Synthesis and electrochemical study of nanoporous Pd-Ag alloys for hydrogen sorption

Article abstract of DOI:10.1016/j.electacta.2010.09.060

We report on the synthesis of novel nanoporous Pd-Ag electrocatalysts using a facile hydrothermal method where the portion of Ag was varied from 0 to 40%. Scanning electron microscopy (SEM) was used to examine the morphologies of the prepared nanoporous materials. Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) were used to directly and indirectly characterize the composition of the formed Pd-Ag nanostructures. X-ray diffraction (XRD) analysis confirmed that the formed Pd-Ag nanomaterials were alloys with a face-centered cubic structure. Electrochemical methods were used to study the capacity and kinetics of hydrogen sorption into the nanoporous Pd and Pd-Ag alloys. The nanoporous Pd-Ag alloy with 20% silver possesses the highest capacity for the α phase hydrogen sorption, which is over 4 times higher than the pure nanoporous Pd. The combination of the enhanced α phase hydrogen sorption capacity and diminishing of the α- and β-phase transition makes the nanoporous Pd-Ag alloys promising for hydrogen selective membranes and hydrogen dissociation catalysts.

Full text of DOI:10.1016/j.electacta.2010.09.060

Electrochimica Acta 56 (2010) 61–67
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical study of nanoporous Pd–Ag alloys for hydrogen
sorption
Shuai Chen, Brian D. Adams, Aicheng Chen∗,1
Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on the synthesis of novel nanoporous Pd–Ag electrocatalysts using a facile hydrothermal
method where the portion of Ag was varied from 0 to 40%. Scanning electron microscopy (SEM) was
used to examine the morphologies of the prepared nanoporous materials. Energy dispersive X-ray spec-
troscopy (EDS), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) were used
to directly and indirectly characterize the composition of the formed Pd–Ag nanostructures. X-ray diffrac-
tion (XRD) analysis confirmed that the formed Pd–Ag nanomaterials were alloys with a face-centered
cubic structure. Electrochemical methods were used to study the capacity and kinetics of hydrogen sorp-
tion into the nanoporous Pd and Pd–Ag alloys. The nanoporous Pd–Ag alloy with 20% silver possesses
the highest capacity for the ␣ phase hydrogen sorption, which is over 4 times higher than the pure
nanoporous Pd. The combination of the enhanced ␣ phase hydrogen sorption capacity and diminishing
of the ␣- and ␤-phase transition makes the nanoporous Pd–Ag alloys promising for hydrogen selective
membranes and hydrogen dissociation catalysts.
Received 9 August 2010
Received in revised form
21 September 2010
Accepted 21 September 2010
Available online 25 September 2010
Keywords:
Hydrogen purification
Hydrogen storage
Pd–Ag alloys
Nanoporous material
Hydrothermal method
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Palladium and its alloys are classical materials for storing hydro-
gen above room temperature and show fast adsorption/absorption
Hydrogen, because of its large combustion heat (287 kJ/mole)
and its environmentally compatible by-product, water, is being
extensively researched as an alternative to fossil fuels [1–4]. Stor-
ing hydrogen in a gas form or handling hydrogen in a liquid state
at a critical low temperature (32.97 K) presents difficult challenges
for its practical usage. Hydrogen storage alloys provide one of the
best ways to store hydrogen compactly and safely as a consequence
of the high stability of their hydride [5–11]. Driven by the need
to develop hydrogen absorption materials, studies on the inter-
action between metals and hydrogen have become increasingly
important. Electrochemical study provides a feasible approach to
determine the thermodynamic and kinetic parameters of hydrogen
sorption, which is important for the development of new materials
for hydrogen storage. Most studies have been focused on finding
the optimal properties of the metallic materials in terms of fast
hydriding/dehydriding kinetics, large hydrogen-uptake capacity,
high cycle stability, and reasonable production costs [12–14]. Poor
reversibility and slow kinetics are the main problems of many metal
hydride systems such as MgH2 and NaAlH4 [3].
rates [13,15–17]. The palladium/hydrogen system has been inten-
sively investigated both in the gas phase and under electrochemical
conditions, due to its potential applications in hydrogen storage,
metal-hydride batteries and hydrogen purification [18–21]. How-
ever, the use of Pd as the sole metal hydride for hydrogen storage is
not practical because of its high cost and the hydrogen embrittle-
ment caused by the phase transition from ␣ to ␤ in pure palladium
[22]. Pd-based alloys, on the other hand, offer a class of attractive
materials for studying metal hydrides because of the high solubil-
ity and permeability of hydrogen compared to pure Pd as well as
the reduced cost if cheaper metals are added [23–29]. Great atten-
tion has been paid to nanostructured materials because of their
high surface areas and significantly different properties compared
to conventional materials with coarse grains [30–33]. For instance,
recent study has revealed that the electrochemical response of
hydrogen at Pd is critically dependent on the size and structure
of the Pd surface [34]. The analytical response can be easily tai-
lored in favor of either Had oxidation or Hab oxidation by nanoscale
tuning the coverage of Pd on Au nanoparticles. Nanostructured
materials show distinct advantages in their hydrogen uptake char-
acteristics compared with their bulk counterparts. Rapid hydrogen
diffusion has been reported to occur in nanostructured materials,
greatly improving the kinetics for hydrogen absorption and desorp-
tion [35–38]. It has been reported that nanoparticles exhibit dilated
lattices that would result in larger interstitial volumes for hydrogen
∗
Corresponding author. Tel.: +1 807 3438318; fax: +1 807 346 7775.
E-mail address: aicheng.chen@lakeheadu.ca (A. Chen).
1
ISE member.
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.09.060

62
S. Chen et al. / Electrochimica Acta 56 (2010) 61–67
storage and better storage characteristics [6,17]. The characteristic
hydrogen diffusion is automatically reduced due to the dilation of
the lattice, leading to faster kinetics for hydrogen absorption and
desorption [39,40].
photoelectron spectra were collected using a Thermo Scientific K-
Alpha XPS spectrometer. All the samples were run at a take-off
angle (relative to the surface) of 90◦. A monochromatic Al K␣ X-ray
source was used, with a spot area of 400 ␮m. Charge compensation
was provided and the position of the energy scale was adjusted to
place the main C 1s feature (C–C) at 284.6 eV. All data processing
was performed using XPSpeak software.
Although palladium and silver have been widely used in elec-
trochemical studies as electrodes, little attention has been directed
to the synthesis and study of nanostructured palladium and silver
alloys [41–44]. The relative low cost and strong structural prop-
erties of silver make it an attractive material to combine with
palladium for hydrogen purification and storage. In the present
study, for the first time, nanoprous Pd–Ag alloys with different
amounts of Ag, varied from 0 to 40 at.%, were synthesized using a
facile hydrothermal method. The behaviour and characteristics of
hydrogen absorption of the nanoporous Pd–Ag alloys were studied
and compared with the pure nanoporous Pd. The effects of sweep
rate, electrode potential, and composition of Pd–Ag on hydrogen
sorption have been systemically evaluated. Pd–Ag alloy is the most
commonly used material for hydrogen extraction in industry [12].
The nanoporous Pd–Ag alloys fabricated in this study can be treated
as a model system for other hydrogen sorbing materials; the knowl-
edge gained from the present study provides insights in the design
of efficient Pd-based catalysts for hydrogen purification and stor-
age.
2.4. Electrochemical study of nanoporous Pd–Ag alloys
A VoltaLab PGZ402 potentiostat was used in this work. All exper-
iments were conducted in 0.1 M HClO4 solution, deoxygenated by
the continuous passage of ultra-pure Ar gas either into the elec-
trolyte before electrochemical measurements or over the top of
the electrolyte during electrochemical measurements. A three-
electrode cell was used with a saturated calomel electrode (SCE)
connected to the cell through a salt bridge as the reference elec-
trode, a Pt wire coil as the counter electrode and the prepared
Ti/Pd–Ag alloys (1 cm2) as the working electrodes. At the begin-
ning of the absorption experiments, each of the Pd–Ag electrodes
was cycled continuously through the potential region of hydrogen
adsorption and absorption until an invariant voltammogram was
obtained on further scanning. Data acquisition and analysis were
performed using VoltMaster 4 software. All the experiments were
carried out at room temperature, 22 2 ◦C.
2. Experimental
2.1. Materials
3. Results and discussion
Ammonium formate (Aldrich, 99.99%) and ethylene glycol were
used as the reducing agent. Pd(NO3)2·xH2O (Aldrich) and AgNO3
(Baker) were used to prepare precursor solutions for the synthe-
sis of the Pd–Ag nanostructures. Pure water (18.2 Mꢀ cm) was
obtained from a Nanopure Diamond® water purification system.
All other chemicals were of reagent grade.
3.1. Surface morphology, composition, and structure of the
prepared Pd–Ag electrodes
The surface morphology of the synthesized Pd–Ag alloys was
examined by SEM at a magnification of 15,000. Fig. 1a presents a
typical SEM image of the Pd–Ag15% sample. All the Pd and Pd–Ag
samples fabricated in this study possess nanoporous structures,
similar to the Pd–Ag15% sample (Fig. 1a), consisting of irregular
pores ranging from several to hundreds of nanometers in diame-
ter. It is expected that the porous structures possess a high surface
area, which is desirable for hydrogen sorption and storage. The
EDS spectra of the seven nanoporous samples (pure Pd, Pd–Ag10%,
Pd–Ag15%, Pd–Ag20%, Pd–Ag25%, Pd–Ag30% and Pd–Ag40%) are
presented in Fig. 1b. The peaks marked by a star are derived from
the Ti substrates. Two Pd peaks and two Ag peaks are observed for
all the nanoporous Pd–Ag samples. As expected, the intensity of
the Ag peaks progressively increases from Pd–Ag10% to Pd–Ag40%.
Quantitative analysis of these EDS spectra shows that the composi-
tions of all the Pd–Ag samples are consistent with the compositions
of the Pd and Ag precursors initially added to the hydrothermal
vessels. This is further confirmed by our ICP analysis. Table 1 dis-
plays the ICP results for the amount of precursors remaining in
solution (i.e. not reduced) after the hydrothermal reduction. These
experimental results demonstrate that the reduction agents cho-
sen in this study can effectively reduce the Pd2+ and Ag+ precursors
under the hydrothermal condition and that the composition of the
formed nanoporous Pd–Ag alloy can be easily controlled using this
proposed new method.
2.2. Synthesis of Pd–Ag nanostructures
A series of Pd–Ag nanostructures with different compositions
of Ag ranging from 0 to 40 at.% were directly grown onto Ti sub-
strates using a hydrothermal method [45,46]. Ti plates (99.2%,
1.25 cm × 0.80 cm × 0.5 mm) were washed by sonication in acetone
followed by pure water (18.2 Mꢀ cm), then etched in an 18 wt% HCl
solution at 85 ◦C for 30 min, and finally rinsed with pure water. To
fabricate the Pd–Ag nanostructures, the pre-etched Ti plates were
placed in Teflon vessels containing 10 ml of an aqueous mixture of
inorganic metal precursors and the reducing agent; and then the
containers were heated at 180 ◦C for 2 h. In all cases, the amounts
of the reducing agent of ammonium formate and ethylene glycol
added were kept constant at 10 mM and 2.5 M, respectively. Vary-
ing amounts of the Pd(NO3)2·xH2O and AgNO3 precursors were
added to obtain the desired ratio of Pd to Ag. After cooling to room
temperature, the Pd–Ag coated Ti plates were finally dried in a
vacuum oven at 40 ◦C.
2.3. Surface morphology and composition of Pd–Ag
nanostructures
X-ray photoelectron spectroscopy (XPS) was performed to ana-
lyze the surface composition and the electronic interaction of the
Pd–Ag alloys. Fig. 2 shows the high resolution XPS spectra for Pd(3d)
and Ag(3d) of the sample Pd–Ag20%. The spectrum of Pd(3d) shows
a doublet peaks located at a low binding energy (3d3/2) at 334.4 eV
and at a high binding energy (3d5/2) at 339.7 eV (Fig. 2a), indicating
the presence of Pd in the metallic state Pd0 and higher oxidization
states. The binding energies of the Ag(3d3/2) and Ag(3d5/2) peaks
(Fig. 2b) were measured at 366.5 eV and 372.4 eV, respectively.
Following the electrode preparation, the surface morphology
and composition of the coatings were characterized using a JEOL
5900LV scanning electron microscope (SEM) and X-ray energy
dispersive spectrometry (EDS). The concentrations of silver and
palladium in the solution after the hydrothermal reduction pro-
cess were also analyzed with inductively coupled plasma (ICP). The
X-ray diffraction (XRD) patterns were recorded on a PW 1050-3710
˚
diffractometer using a Cu K˛ (␭ = 1.5405 A) radiation source. X-ray

S. Chen et al. / Electrochimica Acta 56 (2010) 61–67
63
Table 1
ICP results of the concentrations of Pd2+ and Ag+ remaining in solution after the hydrothermal reductions. Initial concentrations were calculated based on the amount of
Pd(NO3)2 and AgNO3 added to the hydrothermal vessels.
Sample
Pd (initial)/ppm
Pd (final)/ppm
Ag (initial)/ppm
Ag (final)/ppm
Molar ratio Pd:Ag
Pd
532.10
478.89
452.29
425.68
399.08
372.47
319.26
0.02
0.06
0.03
0.18
0.03
0.13
0.04
0.00
53.93
80.90
107.87
134.84
161.80
215.74
0.00
0.00
0.00
0.03
0.02
0.00
0.11
100:0
90:10
85:15
80:20
75:25
70:30
60:40
PdAg10%
PdAg15%
PdAg20%
PdAg25%
PdAg30%
PdAg40%
Using the area of the fitted curves for the Pd and Ag in both the
metallic and the higher oxidation states, the ratio of metal/metal
oxide was calculated. It was found that 92% of the Pd and 97%
of the Ag were in their metallic states, further showing that the
hydrothermal method employed in this study was efficient for the
preparation of nanoporous Pd–Ag alloys. The actual atomic compo-
sition of Ag was also estimated based on the area under the peaks
to be 17.74%, which is close to the nominal Pd:Ag ratio of 80:20.
X-ray diffraction was used to characterize the phase structure
of the samples. Fig. 3a presents the XRD patterns of the nanoporous
Pd and Pd–Ag samples, consistent with a face centered-cubic (fcc)
unit cell. The 2ꢁ values of 40.02◦, 46.56◦, 68.04◦ and 82.05◦ for the
nanoporous Pd can be indexed to the diffractions of the (1 1 1),
(2 0 0), (2 2 0) and (3 1 1) planes of Pd, respectively (JCPDS file no.
46-1043). The absence of Ag peaks in the XRD patterns of the
nanoporous Pd–Ag samples suggests that there was no Pd and
Ag crystalline metal phase separation and that Pd–Ag bimetallic
alloyed structures were formed. In addition, all the peaks slightly
shifted to a lower angle position due to the incorporation of increas-
ing amounts of the larger Ag atoms into the Pd fcc lattice, indicating
that the lattices in the nanoporous Pd–Ag were expanded. The fcc
lattice parameter can be calculated from the diffraction peak posi-
tions. In the XRD patterns the (2 2 0) peak was used to calculate the
lattice parameter a using the following equation [47]:
√
2ꢂK˛
a =
(1)
sin ꢁmax
3500
(a)
Pd3d
5/2
3000
2500
2000
1500
1000
500
Pd3d
3/2
0
350
345
340
335
330
325
B.E./eV
1800
1600
1400
1200
1000
800
(b)
Ag3d
5/2
Ag3d
3/2
375
370
365
360
B.E./eV
Fig. 2. XPS spectra of the Pd(3d) (a) and Ag(3d) (b) regions for the Pd–Ag20% sample.
The green dots, dashed lines, and red, blue, and black solid lines represent the raw
data, baseline, individual components (zero and high oxidation states) and total fit,
respectively. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Fig. 1. (a) Typical SEM image at 15,000× magnification of the Pd–Ag surfaces with
normalized atomic ratios of Pd/Ag of 85:15; (b) the EDS spectra of the nanoporous
Pd and Pd–Ag alloys. The peaks labelled with * are derived from the Ti substrate.

64
S. Chen et al. / Electrochimica Acta 56 (2010) 61–67
(a)
PdAg40%
10
5
PdAg30%
PdAg25%
PdAg20%
PdAg15%
PdAg10%
0
Pd100%
-5
PdAg10%
PdAg20%
PdAg30%
Pd(100%)
-10
20
40
60
80
-.2
0.0
.2
.4
2Theta (degrees)
Potential/V vs.SCE
(b)
0.400
0.395
0.390
Fig. 4. Cyclic voltammograms of the PdAg electrodes recorded in 0.1 M HClO4 at a
scan rate of 20 mV/s.
Pd had a strong impact on the capacity of hydrogen sorption of the
Pd–Ag nanomaterials.
3.3. Sweep rate dependence of hydrogen electrosorption in the
Pd–Ag nanostructures
To illustrate the effect of the sweep rate on the hydrogen elec-
trosorption, Fig. 5a presents the CV curves of the nanoporous
Pd–Ag20% alloy recorded with the sweep rates varied from 5
to 50 mV/s. A well-defined hydrogen sorption peak centered at
−285 mV is observed at the low sweep rate (5 mV/s). The total
charge, QH, due to hydrogen adsorption and absorption into the
Pd–Ag nanostructures versus the potential scan rate is shown
in Fig. 5b. The amount of hydrogen sorbed into the nanoporous
Pd–Ag alloy, calculated from the charge of the hydrogen desorp-
tion/oxidation peaks, was found to be dependent on the sweep
rate used in the cyclic voltammetric experiments. Increasing the
potential scan rate results in a decrease in the hydrogen discharge,
showing that a low sweep rate is preferential for determining the
hydrogen adsorbing/absorbing capacity of the Pd–Ag nanostruc-
tures.
0
10
20
30
40
Atomic Composition of Ag %
Fig. 3. (a) XRD patterns of the prepared Pd and PdAg films; (b) Vegard’s plot showing
the dependence of the fcc lattice constant calculated from the (2 2 0) peaks of each
XRD pattern in (a) using Eq. (1) on the normalized atomic composition of Ag.
where a is the lattice constant, ꢂ is the wavelength of X-ray radia-
tion (Cu K˛ = 0.15405 nm), and ꢁ is the location of the (2 2 0) peak
in radians. A plot of the lattice constant versus the normalized
atomic composition of Ag of the nanoporous Pd–Ag alloys (Vegard’s
plot) is shown in Fig. 3b. The lattice constant of the Pd–Ag alloys
linearly increases with the increase of the Ag component. The lat-
tice constant of the nanoporous Pd is 0.389 nm, which increases to
0.400 nm for the Pd–Ag40% sample, showing a significant dilation
of the lattice constant with increasing amounts of Ag.
3.4. Influence of the electrode potential and the composition of
the Pd–Ag alloys on the capacity of hydrogen electrosorption
The electrosorption of hydrogen into the nanoporous Pd–Ag
alloys was further examined at different electrode potentials varied
from −275 to −175 mV versus SCE. The potential was first held at
a constant potential for a period of time; linear voltammetry was
then run starting from the held potential to +200 mV. The effect
of the time held at different potentials on the hydrogen discharge
was investigated, revealing that a period of 5 min was long enough
to obtain complete saturation of hydrogen into the nanoporous Pd
and Pd–Ag alloys. For comparison, Fig. 6a and b presents the linear
voltammograms of the nanoporous Pd and Pd–Ag20%, respec-
tively, recorded at the scan rate of 20 mV/s after being held at
each of the pre-selected potentials for 5 min. The intensity of the
hydrogen desorption/oxidation peak strongly depends on the held
electrode potential; the lower the potential, the larger the peak.
Similar behaviour is also observed for other nanoporous Pd–Ag
alloys, reflecting the fact that, under the electrochemical condi-
tions, the amount of hydrogen sorbed into the Pd and Pd–Ag alloys
is potential dependent. A lower electrode potential corresponds to
a higher hydrogen pressure in gas-phase experiments [8,26]. For
pure Pd, the absorption of hydrogen produces two different phases
3.2. General cyclic voltammetric (CV) behaviour of the Pd–Ag
nanostructures
For a general hydrogen electrosorption characterization of the
nanoporous Pd and Pd–Ag alloys, two cycles of the CVs in the range
of −300 to 400 mV were recorded in 0.1 M perchloric acid at a scan
rate of 20 mV/s; the second cycle was presented in Fig. 4. For com-
parison, Fig. 4 displays the CV curves of the nanoporous Pd and
Pd–Ag alloys with 10, 20 and 30 at.% Ag. As the high current caused
by hydrogen absorption dominates and covers the adsorption pro-
cesses, it is difficult to decouple the adsorption process from the
absorption process [18]. A large broad peak due to the desorption
of hydrogen appears between −300 and 0.0 mV when scanning the
electrode potential from −300 mV to +400 mV. The integrated peak
intensity for the hydrogen desorption/oxidation (i.e. the discharge)
significantly increased when the Ag content was increased from 0%
to 20%. Further increasing the Ag amount to 30%, the hydrogen dis-
charge decreased, showing that the amount of Ag incorporated into

S. Chen et al. / Electrochimica Acta 56 (2010) 61–67
65
20
(a)
(a)
18
10
5
50mV/s
20mV/s
16
-175mv
-200mv
-225mv
-250mv
-275mv
10mV/s
14
5mV/s
12
10
8
0
6
-5
4
2
-10
0
-0.4
-0.2
0.0
0.2
0.4
-2
-.3
-.2
-.1
0.0
.1
.2
.3
Potential/V
Potential/Vvs.SCE
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
14
12
10
8
(b)
(b)
-175mV
-200mV
-225mV
-250mV
-275mV
6
4
2
0
0
10
20
30
40
50
60
sweep rate mV/s
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Potential/Vvs.SCE
Fig. 5. (a) Cyclicvoltammogramsof thePd–Ag20%electrode recorded in 0.1 M HClO4
at various scan rates. (b) Charge due to hydrogen desorption versus sweep rate
calculated by integrating the area under the anodic peaks in (a).
Fig. 6. Anodic sweeps of the desorption of hydrogen from (a) Pd to (b) Pd–Ag20%
after holding the potential at various cathodic limits for 5 min in 0.1 M HClO4. The
scan rate was 20 mV/s.
(␣ phase and ␤ phase) [48]. At low concentrations of hydrogen, the
␣-phase appears, which possesses a lattice constant very similar
to pure Pd. At high concentrations of hydrogen (metal hydride),
the ␤ phase forms, resulting in an increase in the lattice constant.
The large expansion of the lattice constant can cause cracking of
the membrane (hydrogen embrittlement). As seen in Fig. 6a, for
the nanoporous Pd, a significant increase of the peak intensity
is observed when the held potential was changed from −225 to
−250 mV, which corresponds to the transition from the ␣ phase
to ␤ phase hydrogen sorption. However, no sharp transition was
observed for the nanoporous Pd–Ag20% (Fig. 6b). Note that the peak
intensity of the nanoporous Pd–Ag20% is much higher than that of
the pure nanoporous Pd when the potential was held at −225 mV.
To quantitatively examine the effect of the held potential on
the capacity of the hydrogen sorption into the nanoporous Pd
and Pd–Ag alloys, the total hydrogen discharge was calculated
by integrating the hydrogen desorption/oxidation peak shown in
Fig. 6. Fig. 7a depicts the dependence of the electrochemically mea-
sured hydrogen absorption capacity, expressed as the integrated
hydrogen discharge, on the different absorption potentials for the
nanoporous Pd–Ag alloys with different Ag composition varied
from 0 to 40%. The plots can be divided into two sections. The
potential range above −225 mV corresponds to ␣ phase hydrogen
sorption; while the potential below −225 mV leads to the ␤ phase
hydrogen sorption. Increasing the composition of Ag from 0 to 40%
decreases the ␤ phase hydrogen sorption capacity and diminishes
the ␣ to ␤ phase transition. In contrast, increasing the composition
of Ag increases the ␣ phase hydrogen sorption capacity. After reach-
ing the maximum capacity, further increase of the amount of Ag
results in a decrease of the hydrogen sorption capacity. For compar-
ison, Fig. 7b presents the hydrogen sorption capacity at −225 mV
versus the composition of Ag of the nanoporous Pd–Ag alloys. When
the Ag content is increased from 0 to 20%, the hydrogen sorption
significantly increases. Further increasing the Ag content from 20%
to 40%, the hydrogen sorption capacity decreased. The nanoporous
Pd–Ag20% possesses the highest capacity for the ␣ phase hydrogen
sorption, which is over 4 times larger than that of the nanoporous
Pd. This can be attributed to the dilation of lattice constant result-
ing from the incorporation of larger Ag atoms into the Pd fcc lattice
as seen in Fig. 3.
3.5. Dependence of hydrogen sorption time on the electrode
potential and composition of Pd–Ag
The kinetics of hydrogen sorption into the nanoporous Pd–Ag
alloys was further investigated using chronoamperometry. First,
the Pd–Ag electrode was held at +200 mV for 30 s, where neither
hydrogen absorption nor adsorption occurs. The electrode potential
was then stepped down to the hydrogen sorption region between
−175 and −275 mV; and the corresponding chronoamperometric
(i–t) curves were recorded (not shown here) in order to determine
the time needed for a steady-state saturation of the nanoporous
Pd–Ag electrodes with hydrogen. As shown in Fig. 8, the satura-
tion time strongly depends on the composition of the nanoporous

66
S. Chen et al. / Electrochimica Acta 56 (2010) 61–67
600
500
400
300
200
100
0
time. The increase of the amount of Ag results in a decrease of tm. For
(a)
the nanoporous Pd, when the potential was changed from −225 to
−250 mV, the time required for hydrogen saturation was increased
from 15 to 215 s. This is consistent with the study of the hydrogen
sorption capacity shown in Fig. 7a, where a sharp ␣ to ␤ phase tran-
sition was observed at −250 mV. The above results indicate that the
␣ to ␤ phase transition is the rate determining step and that the
addition of Ag not only increases the ␣ phase hydrogen sorption
capacity, but also improves the kinetics of hydrogen sorption.
Pd100%
PdAg10%
PdAg15%
PdAg20%
PdAg25%
PdAg30%
PdAg40%
4. Conclusions
In this study, we have successfully synthesized nanoporous
Pd–Ag alloys with the Ag content varied from 0 to 40% using
a facile hydrothermal method. The method is very effective and
can easily control the composition of the formed Pd–Ag nanos-
tructures. Our XRD analysis shows that the lattice constant of
the nanoporous Pd–Ag alloys increases with the increase of the
amount of Ag. Electrochemical methods have been employed to
systemically study the fabricated nanoporous Pd–Ag electrodes,
showing that the hydrogen sorption into the nanoporous Pd–Ag
alloys strongly depends on the composition of Pd–Ag and the
applied sorption potential. Hydrogen sorption into the nanoporous
Pd occurs in two distinct phases (␣ phase and ␤ phase). The addi-
tion of Ag greatly increases the ␣ phase hydrogen sorption capacity
and diminishes the ␣- and ␤-phase transition due to the dilation
of the lattice constant. The nanoporous Pd–Ag alloy with 20% silver
content possesses the highest capacity for the ␣ phase hydrogen
sorption at −225 mV, which is over 4 times higher than the pure
nanoporous Pd. Our study has also shown that the phase transition
is the rate limiting step in the hydrogen absorption process and,
therefore, with the addition of Ag, the kinetics is much faster. The
combination of the enhanced ␣ phase hydrogen sorption capac-
ity and diminishing of the ␣- and ␤-phase transition makes the
nanoporous Pd–Ag alloys attractive for hydrogen selective mem-
branes and hydrogen dissociation catalysts.
-280
-260
-240
-220
-200
-180
-160
Potential/mVvs.SCE
300
250
200
150
100
50
(b)
0
10
20
30
40
Ag content (Atomic%)
Fig. 7. (a) The overall hydrogen desorption charge, QH, normalized by the mass of the
Pd and Pd–Ag alloys versus different potentials. (b) The overall hydrogen desorption
charge, QH, after hydrogen electrosorption at −225 mV versus the normalized atomic
composition of Ag.
Acknowledgements
PdAg alloys and the applied potential. For all the nanoporous Pd
and Pd–Ag electrodes, the maximum time (tm) required to achieve
the hydrogen saturation occurs at the same potential (−250 mV),
where the ␤ phase hydrogen sorption takes place. The tm for the
nanoporous Pd is much higher than that for the nanoporous Pd–Ag
electrodes. The addition of Ag dramatically lowers the maximum
This work was supported by a Discovery Grant from the Natu-
ral Sciences and Engineering Research Council of Canada (NSERC).
A. Chen acknowledges NSERC and the Canada Foundation of Inno-
vation (CFI) for the Canada Research Chair Award in Material and
Environmental Chemistry. We also thank the Surface Interface
Ontario/Chemical Engineering & Applied Chemistry at the Uni-
versity of Toronto for carrying out the XPS analysis of the Pd–Ag
samples.
250
Pd
References
PdAg10%
200
150
100
50
PdAg15%
PdAg20%
PdAg25%
PdAg30%
PdAg40%
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