Nano-structured PdxPt1-x/Ti anodes prepared by electrodeposition for alcohol electrooxidation

Article abstract of DOI:10.1016/j.electacta.2009.04.048

Nano-structured Pd_xPt_1-x (x = 0-1) composite catalysts supported on Ti substrate are successfully prepared by electrodeposition method, and the morphology and phase of the catalysts are analyzed by field emission scanning electron mi

Full text of DOI:10.1016/j.electacta.2009.04.048

Electrochimica Acta 54 (2009) 5486–5491
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Nano-structured PdxPt1−x/Ti anodes prepared by electrodeposition for alcohol
electrooxidation
Jinlin Lua, Shanfu Lua, Deli Wanga, Meng Yanga, Zili Liub, Changwei Xub,∗, San Ping Jianga,∗∗
a School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore
b School of Chemistry and Chemical Engineering, Guangzhou University, No. 601 Huangpudadao, Guangzhou 510006, Guangdong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nano-structured PdxPt1−x (x = 0–1) composite catalysts supported on Ti substrate are successfully pre-
pared by electrodeposition method, and the morphology and phase of the catalysts are analyzed by
field emission scanning electron microscope (FE-SEM) and X-ray energy dispersion spectroscopy (EDS).
The activity and stability of the PdxPt1−x/Ti composite catalysts are assessed for the electrooxidation of
alcohols (methanol, ethanol and 2-propanol) in alkaline medium using cyclic voltammetry and chronoam-
perometry techniques. The results show that the Pd and Pt form PdxPt1−x nano-structured composite
catalysts, uniformly distributed on the Ti substrate. The electrocatalytic activity and stability of the
PdxPt1−x nanocatalysts depend strongly on the atomic ratios of Pd and Pt. Among the synthesized cat-
alysts, the Pd0.8Pt0.2/Ti displays the best catalytic activity and stability for the electrooxidation reaction
of alcohols investigated in alkaline medium under conditions in this study, and shows the potential as
electrocatalysts for direct alcohol fuel cells.
Received 21 January 2009
Received in revised form 5 March 2009
Accepted 19 April 2009
Available online 3 May 2009
Keywords:
Direct alcohol fuel cells
PdPt
Electrodeposition
Alcohol electrooxidation
Ethanol
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
ates causes the rapid decrease in the oxidation currents with time
[20–22]. It has been shown that the electrooxidation of ethanol is
Direct alcohol fuel cells (DAFCs) based on liquid fuels have
attracted much attention as power sources for portable electronic
devicesand fuel-cellvehicles duetothemuchhigher energy density
of liquid fuels than gaseous fuels such as hydrogen [1–4]. Among the
different possible alcohols, methanol is the most promising liquid
fuel because it has several advantages, such as an abundant avail-
ability at low cost, easy handling, transportation and storage, high
theoretical density of energy comparable to that of gaseous hydro-
gen. Other alcohols, such as ethanol, propanol, etc. have also been
considered as fuels in fuel cells [5–12].
Almost all low-temperature polymer electrolyte membrane fuel
cells (PEMFCs) use platinum or platinum alloys as electrocatalyst
materials. Carbon supported and unsupported PtRu alloys are com-
monly used as electrocatalysts for methanol oxidation in direct
methanol fuel cells (DMFCs) [13–16]. PtSn bimetallic catalysts for
the electrooxidation of ethanol have been investigated by several
research groups [17–19]. However, they have a limited ability to
break the C–C bond. Besides adsorbed CO, several intermediates
with C–C bonds have been reported for the ethanol oxidation.
The partial blocking of the catalyst surface by these intermedi-
more sluggish than that of methanol oxidation reaction on Pt-based
catalysts in acidic medium [5,23]. In addition, the high price and
limited source of Pt constitute a major barrier to the development of
DAFCs. Therefore, there are several approaches employed to reduce
the amount of platinum used in fuel cells, including new MEA fabri-
cation methods, introducing a new supporting material to produce
the maximum platinum activity, discovering a new catalyst or an
alloyed catalyst based on platinum, etc. [24].
On the other hand, if DAFCs are operated in an alkaline instead
of an acidic medium, the kinetics of the electrode reactions will be
significantly enhanced and Pt-free catalysts with low cost can be
used [25,26]. It is reported that the greater availability of OH− ions
and a higher OH− coverage on the electrode surface will contribute
to the considerable increase in the kinetics of the alcohol oxidation
reaction in alkaline medium [27]. There is an increasing interest in
the research of alkaline membrane fuel cells due to the fact that
the electrode reaction is significantly facile and fast at the cath-
ode as well as at the anode and cheaper non-noble metal catalysts
can be used, thus significantly reducing the cost of fuel cells. It has
long been known that the anodic oxidation of methanol in alka-
line medium is much faster than that in acidic medium. We have
reported that Pd has higher electrocatalytic activity and better sta-
bility for the electrooxidation of ethanol and 2-propanol than Pt in
alkaline medium [20,28–33].
∗
Corresponding author. Tel.: +8620 39366908.
Corresponding author.
E-mail addresses: cwxuneuzsu@126.com (C. Xu), mspjiang@ntu.edu.sg
∗∗
It is well known that the electrodeposition is one of the most
efficient methods for the growth of metal nanoparticles. This is a
(S.P. Jiang).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.04.048

J. Lu et al. / Electrochimica Acta 54 (2009) 5486–5491
5487
powerful technique for the deposition of many metals since it is
rapid and facile, allowing easy control of the nucleation and growth
of metal nanoparticles with various sizes, shapes and distributions
[20,34]. In this paper, PdxPt1−x nanoparticles with different atomic
ratios were electrodeposited on the Ti substrate surface and applied
as electrocatalysts for methanol, ethanol and 2-propanol oxidation
in alkaline medium. There are several reports on the novel electrode
structures for DMFCs based on Ti mesh [35,36] due to its excellent
conductivity and chemical stability.
2. Experimental
The chemical reagents (methanol, ethanol, 2-propanol, potas-
sium hydroxide, ammonia, hydrochloric acid, hydroperoxide, PdCl2
and H2PtCl6·xH2O, etc.) used in this experiment were analytical
grade purity and Ti foils were 99.99% purity. All the above mate-
rials were purchased from Sigma–Aldrich. The solutions used were
freshly prepared with deionized water of ≥18.2 Mꢀ cm.
The PdPt electrocatalysts on Ti substrates surface were synthe-
sized by the electrodeposition method using a solution containing
designed ratio of Pd and Pt. The surfaces of the Ti foils were polished
with 50 nm alumina grinding powers, and then the Ti foils were
rinsed by a mixture solution of HCl (7.4 wt%) and H2O2 (5.0 wt%)
for 10 min to remove oxidized layer and contaminations. After that,
the Ti foils were dipped into the precursor solution. The pH value of
precursor solution was controlled at 9.5 and the metal ions concen-
tration (Pd2+ and Pt4+) was controlled at 5 mM with the designated
Pd2+ and Pt4+ stoichiometric composition. The electrodeposition
was performed by cyclic voltammetry (CV) from 0 to −0.8 V at a
scan rate of 5 mV s−1 and the process was repeated five times.
The electrochemical experiments were carried out using Autolab
PGSTAT30 electrochemical workstation. A standard three-electrode
cell was used and controlled at 25 ◦C using a water bath during the
experiments. The cell consisted of a titanium foil of 0.5 cm2 coated
by Pd, Pt and PdxPt1−x catalyst as the working electrode, a Pt foil of
3 cm2 as the counter electrode, and a saturated calomel electrode
(SCE, 0.241 V versus SHE) as reference electrode. A salt bridge with a
Luggin capillary closing to the working electrode was used between
the cell and the reference electrode.
Fig. 1. FE-SEM image of the Pd0.8Pt0.2/Ti catalyst and the inset is the histogram of
the particle size distribution.
in Fig. 1. The atomic ratio calculated from the relative intensity of
the EDS spectra is in excellent agreement with that of the targeted
ratio during the electrodeposition process, proving that the elec-
trodeposition is an easy and feasible method to control the catalyst’s
composition and to produce nano-structured and composite of PdPt
electrocatalysts on the Ti substrate.
3.2. Methanol electrooxidation on PdxPt1−x/Ti electrodes
Fig. 3 shows the CV curves of the methanol oxidation in a N2-
saturated 1.0 M KOH solution containing 1.0 M methanol on the
as prepared PdxPt1−x/Ti electrodes. The CV curves were measured
at a sweep rate of 50 mV s−1 in the potential range from −0.9 to
0.3 V. The reaction kinetics parameters of the PdxPt1−x/Ti catalysts,
including the onset potential (Es), the peak potential (Ep), the peak
current density (jp) and the current density at −0.4 V (j−0.4 V) are
given in Table 1. The alcohol electrooxidation on the electrodes is
characterized by two well-defined current peaks in the forward and
reverse scans. The forward scan peak currents present the elec-
trochemical oxidation reaction of the freshly chemisorbed species
coming from alcohol solution (i.e., the methanol in this case). The
reverse scan peak is primarily related with removal of carbonaceous
species which are not completely oxidized in the forward scan. The
magnitude of current density at −0.4 V in the forward scan indi-
cates the catalytic activity of the electrocatalysts for the methanol
oxidation reaction. The comparison of the electrochemical perfor-
mances of Pd/Ti, Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti, Pd0.2Pt0.8/Ti and Pt/Ti
as electrocatalysts for methanol oxidation is shown in Table 1. The
highest peak current density at −0.4 V obtained for the methanol
oxidation is 2.1 mA cm−2 on Pd0.8Pt0.2/Ti, which is four times higher
FE-SEM (JEOL JSM-6340F, Japan) was performed to characterize
the morphology and the particle size distribution of the catalyst.
EDS (Oxford, UK) was used to determine the atomic ratio of Pd and
Pt in PdxPt1−x
.
3. Results and discussions
3.1. Physical characterizations of PdxPt1−x/Ti electrocatalysts
The morphologies and size distributions of Pd0.8Pt0.2/Ti catalysts
were observed by FE-SEM and the result is given in Fig. 1. It can be
observed that the particles of the catalysts are spherical in shape
and there is no obvious conglomeration, which means that PdPt cat-
alysts possess a good three-dimensional structure and good specific
surface area. The average particle size is ∼10 nm, quite uniform and
narrow-dispersed (see the inset of Fig. 1). We also found that the
morphology and particle size of the PdPt electrocatalysts did not
change significantly with the Pd/Pt atomic ratio of the catalysts.
The element composition analysis of the catalysts was con-
ducted by EDS and the results are given in Fig. 2. As shown in Fig. 2,
Pt and Pd elements can be distinctly detected in the respective Pt
and Pd catalyst and can also be observed in the bimetallic cata-
lysts (Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti and Pd0.2Pt0.8/Ti). Ti element still
showed the relatively high diffraction peaks, which means the sur-
face of the Ti substrate have not been completely covered by the
PdPt catalyst layer. This is consistent with the SEM results as shown
Table 1
The electrochemical performances of as-synthesized catalysts for the methanol elec-
trooxidation in a 1.0 M methanol + 1.0 M KOH solution at a sweep rate of 50 mV s−1
298 K.
,
Catalysts
Es/V
Ep/V
jp/mA cm−2
j at −0.4 V/mA cm−2
Pd/Ti
−0.51
−0.53
−0.53
−0.53
−0.52
−0.22
−0.23
−0.25
−0.24
−0.27
7.7
22.3
13.2
7.4
0.5
2.1
0.7
0.8
1.0
Pd0.8Pt0.2/Ti
Pd0.5Pt0.5/Ti
Pd0.2Pt0.8/Ti
Pt/Ti
5.4

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J. Lu et al. / Electrochimica Acta 54 (2009) 5486–5491
Fig. 4. Chronoamperometric curves for the methanol oxidation measured at −0.4 V
on Pd/Ti, Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti, Pd0.2Pt0.8/Ti and Pt/Ti electrodes in a 1.0 M
methanol + 1.0 M KOH solution at 298 K.
than that on Pd/Ti electrode and two times higher than that on Pt/Ti
electrode. The onset potential (Es) of methanol oxidation is −0.55 V
on Pd/Ti and −0.56 V on Pt/Ti electrode and it negatively shifts to
−0.68 V on Pd0.8Pt0.2/Ti electrodes. More than 100 mV reduction on
the onset potential for anodic reaction is significant for liquid fuel
cell. The change in the onset potential shows an improvement in
the kinetics due to synergistic effect by the interaction between Pd
and Pt for the methanol electrooxidation reaction.
Fig. 4 is the chronoamperometry (CA) curves showing the
different stabilities of the PdxPt1−x/Ti electrodes for methanol elec-
trooxidation. The potential was constantly held at −0.4 V and the
change in current with time was recorded and plotted. As can be
seen, there is a continuous current drop with test time elapsing,
very rapid at the initial stage and then followed by a much slower
decay. The rapid current decay means the quick poisoning effect
on the catalyst surface, indicating the low tolerance to poisoning
by carbonaceous or CO intermediates formed during the methanol
oxidation reactions. As shown from Fig. 4, the Pd0.8Pt0.2/Ti electrode
shows the best stability and this is consistent with the high catalytic
activity of the catalyst (see Fig. 3).
3.3. Ethanol electrooxidation on PdxPt1−x/Ti electrodes
Fig. 2. EDS curves of (a) Pd/Ti catalyst, (b) Pd0.8Pt0.2/Ti catalyst and (c) Pt/Ti catalyst.
The ethanol electrooxidation reactions on the PdxPt1−x/Ti elec-
trodes were performed in 1.0 M ethanol + 1.0 M KOH solution at a
scan rate of 50 mV s−1. The curves were shown in Fig. 5 and the
corresponding performance data of Es, Ep, jp and j
are listed in
−0.4 V
Table 2. The catalytic activity and stability of Pd0.8Pt0.2/Ti is higher
than that of Pd/Ti. The jp of Pd0.8Pt0.2/Ti is 20.0 mA cm−2, which
is much higher than 11.1 mA cm−2 of Pd/Ti and Pt/Ti. The high-
est peak current density at −0.4 V obtained for ethanol oxidation
is 8.8 mA cm−2 on Pd0.5Pt0.5/Ti, which is about two times higher
than that on Pd/Ti electrode and six times higher than that on Pt/Ti
electrode. The onset potential (Es) of ethanol oxidation is −0.66 V
Table 2
The electrochemical performances of as-synthesized catalysts for the ethanol elec-
trooxidation in a 1.0 M ethanol + 1.0 M KOH solution at a sweep rate of 50 mV s−1
298 K.
,
Catalyst
Es/V
Ep/V
jp/mA cm−2
j at −0.4 V/mA cm−2
Pd/Ti
−0.68
−0.69
−0.74
−0.64
−0.63
−0.26
−0.24
−0.42
−0.29
−0.30
11.1
20.0
9.2
4.7
1.9
4.8
6.4
8.8
2.5
1.3
Pd0.8Pt0.2/Ti
Pd0.5Pt0.5/Ti
Pd0.2Pt0.8/Ti
Pt/Ti
Fig. 3. Cyclic voltammograms of Pd/Ti, Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti, Pd0.2Pt0.8/Ti and
Pt/Ti electrodes in a 1.0 M methanol + 1.0 M KOH solution at a scan rate of 50 mV s−1
and 298 K.

J. Lu et al. / Electrochimica Acta 54 (2009) 5486–5491
5489
Fig. 5. Cyclic voltammograms of Pd/Ti, Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti, Pd0.2Pt0.8/Ti and
Pt/Ti electrodes in a 1.0 M ethanol + 1.0 M KOH solution at a scan rate of 50 mV s−1
and 298 K.
Fig. 6. Chronoamperometric curves for the ethanol oxidation, measured at −0.4 V
on Pd/Ti, Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti, Pd0.2Pt0.8/Ti and Pt/Ti electrodes in a 1.0 M
ethanol + 1.0 M KOH solution at 298 K.
on Pd/Ti and −0.64 V on Pt/Ti electrode and it negatively shifts to
−0.79 V on Pd0.5Pt0.5/Ti electrodes, more than 100 mV reduction
on the onset potential for the ethanol electrooxidation reaction as
compared to that on pure Pd and Pt. The change in the onset poten-
tial shows an improvement in the kinetics due to synergistic effect
by the interaction between Pd and Pt for ethanol electrooxidation.
The results also show that Pd/Ti has higher catalytic activity than
Pt/Ti for ethanol oxidation in alkaline media, similar to the previ-
ous work [29,30]. The ratio of Pd to Pt in the PdxPt1−x/Ti catalysts
affects the catalytic activity for ethanol oxidation. The best perfor-
mance was found when the ratio of Pd to Pt is from 0.5:0.5 to 0.8:0.2
in the PdxPt1−x/Ti. The Pd0.8Pt0.2/Ti has the highest electrooxidation
current and Pd0.5Pt0.5/Ti has the most negative onset potential for
ethanol electrooxidation in alkaline medium. This has an important
implication for the development of DEFC because ethanol is a more
attractive and friendly fuel.
The stability for the ethanol electrooxidation on PdxPt1−x/Ti
electrodes was investigated with CA method. The results are shown
in Fig. 6. The current decays rapidly on Pt/Ti, however, the current
decays relatively slowly on Pd/Ti. That results show that Pd/Ti has
better steady-state electrocatalytic activity than Pt/Ti for ethanol
oxidation in alkaline media. The results are same to the previous
work [29,30]. The ratio of Pd to Pt in the PdxPt1−x/Ti catalysts affects
the stability for ethanol oxidation. The catalytic activity and sta-
bility increases with the increase in the amount of Pd. The best
performance was found when the ratio of Pd to Pt is 0.8:0.2 in
the PdxPt1−x/Ti. The catalytic activity and stability of Pd0.8Pt0.2/Ti
is higher than that of Pd/Ti.
3.4. Propanol electrooxidation on PdxPt1−x/Ti electrodes
Fig. 7 illustrates the CV curves of 2-propanol electrooxidation on
the PdxPt1−x/Ti electrodes. The corresponding performance data of
Pure Pt/Ti displays the lowest catalytic activity for ethanol oxi-
dation in alkaline medium. The overall oxidation reaction equation
for the direct ethanol fuel cell (DEFC) in alkaline medium is:
Es, Ep, jp and j
are listed in Table 3. Similarly to methanol and
−0.4 V
ethanol oxidation, the electrooxidation of 2-propanol is also char-
acterized by two well-defined current peaks in the forward and
reverse scans. The highest peak current density at −0.4 V obtained
CH3CH2OH + 12OH− → 2CO2 + 9H2O + 12e−
(1)
However, in reality the oxidation reaction of ethanol proceed
in multiple steps. According to the literature, the electrooxi-
dation reaction of ethanol is considered to be very complex
because several reaction products and intermediates will be
formed for reaction on Pt-based catalysts. High yields of par-
tial oxidation products such as CH3CHO and CH3COOH are
formed at the catalysts and these parallel reactions will cause a
considerable reduction of the fuel efficiency and produce unde-
sirable by-products [37]. The most possible anodic reactions of
the ethanol oxidation in alkaline medium could be written as
follows:(2)CH3CH2OH + 3OH− → CH3COads + 3H2O + 3e−
OH− ↔ OHads + e−
(3)
CH3COads + OHads → CHCOOH
(4)
The reactive intermediates formed during the ethanol dehydro-
genation react with OHads formed by the OH− anion adsorption
to produce acid in acidic solution or anion in alkaline solution.
The acetic acid formed which is the main intermediate for ethanol
oxidation in alkaline medium may not be further oxidized. The
breaking of the C–C bond in ethanol for total oxidation to CO2 is a
major problem in the electrochemical oxidation reaction of ethanol
[38].
Fig. 7. Cyclic voltammograms of Pd/Ti, Pd0.8Pt0.2/Ti, Pd0.5Pt0.5/Ti, Pd0.2Pt0.8/Ti and
Pt/Ti electrodes in a 1.0 M 2-propanol + 1.0 M KOH solution at a scan rate of 50 mV s−1
and 298 K.

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J. Lu et al. / Electrochimica Acta 54 (2009) 5486–5491
Table 3
fuel cells. The results in the present study clearly show that the
Pd0.8Pt0.2/Ti has best activity and stability for the electrooxidation
of methanol, ethanol and 2-propanol and is a promising catalyst
for DAFCs in alkaline medium. Nevertheless, more detailed stud-
ies of the mechanism and intermediate and product formation
will be in the future works, which can be expected to give an
important insight into the factors that determine the relative rates
of the multiple pathways for alcohol electrooxidation in alkaline
medium.
The electrochemical performances of as-synthesized catalysts for the 2-propanol
electrooxidation in a 1.0 M 2-propanol + 1.0 M KOH solution at a sweep rate of
50 mV s−1, 298 K.
Catalyst
Es/V
Ep/V
jp/mA cm−2
j at −0.4 V/mA cm−2
Pd/Ti
−0.65
−0.68
−0.70
−0.69
−0.69
−0.34
−0.32
−0.36
−0.35
−0.31
3.3
4.7
1.9
0.6
0.3
2.8
3.7
1.8
0.6
0.2
Pd0.8Pt0.2/Ti
Pd0.5Pt0.5/Ti
Pd0.2Pt0.8/Ti
Pt/Ti
4. Conclusions
for 2-propanol oxidation is 3.7 mA cm−2 on Pd0.8Pt0.8/Ti electrode.
The stability of the PdxPt1−x/Ti electrodes for 2-propanol electroox-
idation was also studied by CA method and the results were shown
in Fig. 8. Similarly, the currents decay fast initially and reach a steady
state after tested for some period. From the results, the Pt/Ti elec-
trode shows the worst stability for 2-propanol electrooxidation in
alkaline medium. Pd/Ti has a better steady-state activity than Pt/Ti
for the 2-propanol oxidation reaction in alkaline media. The ratio of
Pd to Pt in the PdxPt1−x/Ti catalysts also affects the catalytic activity
and stability for 2-propanol oxidation. The best performance was
obtained when the ratio of Pd to Pt is 0.8: 0.2 in the PdxPt1−x/Ti. The
catalytic activity and stability of Pd0.8Pt0.2/Ti is higher than that of
Pd/Ti. The Pd0.8Pt0.2/Ti manifests the highest current value in the
catalysts. The current become relative stable after about 10 min,
exhibiting that adsorption and desorption of intermediates on the
catalysts surfaces reach a temporary equilibrium.
By comparing to methanol, ethanol and 2-propanol electroox-
idation, Pd/Ti shows higher activity and better stability than Pt/Ti
for the electrooxidation reactions of ethanol and 2-propanol, while
Pt/Ti shows a better stability towards the methanol electrooxi-
dation as compared to Pd/Ti in alkaline medium. The bimetallic
catalyst, Pd0.8Pt0.2/Ti displays the highest catalytic activity and
the best stability for the electrooxidation of alcohols investigated.
Meantime, through the comparison of the electrochemical param-
eters, the peak current densities of the bimetallic catalysts reduce
monotonously with the decreasing of the Pd content. It shows that
the composition of the bimetallic catalysts possesses an obvious
influence on the catalytic activity, and the presence of the second
component exhibits evident enhancing effect, which can be inter-
preted by the bifunctional mechanism or ligand effect involved in
the alcohol electrooxidation [39].
Nano-structured bimetallic PdxPt1−x catalysts supported on Ti
substrate with varying Pd:Pt atomic ratios have been prepared
by electrodeposition. The electrodeposition method is found to be
convenient for synthesizing nanocomposite catalysts on metal sub-
strate surface. The particle size of the as synthesized bimetallic
catalysts is ∼10 nm and uniform distributed without coagulation.
The electrochemical results indicate that the molar ratio of Pd
and Pt possesses the evident influence to the catalytic activities.
Pd0.8Pt0.2/Ti displays the highest electrocatalytic activity and it
presents the best tolerance to carbonaceous intermediates for the
investigated alcohols in alkaline medium.
Acknowledgements
This work was financially supported by the National Nat-
ural Science Foundations of China (20776031), the Natural
Science Foundations of Guangdong Province (07300877) and
the Guangzhou Key Laboratory of Hydrogen and Green Catalyst
(2008121108), Guangzhou University Innovative Research Team
(2008127), Guangzhou University.
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