Temperature effect on the electrode kinetics of ethanol oxidation on Pd modified Pt electrodes and the estimation of intermediates formed in alkali medium

Article abstract of DOI:10.1016/j.electacta.2010.08.029

Ethanol has been recognized as the ideal fuel for direct alcohol fuel cell (DAFC) systems due to its high energy density, non-toxicity and its bio-generation. However the complete conversion of ethanol to CO₂ is still met with challenges, due t

Full text of DOI:10.1016/j.electacta.2010.08.029

Electrochimica Acta 55 (2010) 9097–9104
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Temperature effect on the electrode kinetics of ethanol oxidation on Pd modified
Pt electrodes and the estimation of intermediates formed in alkali medium
S.S. Mahapatra, A. Dutta, J. Datta∗
Department of Chemistry, Bengal Engineering & Science University, PO-B. Garden, Shibpur, Howrah 711 103, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 June 2010
Received in revised form 8 August 2010
Accepted 9 August 2010
Available online 14 August 2010
Ethanol has been recognized as the ideal fuel for direct alcohol fuel cell (DAFC) systems due to its high
energy density, non-toxicity and its bio-generation. However the complete conversion of ethanol to CO2 is
still met with challenges, due to dearth of suitable catalysts for the electro-oxidation. In the present work
the effect of temperature on the catalytic oxidation of ethanol in alkaline medium over electrodeposited
Pt and Pt–Pd alloyed nano particles on carbon support and also on the product formation during the
course of reaction have been studied within the temperature range of 20–80 ◦C. The information on
surface morphology, structural characteristics and bulk composition of the catalyst was obtained using
SEM, XRD and EDX. BET surface area and pore widths of the catalyst particles were calculated by applying
the BET equation to the adsorption isotherms. The electrochemical techniques like cyclic voltammetry,
chronoamperometry and impedance spectroscopy were employed to investigate the electrochemical
parameters related to electro-oxidation of ethanol in alkaline pH on the catalyst surfaces under the
influence of temperature. The results show that the oxidation kinetics of ethanol on the alloyed Pt–Pd/C
catalysts is significantly improved compared to that on Pt alone. The observations were interpreted in
terms of the synergistic effect of higher electrochemical surface area, preferred OH− adsorption on the
surface and the ad-atom contribution of the alloyed matrix. A pronounced influence of temperature on the
reaction kinetics was manifested in the diminution of charge transfer resistance and activation energy of
the ethanol oxidation with Pd incorporation into the Pt matrix, ensuring greater tolerance of the alloyed
Keywords:
Ethanol electro-oxidation
Pt–Pd/C catalyst
Electrode poisoning
Temperature dependence
Product analysis
−2
catalyst towards ethanolic residues. The higher yield of the reaction products like acetate and CO3
on the alloyed catalyst compared to Pt alone in alkaline medium, as estimated by ion chromatography,
further substantiates the catalytic superiority of the Pt–Pd/C catalyst over Pt/C.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
nearly CO2 neutral since the CO2 produced in the process is con-
sumed for biomass growth, offering a nearly closed carbon loop
In recent times ethanol is strongly recommended for direct
use in electrochemical power sources like direct alcohol fuel cell
(DAFC). The low molecular weight liquid fuels such as methanol
[1–3] and ethanol [4–6] have advantages over pure hydrogen,
because these alcohols can easily be handled, stored and trans-
ported using the present gasoline infrastructure with only slight
modifications and can be used directly without the necessity of
reforming. Furthermore, their energy densities are comparable to
that of gasoline, varying from 6.1 kWh kg−1 for methanol to about
8.01 kWh kg−1 for ethanol. In comparison to methanol, ethanol is
less toxic and can be easily produced in huge quantity by the fer-
mentation of sugar-containing raw materials from agriculture. In
fact, the use of bio-ethanol has the significant advantage of being
making ethanol an eco-friendly and renewable fuel. Moreover the
attraction for ethanol being used in fuel cells is due to its low
permeability across the membrane between the anode and cath-
ode chambers. Though, ethanol is a promising candidate to replace
methanol in fuel cells due to its higher energy density that cor-
responds to 12 electrons per molecule, the major issue in the
electrocatalysis of this alcohol toward complete conversion still
remains unresolved and limits the power density of the direct
ethanol fuel cell (DEFC).
Although much effort has been taken for the development of
DAFCs in acid medium, studies on alkaline fuel cells using small
organic molecules are relatively less. However, in recent times
interest in the alkaline fuel cell has arisen in view of its low cost,
wider selection of possible electrocatalysts and expected faster
kinetics of the proton transfer reactions as well as for the oxygen
reduction [7–9]. The effective use of Pd-based catalyst in alkaline
medium, for complete oxidation of ethanol has been the interest of
a number of research groups [10–14]. The facile adsorption of the
∗
Corresponding author. Tel.: +91 33 2668 4561/62/63x514;
fax: +91 33 2668 2916.
E-mail address: jayati datta@rediffmail.com (J. Datta).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.08.029

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S.S. Mahapatra et al. / Electrochimica Acta 55 (2010) 9097–9104
organic molecules on Pt surface and simultaneous activation of Pd
surfaces by the hydroxyl species in alkaline medium are expected
to produce synergistic effect in improving the kinetics of ethanol
oxidation in Pt–Pd binary system. Furthermore the rate and extent
of alcohol oxidation in a fuel cell environment can be substantially
controlled by regulation of cell temperature [12–14]. In fact the
application of alkaline electrolytes to the DEFCs’ functioning at opti-
mum temperature could lead to the reduction in catalyst loading,
and ultimately allow the use of less expensive non-precious metal
catalysts.
In this paper, the temperature effect on the electrocatalysis of
ethanol oxidation has been investigated with Pt/C and Pt–Pd/C
electrodes in alkaline medium. Different electrochemical tech-
niques like cyclic voltammetry (CV), chronoamperometry (CA) and
electrochemical impedance spectroscopy (EIS) were employed for
the study. Apparent activation energies (Ea(app)) for the oxida-
tion reaction were determined at different potentials within the
temperature range of 20–80 ◦C. Presuming that, in alkali media,
the oxidation of ethanol proceeds through intermediate forma-
tion of the anions like acetate and end products like carbonate,
ion exchange chromatography was employed to quantify these
reaction products at different temperatures and to assess the elec-
trocatalytic efficiency of the alloyed (Pt–Pd/C) electrodes over Pt
alone.
were calculated by applying the BET equation to the adsorption
isotherms.
2.2. Electrochemical measurements
Electrochemical measurements like cyclic voltammetry and
chronoamperometry were carried out in the standard three-
electrode cell at different temperatures with the help of computer
controlled AUTOLAB 30 Potentiostat/Galvanostat from Ecochemie
B.V., The Netherlands. The cell temperature was thermostated
within the range of 20–80 ◦C. The working solutions were deaerated
by purging with nitrogen (XL grade).
Electrochemical impedance spectroscopy was conducted using
the same AUTOLAB 30 PGSTAT coupled with a frequency response
analyzer (FRA) module. EIS was performed with amplitude of 5 mV
for frequencies ranging from 40 kHz to 100 mHz. Each scan con-
tained about 100 data points (20 data points per decade). The
impedance spectra were fitted to an equivalent circuit model using
a non-linear fitting program.
A mercury-mercuric oxide (MMO) reference electrode and a
platinum foil (2 cm × 1 cm) counter electrode were used for all
electrochemical experiments. All the potential in this paper are ref-
erenced to the standard hydrogen electrode (SHE) at cell operating
temperature and the current is normalized to the geometric surface
area. The temperature correction for Hg/HgO reference electrode in
alkaline medium was made following Eq. (1) [16].
2. Experimental
ET = 0.130 − 1.25 × 10−3
T
(1)
2.1.1. Materials
where T is temperature in degree centigrade.
Ethanol was purified using standard procedure [15] and was
stored over a Type 4A molecular sieve beads. The other reagents
such as NaOH and HCl were of GR grade purity (Merck) and
H2PtCl6·6H2O and PdCl2 were obtained from Arora Matthey Ltd.
The graphite block (saw cut finish grade) used as the catalyst sub-
strate was procured from Graphite India Limited. All the solutions
were freshly prepared with Milli-Q water.
2.2.1. Ion-chromatographic analysis
Aliquots from the working electrolyte were analyzed by ion
chromatography after polarization at constant potential for 2 h at
selected temperature of 20–80 ◦C. Metrohm’s Advanced Modular
Ion Chromatography systems and a Metrosep A Supp 5-250 col-
umn were used for analyzing the acetate and carbonate ions. 1 ml
sample of electrolyte solution was diluted with 19 ml of filtered
Milli-Q water and 25 ␮l of this sample was injected into the ion
chromatograph.
2.1.2. Preparation of Pt/C and Pt–Pd/C electrocatalysts
Electrochemical deposition of Pt and co-deposition of Pt and
Pd were made on coupons of graphite samples with surface area
of 0.65 cm2. Electrodeposition of Pt was carried out using 0.05 M
H2PtCl6·6H2O solutions. Both H2PtCl6·6H2O and PdCl2 solutions
were prepared in 2.0 M HCl. For co-deposition of Pt and Pd, an
equimolar mixture of 0.05 M H2PtCl6·6H2O and 0.05 M PdCl2 was
used. For each case electrodeposition was performed at room
temperature under galvanostatic control by applying 5 mA cm−2
current with the help of computer controlled PG Stat (AUTOLAB
30) for 10 min duration. Catalyst loading for each electrode was
3. Results and discussion
3.1. Structural characterization of Pt–Pd/C electrocatalyst
The surface morphology of the catalyst was revealed through
scanning electron microscopy. Pt–Pd particles show better disper-
sion than Pt on the graphite substrate as shown in Fig. 1a,b. As
platinum and palladium were electro-co-deposited on the graphite
surface both the metals were grown together on the same sphere
making an assembly of ad-atoms in the form of agglomerated par-
ticles. The size of the particles does not show much variation and
were found to remain within the range of 80–100 nm. The catalyst
particles exhibited XRD pattern of a typical face-centered-cubic lat-
tice structure as shown in Fig. 2. The strong diffraction peaks at
the Bragg angles of 40.08◦, 46.67◦, 68.02◦, 82.08◦ and 86.45◦ cor-
responded to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) facets of
Pt–Pd crystals. Alloying of Pt with Pd does not change the diffraction
pattern. However with the incorporation of Pd into the fcc struc-
ture of Pt, the diffraction peaks were shifted to higher values of 2ꢁ,
indicating contraction of the lattice. No characteristic peaks of Pt or
Pd oxides were detected. The (1 1 1) peak was used to calculate the
particle size of the Pt–Pd crystal according to the Debye–Scherrer
equation. The average particle size in the Pt–Pd/C catalyst matrix
was found to be 6.5 nm, although the morphology could only reveal
agglomeration of smaller particles throughout the matrix. The bulk
maintained at 2 mg cm−2
.
2.1.3. Physico-chemical characterization of the electrocatalysts
Bulk composition of the Pt–Pd/C electrocatalyst was determined
by energy dispersive X-ray spectroscopy (EDX) while scanning
electron microscopy (SEM) was employed to reveal the surface
morphology. All these measurements were done with a JSM-6700F
FESEM at an accelerating potential of 5 kV. In order to obtain infor-
mation about the surface and bulk structure of the catalyst, X-ray
diffraction (XRD) study was carried out with the help of Philips
PW 1140 parallel beam X-ray diffractometer with Bragg–Bretano
˚
focusing geometry and monochromatic Cu K␣ radiation (ꢀ = 1.54 A).
The Pt/C and Pt–Pd/C catalysts were subject to surface area and pore
structure analysis by employing Quantachrome instruments Model
AS1-CT-9. Adsorption isotherms were determined by N2 adsorption
over the catalyst sample at 77 K. BET surface area and pore widths

S.S. Mahapatra et al. / Electrochimica Acta 55 (2010) 9097–9104
9099
Fig. 1. (a–c) Scanning electron micrographs of (a) Pt, (b) Pt–Pd/C catalysts and (c) EDX spectrum of Pt–Pd/C catalyst co-electrodeposited by applying 5 mA cm−2 current for
10 min duration.
compositions of electrocatalysts were investigated using EDX spec-
troscopy and as shown in Fig. 1c the atomic ratio of Pt to Pd was
found to be 1:2 irrespective of the relative concentration of the pre-
cursor salts in the preparative electrolytic bath. The comparatively
high loading of Pd in the Pt–Pd/C catalyst is ascribed to the higher
reduction potential of Pd2+/Pd compared to Pt4+/Pt.
at the same time micropore area of the Pt–Pd/C catalyst was also
increased almost twice the value obtained for Pt/C catalysts. The
inset bar diagram in Fig. 4 demonstrates the increase of porosity
with alloying.
3.2. Effect of temperature on ethanol oxidation
To get an overview of the catalyst surface area and the textural
properties of the catalysts, adsorption phenomenon was recorded
employing BET method. Nitrogen adsorption and desorption
isotherms recorded for Pt and Pt–Pd catalysts are representative
of typical reversible curve of type III pattern displayed accord-
ing to the Brunauer–Deming–Deming–Teller (BDDT) classification
[17] and are illustrated in Fig. 3. The pore sizes of the catalysts are
found to cover both the mesopore and micropore ranges as shown
in the pore size distribution curve (Fig. 4). On introduction of Pd
to the Pt matrix, the surface area was found to increase, indicat-
ing lattice contraction and narrow size distribution of the metal
crystallites. Both the BET surface area and external surface area of
the alloyed catalyst were increased compared to the single Pt and
Fig. 5a,b represents the cyclic voltammograms for the EOR on
Pt/C and Pt–Pd/C in 0.5 M NaOH containing 1.0 M ethanol at tem-
peratures of 20, 40, 60 and 80 ◦C. The Pt/C and Pt–Pd/C catalysts
show significantly different CV profiles particularly at higher tem-
peratures. The ethanol oxidation current obtained with Pt/C (peak
‘a’) in the forward sweep was raised by a factor of 1.3 as the tem-
perature increased from 20 to 80 ◦C (shown in Fig. 5a), while the
Fig. 3. Nitrogen adsorption (filled) and desorption (unfilled) isotherms at 77 K for
Pt and Pt–Pd catalysts.
Fig. 2. X-ray diffraction (XRD) patterns of Pt/C and Pt–Pd/C catalysts.

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Fig. 6. Variation of anodic peak current, ip for electro-oxidation of ethanol on Pt/C
and Pt–Pd/C catalysts with temperature. The inset is the variation of forward and
backward peak current ratio (IF/IB) of ethanol oxidation on Pt/C and Pt–Pd/C elec-
trodes with different temperatures.
Fig. 4. Pore size distributions calculated from nitrogen adsorption–desorption
isotherms for different catalysts. The inset is the different surface areas for Pt and
Pt–Pd catalysts.
negative going sweeps display sharp vertical increase of the reverse
oxidation peak (‘b’) which are more prominent at higher temper-
atures. This sharp rise in current on Pt/C is indicative of the fact
that the ethanolic residues remaining un-oxidized on the surface
during the forward sweep can only be oxidized by the Pt oxide
formed at higher potentials. As the temperature was increased, the
potential differences between ‘a’ and ‘b’ peaks were reduced sug-
gesting that surface activation of Pt/C by OH species is facilitated
at higher temperatures. The influence of temperature is also sig-
nificant for ethanol oxidation on Pt–Pd/C systems. Considering the
voltammograms of Pt–Pd/C system as shown in Fig. 5b obtained
in the same temperature range, the oxidation current was found to
increase 2.5 times, the value obtained with Pt/C. Also the CV profiles
are uniquely featured with the appearance of the second oxidation
peak ‘a2’ in the positive going sweep at temperatures 60 and 80 ◦C.
In contrast to Pt, the Pt–Pd/C catalyst suffers a diminution in the
reverse oxidation current with the rise in anodic peak intensity.
This behavior clearly demonstrates that the catalytic performance
of Pt–Pd system fairly exceeds the level attained by Pt alone.
The pronounced influence of temperature on the electrocatal-
ysis of Pt–Pd/C system as compared to Pt/C is further clarified
with the variations of ips and Eps with temperature as shown in
Figs. 6 and 7 respectively. For Pt/C catalyst, the anodic peak current
Fig. 5. (a–b) Cyclic voltammograms for ethanol electro-oxidation on (a) Pt/C and
(b) Pt–Pd/C catalysts in a mixture of 0.5 M NaOH and 1.0 M ethanol at different
Fig. 7. Variation of anodic peak potential, Ep for electro-oxidation of ethanol on Pt/C
and Pt–Pd/C catalysts with temperature.
temperatures. Sweep rate: 50 mV s−1
.

S.S. Mahapatra et al. / Electrochimica Acta 55 (2010) 9097–9104
9101
Fig. 8. Variation of onset potential (Eonset) for ethanol oxidation on Pt/C and Pt–Pd/C
electrodes with temperature.
(ip) and anodic peak potential (Ep) were practically invariant with
temperature (Fig. 6), whereas a substantial increase in anodic peak
current with positive shift of potential was observed for Pt–Pd/C
electrode (Fig. 6). A considerable negative shift in the onset (Eonset
)
potential was observed in the alloyed electrodes as shown in the
inset of Fig. 5b by magnifying the CV profiles in the onset potential
region. A plot of onset potential with temperature for the catalyst is
also presented in Fig. 8. A constant slope of −3.40 mV/degree tem-
peratures for Pt–Pd/C electrode as compared to −0.9 mV/degree
temperatures for Pt/C clearly demonstrates that the catalytic activ-
ity of the former is improved to a reasonable extent with moderate
increase in temperature. It appears that, with rise in temperature
the binary catalyst surfaces get self activated possibly with the
contact adsorbed OH sheath even at low potentials. The catalytic
tolerance to the intermediate carbonaceous species were further
described, following the nature of the ratio of forward anodic peak
current density (IF) to the reverse anodic peak current density (IB)
for the two catalysts. Higher IF/IB ratio of the alloyed catalyst exhib-
ited throughout the temperature range (Fig. 6 inset) indicates fairly
good oxidation kinetics of ethanol proceeding towards completion
in the alkaline medium and producing carbon dioxide existing as
CO−3 2 in solution, and thus enabling the catalyst surface to remain
free of the poisonous residues.
Chronoamperometric experiments were carried out in 0.5 M
NaOH containing 1.0 M ethanol on Pt/C and Pt–Pd/C at different
temperatures to assess the stability of the electrodes. The ethanol
oxidation current was recorded at potential of −160 mV over a
period of 1000 s and shown in Fig. 9a,b. A decrease of current den-
sity was recorded at the beginning of each experiment for both
Pt/C and Pt–Pd/C electrodes at each temperature. Pt/C electrode
demonstrates rapid current decay and very low current densities
throughout the temperature range, indicating that the Pt/C surface
is prone to poisoning by ethanolic residues. In contrast Pt–Pd/C
electrodes, exhibit comparatively low decay of the current and
reasonably high current densities were maintained at all the tem-
peratures. Thus, Pt–Pd/C electrode conforms to greater tolerance
to ethanolic residues within the temperature range studied. This is
better understood from the log plots of the chronoamperometric
data as shown in insets of Fig. 9a,b.
Fig. 9. (a–b) Chronoamperometric studies of ethanol oxidation on (a) Pt/C and (b)
Pt–Pd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M ethanol at a constant
potential of −160 mV (vs. SHE) at different temperatures.
where (di/dt)t > 500 s is the slope of the linear portion of the current
decay and i0 is the current at the start of polarization back extrap-
olated from the linear current decay. It is evident from Table 1 that
the long-term poisoning effect for ethanol oxidation can be signif-
icantly controlled on Pt–Pd/C whereas Pt/C electrode suffers from
extensive poisoning. However the poisoning rate on Pt is reduced
to some extent at the elevated temperatures. This behavior is in
concordance with the results obtained from voltammetric experi-
ments.
It is established that the ethanol oxidation reaction proceeds
through the formation of reactive intermediates (CH3CO)ads and the
ultimate poisoning species (CO)ads on Pt group of metals [19]. How-
ever in the present case, it appears that reaction in its course does
not follow the same pathway for Pt catalyst and its alloyed form.
This is better understood when we followed the Arrhenius plots to
derive activation energy of the reaction over the temperature range
with Pt and Pt–Pd catalysts.
Long-term poisoning rates (ı) were evaluated from the record of
current decay (Fig. 9) at times greater than 500 s using the following
equation [18].
Fig. 10 shows Arrhenius plots for the ethanol oxidation on the
Pt/C and Pt–Pd/C electrodes at different potentials. Linear rela-
tionships between log i and 1/T is maintained throughout the
temperature range at all potentials. The activation energies are
plotted against the potential in Fig. 11. At the potential range −60
ꢀ
ꢁ
100
i0
di
dt
ı =
×
(% per s)
(2)
t>500 s

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3.3. Effect of temperature on the impedance response
Electrochemical impedance spectroscopy was used to inves-
tigate the kinetics of ethanol oxidation on Pt/C and Pt–Pd/C
electrodes at different temperatures and at a selected potential of
−160 mV (vs. SHE) much beyond the onset potential for both the
catalysts. The Nyquist plots for ethanol oxidation in a solution con-
taining 0.5 M NaOH and 1.0 M ethanol at temperature 20–80 ◦C are
shown in Fig. 12a,b. Behavioral change is observed in the Nyquist
plot for Pt/C and Pt–Pd/C electrodes particularly at lower tempera-
tures.
The electro-oxidation of ethanol on Pt/C electrode shows induc-
tive behavior only at higher temperature which is indicative of
the possible oxidation of the accumulated reaction intermediates
on the catalyst surface at elevated temperatures. In contrast the
Nyquist plots for the Pt–Pd/C electrode show similar features in
the total temperature regime and even at low temperature, semi-
circle is observed in the complex plane in impedance plot which
indicates the presence of resistive component. The equivalent cir-
cuit elements as derived from Nyquist plots for ethanol oxidation
on Pt/C and Pt–Pd/C electrodes at −160 mV (vs. SHE) are summa-
rized in Table 1. In case of the alloyed catalyst the diameter of
the semicircle decreased as temperature increased, and inductive
loop appeared in each case above 40 ◦C. This inductive effect was
Fig. 10. Arrhenius plots of ethanol oxidation current densities on Pt/C and Pt–Pd/C
electrodes at various potentials.
to 90 mV, the apparent activation energy for ethanol oxidation on
Pt/C increases from 12.05 to 19.71 kJ mol−1, whereas on Pt–Pd/C
the same is reduced to half of its value and further decreases from
6.70 to 5.35 kJ mol−1 and remains almost invariant for the rest of
the potential studied. Apparent activation energies for ethanol oxi-
dation on Pt/C were not only higher than those on Pt–Pd/C but also
show higher potential dependence in comparison to Pt–Pd/C cata-
lyst. The reaction on Pt presumably keeps on proceeding towards
ultimate poisoning by PtCO formation, and thus exhibiting an unfa-
vorable kinetics for the complete oxidation of ethanol. However
the reaction of COads with OHads/OH− can occur at high activation
energy only when PtOH is formed at higher potentials producing
carbonates and thus alleviating the problem of surface poison-
ing to some extent at elevated temperatures. On the other hand,
kinetic control is well established on Pt–Pd/C by the reaction of
the carbonaceous intermediates (CH3CO)ads with OHads leading to
the formation of acetate (CH3COO−) ions as well as formation of
carbonates (CO−3 2) by facile C–C bond cleavage at a lower over
potential. These two anionic species exit the pre-electrode layers
and diffuse to the bulk with sufficient ease, thus rendering the sur-
face free from blockage. Eventually, an appreciable reduction in
the activation energy, Ea(app) of ethanol oxidation was observed on
Pt–Pd/C electrode which translates to the higher intrinsic activ-
ity for this electrode compared to Pt and validates its potential for
alkaline DEFCs.
Fig. 12. (a–b) Impedance spectra for ethanol oxidation on (a) Pt/C and (b) Pt–Pd/C
electrodes at different temperatures in a mixture of 0.5 M NaOH and 1.0 M ethanol at
−160 mV (vs. SHE). The solid lines represent the fit to the impedance data according
to the equivalent circuits shown in the insets.
Fig. 11. Dependence of apparent activation energy with potentials for ethanol oxi-
dation on Pt/C and Pt–Pd/C catalysts in a mixture of 0.5 M NaOH and 1.0 M ethanol.

S.S. Mahapatra et al. / Electrochimica Acta 55 (2010) 9097–9104
9103
Table 1
Temperature dependence of EIS parameter and long-term poisoning rate (ı, % per s) for ethanol oxidation on Pt/C and Pt–Pd/C electrode surface at −160 mV.
E.C. parameters
Pt/C
Pt–Pd/C
20 ◦
20 ◦
C
40 ◦
C
60 ◦
C
80 ◦
C
C
40 ◦
C
60 ◦
C
80 ◦
C
Rs (ꢂ)
Rct (ꢂ)
3.58
121.5
5.51
–
0.067
3.17
83.4
3.2
–
0.059
3.15
43.81
3.28
–
3.22
35.45
4.4
4.92
9.73
1.48
–
3.45
5.29
1.31
0.66
0.014
2.43
2.89
1.25
0.45
0.013
1.94
2.48
1.19
0.19
0.011
CPE × 103 (F cm−2
)
L (Hz)
–
Poising rate: ı (% per s)
0.046
0.044
0.023
the carbonaceous intermediates on the catalyst surface, the lower
values of L at high temperature (60–80 ◦C) demonstrates the ability
of the alloyed catalyst to abate surface poisoning. This outstand-
ing catalytic performance of Pt–Pd/C system towards the charge
transfer kinetics of ethanol oxidation, merits the use of the alloyed
catalyst in alkaline DEFCs.
3.4. Estimation of oxidation products found during ethanol
electro-oxidation
The effect of varying temperature on the rate of product for-
mation during oxidation of ethanol was studied over a range
of temperatures (20–80 ◦C) on Pt/C and Pt–Pd/C electrodes. The
concentrations of the end products were determined through
ion-chromatographic analysis of the chronoamperometric aliquots
taken at −160 mV after 2 h. The overall oxidation reaction for the
ethanol in alkaline medium is supposed to be
Fig. 13. Bode plots at a potential of −160 mV (vs. SHE) for Pt–Pd/C catalyst in 1.0 M
CH3CH2OH + 12OH− → 2CO2 + 9H2O + 12e−
(i)
EtOH/0.5 M NaOH at different temperatures.
However, the electro-oxidation reaction of ethanol is considered
to be a complex reaction that proceeds through multiple steps. Sev-
eral reaction products and intermediates are expected to be formed
on the catalyst surface, some of the adsorbed intermediates con-
verted into the end products like acetate and carbonate and the
rest being retained on the catalyst surface may cause considerable
reduction in the fuel efficiency. The mechanism of the oxidation
process on Pd and Pt-based catalyst in alkaline media has been
proposed by a number of researchers [19–22]. It was suggested
that ethanol oxidation reaction follows a dual path mechanism (A
and B) that involve the production of reactive intermediates and
poisoning species.
attributed to the electro-oxidation kinetics of ethanol with fair pos-
sibilities of CO oxidation. Analyses of the corresponding Bode plots
(Fig. 13) reveal that the phase angle ˚ is almost independent of
temperature and confined to the frequency range 40 kHz–40 mHz.
In case of responses obtained at 60 ◦C, the phase angle
maximum is raised to further high level and slightly displaced
towards higher frequency values, justifying the predominance
of pseudo-inductive behavior and increased reaction rate at this
temperature.
Acetate ion
(in alkaline medium)
Ethanol
Reactive intermediates
(A)
(B)
Poisoning Species
CO2
The equivalent circuits compatible with the impedance data for
both the catalyst are presented in the inset of the corresponding
plots in Fig. 12a,b. In this circuit Rs, CPE and Rct represent solution
resistance, constant phase element and charge transfer resistance
associated to the ethanol oxidations primarily involving the disso-
ciative adsorption of the alcohol on the surface.
The Pt–Pd/C electrodes offer only a feeble Rct which is almost
32–67 times less than that offered by Pt alone throughout the
temperature range. There is also significant fall in the Rct values
of Pd incorporated catalyst even at temperature as low as 20 ◦C.
The increase in solution conductivity with rise in temperature
is reflected in the gradual decrement in Rs values with temper-
ature. The CPE impedance coefficients for Pt–Pd catalyst were
substantially reduced compared to the Pt catalyst indicating fairly
reversible kinetics at the electrified interface without much alter-
ation of the double layer structure by the adsorbed carbonaceous
intermediates. For the Pt–Pd catalyst the circuit included induc-
tive resistance R0 and the inductance, L, associated with oxidative
removal of (CO)ads or other parallel pathways leading to acetate for-
mations. Since L−1 can be associated with the oxidation kinetics of
In path A, an ethanol molecule is first adsorbed on active sites
and reacts with (OH)ads species and the resultant (CH3CO)ads is
likely to further react with (OH)ads to produce CH3COOH, which
exits in the form of CH3COO− ions in alkaline solution [21]. On
the other hand path B involves the reaction of the intermediates
forming the poisoning species (CO)ads which may further combine
with (OH)ads/OH− species to produce the carbonates. The oxida-
tion of dehydrogenated intermediates (catalyst-acyl species) was
proposed to be the rate-determining step (v) as furnished in the
following scheme
(CH3CH2OH)sol → (CH3CH2OH)ads
(ii)
(iii)
(iv)
(v)
OH− → (OH)ads + e−
(CH3CH2OH)ads + 3OH− → (CH3CO)ads + 3H2O + 3e−
(CH3CO)ads + (OH)ads → CH3COOH : slow
CH3COOH + OH− → CH3COO− + H2O : fast
(vi)
For this investigation it is proposed that, during the formation
of (CH3COO−)ads at step (vi) the reaction may also take a different

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S.S. Mahapatra et al. / Electrochimica Acta 55 (2010) 9097–9104
high yield of acetate ions. Besides this, the alloyed electrode also
catalyses the oxidations of (CH3COO−)ads at its OH− activated sur-
face facilitating C–C bond cleavage ending up with the ultimate
formation of CO−3 2. In contrast, lower yield of acetate ions with Pt/C
catalyst is due to the low coverage of the Pt surface with (CH3CO)ads
as well as (OH)ads. The appreciably high content of carbonate ions
with the Pt/C electrode is due to high degree of coverage of the cat-
alyst with poisoning species (CO)ads while the presence of Pd in the
Pt–Pd/C catalyst prohibits the building up of (CO)ads on the catalyst
surface resulting in lower carbonate ion content than the expected
value. When compared at temperature 60 ◦C, the acetate yield on
Pt–Pd is found to be 30 times greater than on Pt and nevertheless,
CO−3 2 concentration on this electrode also reaches a value 2.5 times
that on Pt.
4. Conclusion
The Pt–Pd/C catalyst exhibits higher rate of ethanol oxidation in
comparison to Pt/C at all temperatures. For ethanol oxidation, the
onset potentials on Pt–Pd/C catalyst shows considerable negative
shift with temperature and the values are much lower than that
obtained with Pt/C catalyst. The decay in oxidation current den-
sity is nominal for the Pt–Pd/C catalyst at all temperatures while
the Pt/C catalyst shows rapid current decay indicating higher rate
of catalyst poisoning. Pd addition drastically reduces the activa-
tion energies and the charge transfer resistance and promotes the
electrocatalytic activity towards the ethanol oxidation. The rapid
kinetics of the dissociative adsorption in alkali media followed by
facile oxidation of the intermediate species on the alloyed surface
were demonstrated in the inductive behavior of the impedance
record for this catalyst. The beneficial effect of catalysis by Pd addi-
tion to Pt is ascribed to the facile adsorption of OH− on this surface
as furnished in the suggested mechanism and is further reflected by
the very high yield of acetate as well as carbonate obtained during
the oxidation of ethanol on Pt–Pd/C catalyst.
Fig. 14. Temperature dependence of acetate and carbonate formations during the
electro-oxidation of ethanol in a mixture of 0.5 M NaOH and 1.0 M ethanol on Pt/C
and Pt–Pd/C electrodes at −160 mV (vs. SHE) in the temperature range of 20–80 ◦C.
direction in the presence of excess of OH− producing (CH3O)ads and
CO2 in the following way
(CH3COO−)ads + 2OH− → (CH3O)ads + CO2 + H2O + 3e−
(vii)
or
(CH3COOH)ads + 3OH− → (CH3O)ads + CO2 + 3H2O + 3e−
(viii)
Finally the poisoning species (CO)ads may accumulate on the
surface as a result of (CH3O)ads combining with OH− as
(CH3O)ads + 3OH− → (CO)ads + 3H2O + 3e−
(ix)
However at favorable pH condition (CO)ads is likely to get oxi-
dized, producing the ultimate species CO3−2
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