Comparative study on hydrogenation properties of Pd capped Mg and Mg/Al films
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Transcript of Comparative study on hydrogenation properties of Pd capped Mg and Mg/Al films
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 5
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Comparative study on hydrogenation properties of Pd cappedMg and Mg/Al films
Pragya Jain a,*, Ankur Jain a, Devendra Vyas a, D. Kabiraj b, S.A. Khan b, I.P. Jain a
aCentre for Non-Conventional Energy Resources, University of Rajasthan, 14 Vigyan Bhawan, Jaipur 302055, Rajasthan, Indiab Inter University Accelerator Centre, New Delhi 110 067, India
a r t i c l e i n f o
Article history:
Received 15 February 2011
Received in revised form
23 February 2011
Accepted 25 February 2011
Available online 29 March 2011
Keywords:
Metal hydrogen systems
Mg thin films
Hydrogen content
ERDA
* Corresponding author. Tel./fax: þ91 141271E-mail addresses: [email protected]
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.02.144
a b s t r a c t
Recent emergence of Mg as a promising hydrogen storage material with 7.6 wt% hydrogen
encourages study on its thin films to understand physics of storage mechanism. The
present study investigates the variations in hydrogen storage properties of Pd sandwiched
Mg films upon introduction of Al layer. Multilayered stack of Pd/Mg/Pd and Pd/Al/Mg/Pd
were grown on Si substrate using vapor deposition method and further hydrogenated at
150� C under 2 bar H2 pressure for 2 h. Elastic Recoil Detection Analysis (ERDA) technique
with 120 MeV Ag9þ ions was used to obtain hydrogen concentration versus incident ion
fluence. ERDA study reveals that Pd/Mg/Al/Pd films absorb 6.01 � 1018hydrogen atoms/cm2
in comparison to 4 � 1017 atoms/cm2 absorbed by Pd/Mg/Pd system.
Atomic force Microscopy (AFM) and X-ray Diffraction (XRD) techniques were utilized to
analyze the morphological and structural changes in the hydrogenated films. Results
indicate that addition of Al to the base system has led to the formation of Mg(AlH4)2 along
with MgH2 causing an increment in the hydrogen storage capacity and reduction in the
oxygen content.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction difficult to activate. Various efforts have been made to over-
Hydrogen storage for mobile applications such as fuel cell
driven cars has been a very active research area for decades.
Magnesium has been considered as a strong candidate for
these applications because of its high gravimetric (w 7.6 wt%
for MgH2) and volumetric (w 150 kg H2/m2 MgH2) efficiency,
light weight, low cost and abundance on earth crust. Unfor-
tunately, the practical application of Mg is limited by its slow
hydriding/dehydriding kinetics even at high temperatures.
The slow kinetics is attributed to (i) low dissociation rate of
hydrogen onMg surface due to high energy barrier (Eaw 72 kJ/
mol H2) [1] and (ii) slow diffusibility of hydrogen atoms in
MgH2 phase (DH w 10�16 cm2/sec) [2]. Additionally, the Mg
metal is very sensitive to contamination, which makes it
1049., [email protected] (P2011, Hydrogen Energy P
come the thermodynamic and kinetic barriers by alloying Mg
with various elements to alter the crystal structure of the
hydride [3], reducing the particle and grain size via mechan-
ical milling with e.g nanostructured carbon [4], or by addition
of catalytic additives such as Ni, LaNi5 or LaeMgeNi alloys
during milling [5e7].
A possible and better solution would come from the addi-
tion of light and cheap elements like Al. Guo and co-workers
[8,9], have shown that Al addition to MgH2 reduces the
stability of the hydride leading to an improvement in the
dehydrogenation conditions. The heat of formation predicted
for the MgH2þAl system is 28 kJ/mol H2 [10]. Additionally, it
has been found that the thermodynamics and kinetics of
MgeAl as compared to Mg are improved along with resistance
. Jain).ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 53780
toward oxygen contamination [11]. Moreover, there is
a possibility of formation of complex hydrides, such as alanate
compounds, in particular Mg(AlH4)2, which has a storage
capacity of 9.3 wt%.
The best way to understand the reaction mechanism of
MgeAl system is to prepare thin film by sputtering or evapo-
ration method. As in thin film structure, it is easy to control
the thickness, composition, interface and structural order.
Moreover, the co-operative phenomena and the spill over
effects can be induced by synthesis of sandwich structured
films, leading to an improved kinetics [12e14]. Ferrer et al. [15]
investigated the Mg/Al layer sandwiched between Pd/Fe(Ti)
and observed improved storage capacity by the formation of
MgxAly intermetallic. Some authors have reported the
formation of Mg(AlH4)2 from MgeAl thin films under different
conditions [16e18].
This papermainly focuses on the hydrogenation properties
of Mg and Mg/Al thin films sandwiched between Pd layers.
The increase in the hydrogen content has been studied by
Elastic Recoil Detection Analysis (ERDA). X-ray diffraction and
Atomic force microscope have been used to investigate the
structural and morphological changes.
20 30 40 50
0
200
400
600
800
1000
1200
*
*
Δ
a
Inte
nsity
2θ
Δ
Pd*Mg
2. Experiment
2.1. Thin film preparation technique
The thin film sample of Pd/Mg/Pd was prepared by vapor
deposition method at a base pressure of 10�7 mbar. The
evaporation unit is equipped with 3 KW electron gun, for Pd
deposition and two thermal evaporation units used forMg and
Al deposition. 150 nm Mg layer is sandwiched between 20 nm
layers of Pd to protect it against oxidation and to promote
hydrogen dissociation. The deposition rates ofMg and Pdwere
kept constant at 0.15 nm/s and 0.1 nm/s respectively. In the
second sample 50 nm Mg layer is replaced by 50 nm Al layer
deposited by resistive heating at a deposition rate of 0.15 nm/
s, to form Pd/Al/Mg/Pd system. Thus, in the present study two
systems are being investigated (i) as-deposited (AD1) and
hydrogenated (HD1) Pd/Mg/Pd system and (ii) as-deposited
(AD2) and hydrogenated (HD2) Pd/Al/Mg/Pd system.
1500
2000
2500
3000
3500
4000
*
ΔPd
b
Inte
nsity
Δ Mg* Al
2.2. Thin film hydrogenation technique
Hydrogenation of thin film samples was carried out at 150� C
and 2 bar H2 pressure for 2 h in the system described by
Agarwal et al. [19]. Three cycles of hydrogen absorption/
desorption were performed to ensure complete hydrogena-
tion of the films.
20 30 40 50
0
500
1000
*
2θ
Fig. 1 e XRD spectra of as-deposited (a) Pd/Mg/Pd and (b)
Pd/Al/Mg/Pd samples.
2.3. Structural characterization using GI-XRD
The structures of the as-deposited and hydrogenated samples
were studied by GI-XRD technique using monochromated
CuKa radiation of wavelength 1.54060 A with model Brucker
DX 8-Advance.
The spectra were recorded in the 2q range of 20e50� with
scan speed of 0.5�/min and step width of 0.02�. The average
crystallite dimension DP (nm) was calculated using the
formula:
Dp ¼ 0:9lb1=2cosq
(1)
where l is the X-ray wavelength, q is the Bragg Diffraction
angle and b1/2 is the FWHM of the peak after correction for the
instrument broadening.
2.4. Morphological characterization using AFM
The surface morphology of all the samples have been inves-
tigated by AFM (Nanoscope IIIE model from Digital Instru-
ments, USA), in contact mode at room temperature. The scan
area and rate were kept as 5 � 5 mm and 1.526 Hz respectively.
2.5. Hydrogen content measurement using ERDA
Elastic recoil detection analysis (ERDA) measurements for
areal concentration of hydrogen (NH in atoms cm�2) in as-
deposited and hydrogenated films of both systems were
carried out at Material Science beam line in IUAC, New Delhi.
Silver (Ag9þ) beam of energy 120 MeV and current 7e9 nA was
20 30 40 50
50
100
150
200
250
300
350
O
O
Δ
Δ
*
a
2θ
Inte
nsity
MgH2
Δ
Mg5Pd2O
Pd*
PdH0.7
20 30 40 50
0
100
200
300
400
500
600
700
800
Inte
nsity
2θ
ΔΔ
Δ
*
*
Δ
Δ
MgH2
Δ
Pd*Mg(AlH4)2
Al
b
Fig. 2 e XRD spectra of hydrogenated (a)Pd/Mg/Pd and (b)
Pd/Al/Mg/Pd samples.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 5 3781
obtained from 15UD Accelerator. ERDA experiment was per-
formedwith a beamof spot size 1� 1mm2 incident at an angle
of 20�with respect to sample under a base pressure of
4.5 � 10�6 mbar. The H-recoils from the film were detected in
a silicon surface barrier detector (SSBD) kept at 30� recoil
angle, assembled with a 1.5 mm polypropylene stopper foil in
front of it to stop other recoils. The recoils of oxygen present
Table 1 e DebyeeScherer analysis for crystallite sizecalculation of as-deposited and hydrogenated samples.
Specifications Element 2q b1/2 DP(nm)
As-deposited Mg 34.4� 0.228 47.91
Al 38.47� 0.203 43.38
Pd(1) 40.14� 0.332 26.62
Pd(2) 46.84� 0.947 9.55
Hydrogenated MgH2 27.98� 0.229 37.26
Al 38.47� 0.606 24.40
Pd(1) 40.14� 0.495 17.87
PdH0.7 46.7� 1.606 5.63
as impurity in the films were detected in isobutane gas filled
detector placed at 45� recoil angle. The areal concentration of
hydrogen (H) (NH atoms cm�2 � 5%) atoms were calculated
using the following equation:
NH ¼ Ysina
NpdsdU
U
(2)
where, Y is the integral counts obtained by the recoil energy
spectra, a is the target tilt angle,U is the solid angle subtended
by the detector and ds/dU is the Rutherford recoil cross
section. The fluence-dependent NH was estimated from the
on-line data, taken in event-by-event mode [20].
3. Result and discussion
3.1. Effect of hydrogenation on structural properties
In Fig. 1(a) and (b), the patterns of as-deposited samples, AD1
and AD2 respectively are presented. In both the samples,
peaks at around 34.4�, 40.19� and 46.84� characteristic of pure
Mg [002], Pd [111] and [200] respectively were observed. Thus
the profile reveals that the Mg grains prefer the c-axis orien-
tation while no preferred orientation occurs for Pd. No peak
corresponding to MgO was observable, suggesting that the
Fig. 3 e AFM images of as-deposited and hydrogenated Pd/
Mg/Pd samples.
Table 2 e Particle size and roughness calculation fromAFM images of as-deposited and hydrogenated samples.
Sample Particle Size (nm) Roughness (nm)
Pd/Mg/Pd 410.16 31.37
Pd/Mg/PdeH 341.80 38.05
Pd/Mg/Al/Pd 537.11 48.53
Pd/Mg/Al/PdeH 419.92 67.34
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 53782
sandwiched structure effectively protects Mg from oxidation.
In AD2, an additional peak at around 38.4�, characteristic of Al
[111] is also an observant. The XRD pattern of Pd/Mg/Pd (HD1)
sample, exposed to 2 bar H2 at 150� C for 2 h is shown in
Fig. 2(a). It was observed that hydrogenation leads to the
disappearance of pure magnesium [002] peak and appearance
ofmagnesium hydride (MgH2) [110] and [101] peaks, indicating
the transformation from hexagonal close packed Mg to
tetragonal MgH2 phase. The X-ray peak related to Pd hydride
(PdH0.7) was also detected, however most of the Pd remains
unhydrogenated as evident from the presence of an intense
peak corresponding to pure Pd element. This is in agreement
with several reports [21]. However, a broadening in the Pd
peak is noticed, which is caused by particle/grain refining and
lattice strains introduced by several cycles of hydrogenation.
In addition, the formation of MgePd intermetallic (Mg5Pd2)
due to intermixing of Mg and Pd occurring at the interfacial
region was also observed. XRD findings of hydrogenated Pd/
Al/Mg/Pd (HD2) shows the appearance of complex hydride
Mg(AlH4)2 peak along with MgH2. Garcia et al. [22] have made
similar studies on Mg/Al systems of varying composition and
have suggested the formation of complex hydride phase with
composition above 35% Mg but they were unable to observe
any peak corresponding to this hydride in the XRD pattern.
The XRD pattern (Fig. 2(b)) in this work gives a clear evidence
of the formation of complex hydride due to the reaction
Fig. 4 e AFM images of as-deposited and hydrogenated
Pd/Al/Mg/Pd samples.
between Al and Mg at Mg/Al interface under 150� C and 2 bar
H2 pressure, while the Mg atoms located far away from the
interface contributes to the formation of magnesium hydride.
Reduction in Al peak area further supports these observations.
Further, Al addition benefited in the form suggests that it
eliminates the formation of MgxPdy intermetallic phase which
is crucial for hydrogenation, as shown in Fig. 2(b). The
DebyeeScherer analysis for both Pd/Mg/Pd and Pd/Al/Mg/Pd
systems shows a decrease in the crystalline size upon
hydrogenation, which is given in Table 1.
3.2. Effect of hydrogenation on morphological properties
AFM surface analysis provides additional evidence that
hydrogenation process drastically changes surface topog-
raphy. Figs. 3 and 4 shows the AFM images of (a) as-deposited
and (b) hydrogenated Pd/Mg/Pd and Pd/Al/Mg/Pd films
respectively. Table 2 shows that hydrogen loading in thin film
samples causes a reduction in particle size leading to an
enhanced roughness.
3.3. Hydrogen content measurement
The ERDA spectrum of hydrogenated Pd/Mg/Pd and Pd/Mg/Al/
Pd films taken during the first minute of the ERDA measure-
ments is shown in Fig. 5. The area under the hydrogen recoil
600 800 1000 1200
0
10
20
30
40
50
60
70
80
90
100
Energy/Channel (MeV)
H-C
ount
s (a
.u)
(Pd/Mg/Pd) (Pd/Mg/Al/Pd)
Fig. 5 e ERDA spectra of hydrogenated Pd/Mg/Pd (black
line) and Pd/Al/Mg/Pd (red line) samples taken during 1st
minute of the ERDAmeasurement.(For interpretation of the
references to color in this figure legend, the reader is
referred to the web version of this article.)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 5 3783
spectra can be used to obtain the hydrogen concentration NH
atoms/cm2 in the samples at a particular time during experi-
ment [23]. Using Eq. (2), amount of hydrogen absorbed by the
films (NH) under 150� C and 2 bar H2 pressure is calculated for
different ion doses. Fig. 6(a) and (b) represents the plot of NH
(atoms/cm2) v/s incident ion fluence (ions/cm2) after hydro-
genation for both the films. The decrease in the hydrogen
content is due to H loss on ion irradiation during ERDA anal-
ysis [24]. The data is fitted using equation:
NH ¼ NOexpð�sfÞ (3)
where NO is the initial concentration of hydrogen,atoms/cm2,
s is the hydrogen release cross section (cm2) and v is the ion
fluence (ions/cm2) [24]. This type of variation has been studied
by Gupta et al. [24]. According to them the interaction of the
ions with the sample releases hydrogen from a cylindrical
zone. After certain duration of ion beam exposure, the
damaged zone is modified due to hydrogen loss and ion beam
0.00E+000 2.00E+013 4.00E+013 6.00E+013
2.35385E17
6.39843E17
Hyd
roge
n co
nc. (
at/c
m2 )
Fluence(ions/cm2)
1
2
a
0.00E+000 2.00E+013 4.00E+013
6.39843E17
1.73927E18
4.72784E18
b
Hyd
roge
n co
nc. (
at/c
m2 )
Fluence (ions/cm2)
1
2
Fig. 6 e Variation of hydrogen concentration with fluence
of Ag9D 120 MeV ion beam irradiation on hydrogenated (a)
Pd/Mg/Pd and (b) Pd/Al/Mg/Pd films. Solid lines (red) are
linear fits of region 1 and 2 (see text). Error bars show
statistical error in the calculated regions.(For interpretation
of the references to color in this figure legend, the reader is
referred to the web version of this article.)
induced modifications. This leads to the existence of two
curves of different slopes explaining the nature of hydrogen
loss from the sample.
Todeterminethe initialhydrogenconcentration inPd/Mg/Pd
and Pd/Al/Mg/Pd hydrogenated samples, the graphs were
extrapolated to zero ion doses as explained by Singh et al.
[25]. ERDA result shows that the HD1 film absorbs 4.34
� 1017hydrogen atoms/cm2 whereas HD2 absorbs 6.01
� 1018hydrogen atoms/cm2. The present study shows that Al
layer enhances hydrogen content in Pd sandwiched Mg film to
a greater extend than that observed for Pd sandwiched Mg/Ni
andMg/Mg2Ni systems inourpreviouswork [26]. Followingmay
be the reasons responsible for this enhancement:
i) ERDA plots for O-content in hydrogenated Pd/Mg/Pd and
Pd/Mg/Al/Pd films are shown in Fig. 7(a) and (b) respec-
tively. Table 3 summarizes the atomic concentration of
hydrogen and oxygen atoms in both the samples. ERDA
0.00E+000 2.00E+013 4.00E+013 6.00E+013
1.00E+017
1.20E+017
1.40E+017
1.60E+017
1.80E+017
2
1
Oxy
gen
Con
c. (a
tom
s/cm
2 )
Fluence (ions/cm2)
a
0.00E+000 2.00E+013 4.00E+013
3.00E+016
6.00E+016
9.00E+016
1.20E+017
1.50E+017
1.80E+017
2.10E+017
2.40E+017b
Oxy
gen
Con
c. (a
tom
s/cm
2 )
Fluence (ions/cm2)
2
1
Fig. 7 e Variation of oxygen concentration with fluence of
Ag9D 120 MeV ion beam irradiation on hydrogenated (a)
Pd/Mg/Pd (reprint from ref. 27 with permission from
Elsevier) and (b) Pd/Al/Mg/Pd films. Solid lines (red) are
linear fits of region 1 and 2 (see text). Error bars show
statistical error in the calculated regions.(For interpretation
of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Table 3 e ERDA measurements of hydrogen and oxygencontent in hydrogenated Pd/Mg/Pd & Pd/Al/Mg/Pd films.
Sample Areal H-Content(atoms/cm2)
Areal O-Content(atoms/cm2)
Pd/Mg/PdeH 4.00 � 1017 2.83 � 1017
Pd/Mg/Al/PdeH 6.01 � 1018 1.80 � 1017
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 53784
findingsshowlessoxygencontent inPd/Al/Mg/Pdsystem
in comparison to thebase system, suggesting that adding
Al creates a compound with less oxygen concentration
probablydueto the formationof lessdenseoxide layere.g
amorphous alumina with improved hydrogen diffusion
properties compared to close packed MgO [27].
ii) Further, ERDA results well supported by XRD pattern
suggest that at 50%Mg thereoccursa reactionbetweenAl
and Mg at Mg/Al interface under 150� C and 2 bar H2
pressure which leads to the formation of complex
hydride while the Mg atoms located far away from the
interface contributes to the formation of magnesium
hydride.
iii) Al layer also prevents the formation of any MgePd inter-
metallic alloy which was formed in Pd/Mg/Pd system,
resulting in complete conversion of Mg to MgH2 phase.
4. Conclusion
The present study reports that hydrogenation properties of
Pd/Mg/Pd system can be enhanced by the introduction of Al
layer. The XRD patternwell supported by ERDA finding reveals
that the formation of complex hydride phase along with
magnesium hydride is responsible for increase in hydrogen
content from 4.34 � 1017 atoms/cm2 to 6.01 � 1018 atoms/cm2.
Acknowledgments
The authors are thankful to Inter University Accelerator
Centre (IUAC), NewDelhi, India for providing financial support
and permission for availing research facility under Project No.
4133. Special thanks to Mr. S.A.Khan, Scientist IUAC for
support during ERDA experiment.
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