe0.2 and 500 mV for
TiNi0.8Sn0.2. Typical charge and discharge curves can be seen in
Fig. 3. The charging process occurred in the potential range of
about 860–920 mV—the linear part of the charging curve. It was
found that the hydrogen charging process practically ends when
the potential establishes a constant value. Once the constant
potential value fixes hydrogen bobbles evolution takes place.
The discharging plateau was not observed for any of the com-
positions. The capacity as a function of charge/discharge cycle
Fig. 4. Capacity as a function of cycle number for the different as-milled com-
positions.
number is presented in Fig. 4. A discharge capacity increase
with the cycle number is observed for the TiNi, TiNi0.8Co0.2
and TiNi0.8Fe0.2 alloys and a slow capacity decrease for the
TiNi0.8Sn0.2 alloy. Table 1 presents the initial and the capacity
at the 11th discharge cycle for the four alloys studied. The iron-
containing alloy possesses the highest discharge capacity and
this with tin—the lowest. It is necessary to be mentioned that
the type of the curves “discharge capacity versus cycle number”
depends on the alloy composition as well as on the electrode
Table 1
Initial capacity and the capacity at the 11th discharge cycle for the alloys studied
Composition
Initial capacity
mAh/g)
Capacity (mAh/g)
at 11th cycle
(
TiNi
39
23
48
36
52
56
60
30
TiNi0.8Co0.2
TiNi0.8Fe0.2
TiNi0.8Sn0.2
Fig. 2. X-ray diffractograms of the as-milled alloys.

2
00
B. Drenchev, T. Spassov / Journal of Alloys and Compounds 441 (2007) 197–201
Fig. 5. DSC analysis of the as-milled alloys.
Fig. 7. Capacity as a function of cycle number for the different annealed
alloys.
preparation technique (mass ratio between alloy and carbon,
amount of PTFE, degree of homogenization, electrode thick-
Table 2
Initial capacity and the capacity at the 50th discharge cycle for the alloys studied
ness and pressure). Electrodes prepared at different pressures
2
Composition (annealed)
Initial capacity
mAh/g)
Capacity (mAh/g)
at 50th cycle
(
900–1250 kg/cm ) revealed the highest capacity for the sample
(
compacted with the highest pressure.
In order to obtain information about the thermal behavior of
the as-milled hydrogen free alloys they were annealed in DSC
up to 500 C and then analyzed by XRD. Fig. 5 shows DSC
TiNi
4
67
83
79
44
TiNi0.8Co0.2
TiNi0.8Fe0.2
TiNi0.8Sn0.2
0.57
1.68
2.1
◦
curvesofthepowdersandthecorrespondingXRDpatternsofthe
annealed materials can be seen in Fig. 6. There are two endother-
mic effects for all as-milled alloys. There could also be some
exothermic effects combined with the endothermic reactions,
but it is difficult to separate the overlapping processes from the
calorimetric curves measured. The enthalpy changes associated
with the solid state reactions observed by DSC are substantial
ples varies between 6 and 8 nm for the alloys studied. A detailed
study on the nature of the solid state transformations occurring
in the as-milled samples during heating is underway.
The hydrogen absorption properties of the annealed materi-
◦
als (500 C for short time) are different compared to those in the
(
>150 J/g). It is interesting to note that the processes associated
as-milled state. Activation processes and capacity increase were
observed, Fig. 7. It can be seen that for TiNi and TiNi0.8Co0.2
alloys the activation lasts about five charge/discharge cycles
and the maximum capacities attained are 67 and 83 mAh/g,
respectively. The compositions TiNi0.8Fe0.2 and TiNi0.8Sn0.2
have longer activation period and their capacity increase start
after the 15th cycle. The maximum value of the capacity for the
TiNi0.8Fe0.2 and TiNi0.8Sn0.2 is ∼80 and ∼44 mAh/g, respec-
tively. The discharge capacities of the annealed TiNi0.8Co0.2
and TiNi0.8Fe0.2 are almost the same, as the first one is slightly
higher. Table 2 presents the initial capacity and the capacity at
the fiftieth discharge cycle for the alloys studied.
with the DSC effects do not change the microstructure signif-
icantly, i.e. the annealed materials are still in a nanostructured
state, Fig. 6. The average nanocrystals size of the annealed sam-
4. Conclusion
Ni-substituted amorphous and nanocrystalline TiNi alloys
(
TiNi1−xMx, M = Co, Fe, Sn; x = 0 and 0.2) were synthesized by
high-energy ball milling. A favoring effect of Sn on the grain size
refinement of the alloy during milling was found. The as-milled
alloys revealed high thermal stability. Significant microstruc-
tural coarsening of the nanostructured alloys starts at annealing
◦
temperatures above 500 C only.
Fig. 6. XRD diffarctograms of the annealed alloys.

B. Drenchev, T. Spassov / Journal of Alloys and Compounds 441 (2007) 197–201
201
The electrochemical hydrogen capacity of the as-milled
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TiNi0.8Fe0.2 alloyexhibitsthehighestcapacityinas-milledstate,
but after annealing both Fe and Co-containing materials show
almost the same capacity of 80–85 mAh/g.
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