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Carbon supported PtRh catalysts for ethanol oxidation inalkaline direct ethanol fuel cell
S.Y. Shen, T.S. Zhao*, J.B. Xu
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong SAR, China
a r t i c l e i n f o
Article history:
Received 26 May 2010
Received in revised form
20 August 2010
Accepted 25 August 2010
Available online 26 September 2010
Keywords:
Fuel cell
Ethanol oxidation reaction (EOR)
Alkaline direct ethanol fuel cell
PtRh catalyst
The CeC bond cleavage
* Corresponding author. Tel.: þ852 2358 8647E-mail address: [email protected] (T.S. Zh
0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.08.107
a b s t r a c t
Owing to the formation of an oxametallacyclic conformation, the CeC bond cleavage is the
preferential channel for the ethanol dissociation on the Rh surface, the addition of Rh to Pt
can increase the CO2 yield during the ethanol oxidation. However, in acidic media the slow
oxidation kinetics of COads to CO2 limits the overall reaction rate. In this work, we prepare
carbon supported PtRh catalysts and compare their catalytic activities with that of Pt/C in
alkaline media. Cyclic voltammetry tests demonstrate that the Pt2Rh/C catalyst exhibits
a higher activity for the ethanol oxidation than Pt/C does. Linear sweep voltammetry tests
show that the peak current density on Pt2Rh/C is about 2.4 times of that on Pt/C. The
enhanced electro-activity can be ascribed not only to the improved CeC bond cleavage in
the presence of Rh, but also to the accelerated oxidation kinetics of COads to CO2 in alkaline
media.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1. Introduction break the CeC bond at low temperatures [4e6]. Up to now,
In terms of fuel, a direct ethanol fuel cell (DEFC) is more
attractive than a direct methanol fuel cell (DMFC), because
ethanol has higher energy density than methanol
(8.0 kWh kg�1 vs. 6.1 kWh kg�1), is less toxic, and can be
produced in large quantities from agricultural products or
biomass, which will not change the natural balance of carbon
dioxide in the atmosphere in contrast to the use of fossil
fuels [1e3]. However, unlike the methanol oxidation reaction
(MOR) that can almost completely go to CO2, the ethanol
oxidation reaction (EOR) undergoes both parallel and consec-
utive oxidation reactions, resulting in more complicated
adsorbed intermediates and byproducts. Most importantly,
the complete oxidation of ethanol to CO2 requires the cleavage
of the CeC bond, which is between two atoms with little
electron affinity or ionization energy, making it difficult to
.ao).ssor T. Nejat Veziroglu. P
platinum is the best-known material for the dissociative
adsorption of small organic molecules at low temperatures;
PtRu/C and PtSn/C have been widely accepted as the most
effective catalysts for the EOR in acidicmedia [4,7]. Combining
cyclic voltammetry (CV) with in-situ Fourier transform
infrared (FTIR) spectroscopy and differential electrochemical
mass spectroscopy (DEMS), the EOR on PtRu/C and PtSn/C
in acidic media was studied extensively [8e10]. The CV
results showed the addition of Ru or Sn to Pt could increase
the overall reaction rate of the EOR, both lowering the
onset potential and increasing the peak current density;
however, the FTIR and DEMS results demonstrated that as
compared to pure Pt, the PtRu or PtSn catalysts did not help
that much in improving the selectivity for CO2 formation, and
acetaldehyde and acetic acid were dominant products during
the EOR.
ublished by Elsevier Ltd. All rights reserved.
CH2
CH2
O
Rh
a
CH3
C
Pd
OH
b
Scheme 1 e An oxametallacyclic conformation formed
during ethanol adsorbed on an Rh (111) surface (a) and
h2-acetaldehyde formed during ethanol adsorbed on a Pd
(111) surface (b) [14].
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It has recently been reported that rhodium has a great
potential to achieve the CeC bond cleavage during the EOR
[11,12]. Owing to the formation of an oxametallacyclic confor-
mation (Scheme 1a), the CeC bond cleavage is the preferential
channel for the dissociation of ethanol on Rh surface, while
h2-acetaldehyde (Scheme 1b) is preferred on Pt or Pd surfaces
[13e15]. Tacconi et al. [16] investigated the EOR on Ir and Rh
electrodes in acidicmedia by in-situ FTIR technique, and found
that themajor product with the Ir electrodewas acetic acid, but
was CO2 with the Rh electrode. Since Rh is a far less active
catalyst for the EOR, it is usually combinedwith Pt as a catalyst.
The EOR on the PtRh electrodes in acidic media was studied by
both in-situ FTIR and DEMS techniques [17e23]. It was found
that the addition of Rh to Pt could indeed enhance the CO2 yield
during the EOR, but the overall rate of the EOR on the PtRh
catalyst was lower than that on the pure Pt catalyst. There are
two possible reasons that are responsible for the lower rate of
the EOR on the PtRh catalyst in acidic media. First, Rh has less
efficientdehydrogenationability thanPtdoes, leading toa lower
rate of the CeC bond cleavage to form COads, thus lowering the
overall reaction rate. Theother reason is related to theveryhigh
barrier for the COads oxidation caused by the strong COeRh
bonding. It should be recognized that the kinetics of the COads
oxidation can be accelerated at high pH values and a change
from acidic to alkaline media may also facilitate the CeC bond
cleavage during the EOR. In line with this idea, in this work we
prepared carbon supported PtRh catalysts by the microwave-
polyolmethod [24,25], and investigated their catalytic activities
for the EOR in alkaline media. The obtained PtRh/C catalysts
withdifferentPt/RhatomicratioswerecharacterizedwithX-ray
diffraction (XRD), transmission electron microscopy (TEM) and
X-ray photoelectron spectroscopy (XPS). The EOR on the PtRh/C
catalysts in alkalinemediawere examined by the CV and linear
sweep voltammetry (LSV) methods.
2. Experimental
2.1. Synthesis of the PtRh/C catalysts
All the chemicals usedwere of analytical grade. Chloroplatinic
acid hydrate (H2PtCl6$xH2O) and rhodium chloride hydrate
(RhCl3$xH2O) were purchased from Aldrich. Ethylene glycol
(EG), potassiumhydroxide (KOH), and ethanol (CH3CH2OH) (all
from Merck KGaA) were used as received. Vulcan XC-72
carbon (particle size 20e40 nm) was purchased from E-TEK,
while 5 wt.% Polytetrafluoroethylene (PTFE) emulsion was
received from Dupont. Carbon supported PtRh catalysts were
prepared by the microwave-polyol method. The metal
precursors of H2PtCl6$xH2O and RhCl3$xH2O with different
atomic ratios were first completely dissolved in EG/water
(3/1, v/v), carbon powders were then suspended into the
resulting solution under vigorous stirring. After a homoge-
neous suspension was formed, the resulting mixtures were
heated in a household microwave oven (Output: 800 W;
Frequency: 2450 MHz) for 180 s. The so-obtained precipitate
was collected by filtration, washed several times with ethanol
and deionized (DI) water, respectively, and dried at 70 �C in an
oven. For comparison, carbon supported Pt or Rh catalysts
were also prepared with the same method, and within all the
catalysts, a 20 wt.%metal (Pt and Rh) loading was guaranteed.
2.2. Catalyst characterizations
The XRD patterns of the Pt/C, Rh/C and PtRh/C catalysts with
different Pt/Rh atomic ratios were obtained with a Philips
powder diffraction system (model PW 1830) using a Cu Ka
source operating at 40 keV at a scan rate of 0.025�s�1. The TEM
images were obtained by using a high-resolution JEOL 2010F
TEM system operating with a LaB6 filament at 200 kV. The XPS
characterization was carried out with a Physical Electronics
PHI 5600 multi-technique system using Al monochromatic X-
ray at a power of 350 W. The survey and regional spectra were
obtained by passing energy of 187.85 and 23.5 eV, respectively.
2.3. Electrochemical characterizations
Both the CV and LSV tests were carried out using a potentio-
stat (EG&G Princeton, model 273A) in a conventional three-
electrode cell, in which a glass carbon electrode (GCE) with an
area of 0.1256 cm2 was used as the underlying support of the
working electrode, a platinum foil as the counter electrode,
and Hg/HgO/KOH (1.0 mol L�1) (MMO, 0.098 V vs. SHE) as the
reference electrode, which was connected to the cell through
a Luggin capillary. The GCE was modified by depositing
a catalyst layer onto it and served as the working electrode.
The catalyst ink was prepared by ultrasonically dispersing
10 mg of 20 wt.% Pt/C, Rh/C or PtRh/C catalysts in 1.9 mL of
ethanol, to which 0.1 mL of 5 wt.% PTFE emulsion was added.
After 30 min, a homogeneous solution was obtained and
a quantity of 12 mL of the ink was pipetted out on top of the
GCE and dried in air to yield a metal loading of 96 mg cm�2.
Solutions were prepared from analytical grade reagents and
DI water. All the CV and LSV experiments were done at room
temperature and in 1.0 M KOH solution containing 1.0 M
ethanol, which was deaerated by bubbling nitrogen (99.9%) for
30 min in advance. The CV tests were performed between the
potential ranges of �0.926e0.274 V at a scan rate of 50 mV s�1,
while 1 mV s�1 for the LSV tests. The potentials in this paper
all refer to theMMO, and the current densities were calculated
according to the geometric area of the GCE (0.1256 cm2).
Table 1 e Structural characteristics of the Pt/C, Rh/C andPtRh/C catalysts with different Pt/Rh ratios.
Nominalcomposition
Surfacecomposition by
XPS
d111space(�A)
Particle size(nm) by XRD
Pt/C 2.277 1.7
Pt3Rh/C Pt4.4Rh/C 2.271 1.7
Pt2Rh/C Pt2.8Rh/C 2.252 1.8
PtRh/C Pt1.5Rh/C 2.231 2.0
PtRh2/C Pt0.8Rh/C 2.223 2.4
Rh/C 2.199 2.6
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3. Results and discussion
3.1. Physicochemical characterizations
Fig. 1 shows the XRD diffraction patterns of the PtRh/C cata-
lysts, and the diffraction patterns of both Pt/C and Rh/C are
also given for comparison. For all these samples, the first
diffraction peak located at the 2q value of about 25� is referredto the graphite (002) facet of the carbon powder support, and
the other four diffraction peaks are characteristics of the face-
centered cubic (fcc) crystalline structure, corresponding to the
(111), (200), (220) and (311) planes, respectively. It can be
observed that the four diffraction peaks of the PtRh/C cata-
lysts are located at higher 2q values with respect to the same
reflection of Pt/C, while at lower 2q values compared to that of
Rh/C; and the diffraction peaks of the PtRh/C catalysts shift to
higher 2q values with an increase in Rh content, which can be
indexed to the incorporation of a lower d space crystal struc-
ture of Rh (d111 ¼ 2.20) compared to that of Pt (d111 ¼ 2.265).
Such evidence indicates a lattice constriction due to the
incorporation of smaller Rh atoms into the Pt fcc structure,
and suggests the alloy formation between Pt and Rh during
the synthesis of the PtRh/C catalysts [18,19]. The average size
of themetal particles is calculated based on the broadening of
the (220) diffraction peaks according to Scherrer’s equation
[26]:
d ¼ 0:9lB2qcos qmax
(1)
where l represents the wavelength of the X-ray (1.54056 �A), q
is the angle of the maximum peak, and B2q is the width of the
peak at the half height. The particle size and d111 space
parameters of all the samples are presented in Table 1.
The typical TEM images of the Pt/C, Rh/C and Pt2Rh/C
samples are, respectively, shown in Figs. 2aec. As can be seen,
the metal particles on all the three catalysts exhibit a spher-
ical-like shape and are well dispersed on the carbon powder
20 30 40 50 60 70 80 90
graphite (002)
Rh (200)
Rh (200)Rh (220) Rh (311)
graphite (002)Pt (311)Pt (220)
Pt (200)Pt (111)
Rh/C
PtRh2/C
PtRh/C
Pt2Rh/C
Pt3Rh/C
).u.a(ytisnetnI
2θ (degree)
Pt/C
Fig. 1 e XRD diffraction patterns of the Pt/C, Rh/C and PtRh/C
catalysts with different Pt/Rh atomic ratios.
support. The metal particles size distributions of the Pt/C,
Rh/C, and Pt2Rh/C catalystswere, respectively, evaluated from
an ensemble of 100 particles. Both the Pt/C and Pt2Rh/C cata-
lysts show the same metal particle size distribution ranging
from 1.2 nm to 4 nm, and the average metal particle sizes of
Pt/C and Pt2Rh/C are, respectively, 2.0 nm and 2.1 nm; while
Rh/C has a different particle size distribution, which is from
1.6 nm to 4.4 nm, and a larger average metal particle size of
2.5 nm. Fig. 2d shows the high-resolution TEM (HRTEM) image
of the Pt2Rh/C catalyst. It can be seen that the lattice fringes
can be observed across the entire image, indicating that the
prepared PtRh nanoparticles are entirely crystalline. The
d space of one randomly chosen particle, as denoted in Fig. 2d,
is 2.250 �A, very close to the value of 2.252 �A, which was pre-
dicted from the XRD data via Bragg law.
The XPS test was employed to analyze the surface
composition and the oxidation state of themetals on the PtRh/
C catalysts. The surface composition analyses based on the
intensities of XPS peaks for the PtRh/C catalysts are summa-
rized in Table 1. The Pt/Rh atomic ratios obtained by XPS show
some deviation from the nominal ratios in the precursors,
which can be ascribed to the fact that the reduction potential
of Rh3þ/Rh (E0 e 0.43 V) is much lower than that of Pt4þ/Pt(E0e0.74 V) in the presence of Cl� ions, and then the reduction
efficiency of Pt4þ to Pt is higher than that of Rh3þ to Rh during
the simultaneous reduction process, just like the case of the
PtRu/C catalyst [18,27,28]. The XPS spectra of all the Pt-con-
taining samples in the Pt4f region are shown in Fig. 3a, and the
normalized spectra are shown in Fig. 3b. According to Fig. 3,
the shape of the Pt4f XPS spectra are the same for the Pt/C and
PtRh/C catalysts, demonstrating the same distribution of
different Pt chemical states on them. For all the Pt-containing
samples, the Pt4f spectra show a doublet consisting of a high
energy band (Pt4f5/2) at about 74.8 eV and a low energy band
(Pt4f7/2) at about 71.5 eV, respectively, and this unambigously
indicates the existence of metallic state Pt.
3.2. Electrochemical properties
Fig. 4 compares the stabilized CVs of the EOR on the Pt/C, Rh/C
and PtRh/C catalysts in 1.0 M KOH containing 1.0 M ethanol.
For comparison, four parameters, including the onset poten-
tial of ethanol oxidation (Eonset), the anodic peak current
density in the forward scan ( jpeak), the potential correspond-
ing to jpeak (Epeak), and the ratio of the forward anodic peak
Fig. 2 e TEM images of Pt/C (a), Rh/C (b), Pt2Rh/C (c) and HRTEM image of Pt2Rh/C (d).
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current density ( jf) to the backward anodic peak current
density ( jb) ( jf/jb), were extracted from the CVs and are shown
in Table 2. The following observations to the PtRh/C samples
can be made with an increase in Rh content: the Eonset first
decreases and then increases; the jpeak first increases and then
decreases; the Epeak monotonously decreases; and the ratio of
jf/jb monotonously increases. Among all the PtRh/C catalysts,
66 68 70 72 74 76 78 80 82
0
1000
2000
3000
4000
5000
6000
Pt4f5/2 Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/Cs/c
Binding Energy (eV)
Pt4f7/2
a
Fig. 3 e Pt4f XPS spectra of the Pt/C and PtRh/C catalyst with diffe
the Pt2Rh/C catalyst shows the lowest Eonset, the highest jpeak,
a relative lower Epeak, and a relative higher jf/jb ratio toward
the EOR in alkalinemedia. The Eonset on the Pt2Rh/C catalyst is
�0.55 V, which is 50 mV lower than that on Pt/C; the Epeak on
Pt2Rh/C is �0.08 V, 20 mV lower than that on Pt/C; the jpeak on
Pt2Rh/C is 0.172 A cm�2, 0.027 A cm�2 higher than that on Pt/C.
Most attractively, the ratio of jf/jb on Pt2Rh/C is 1.9, twice as
70 72 74 760.0
0.2
0.4
0.6
0.8
1.0 Pt4f5/2
Pt4f7/2
ytisn etnidezila
m roN
Binding Energy (eV)
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C
b
rent Pt/Rh atomic ratios (a) and their normalized spectra (b).
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20ytisnedtnerru
C(
mcA
2-)
Potential (V) vs. MMO
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
Fig. 4 e CVs of the EOR on the Pt/C, Rh/C and PtRh/C
catalysts in 1.0 M KOH D 1.0 M ethanol.
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24 5mV/s 10mV/s 25mV/s 50mV/s 100mV/s 200mV/s
ytisnedtnerruC
(mc
A2-)
Potential (V) vs. MMO
Fig. 5 e CVs of the EOR on the Pt2Rh/C catalyst in 1.0 M
KOH D 1.0 M ethanol at different scan rates, and with the
insert: peak current density vs. square root of scan rate.
0.05
0.06
0.07
)2
Pt/C
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large as that on Pt/C. Usually, the anodic peak in the backward
scan represents the removal of the incompletely oxidized
species formed in the forward scan, and a high ratio of jf/jb can
be an indication of excellent oxidation of ethanol to CO2 and
less accumulation of the carbonaceous residues on the cata-
lyst [29,30]. The CVs of the EOR on the Pt2Rh/C catalyst in 1.0 M
KOH containing 1.0 M ethanol at different scan rates is shown
in Fig. 5, and the insert shows the relationship between the
peak current density and the square root of scan rate. As can
be seen, the peak current densities are linearly proportional to
the square root of the scan rates, suggesting that the EOR on
the Pt2Rh/C catalyst in alkaline media may be controlled by
a diffusion process [31].
The ethanol oxidation kinetics on the Pt/C, Rh/C and PtRh/
C catalysts in alkaline media was examined under the quasi-
steady-state conditions. Fig. 6 shows the LSVs of the EOR on
the Pt/C, Rh/C and PtRh/C catalysts in 1.0 M KOH containing
1.0 M ethanol. The sweep rate is 1mV s�1. As can be seen from
Fig. 6, compared to pure Pt, the addition of Rh to Pt can
significantly improve the ethanol oxidation kinetics in alka-
line media. Four parameters, including the onset potential of
ethanol oxidation (E�onset), the peak current density ( j�peak),
the current density at �0.4 V ( j at �0.4 V), and the current
density at �0.2 V ( j at �0.2 V) were extracted from the LSVs
and are shown in Table 3. For all the PtRh/C catalysts, the
Table 2 e Onset potentials, peak potentials, peak currentdensities and jf/jb ratios of the Pt/C, Rh/C and PtRh/Ccatalysts with different Pt/Rh ratios during the CV tests.
Nominalcomposition
Eonset (V) Epeak (V) jpeak(A cm�2)
jf/jb ratio
Pt/C �0.50 �0.060 0.145 0.9
Pt3Rh/C �0.54 �0.075 0.154 1.6
Pt2Rh/C �0.55 �0.080 0.172 1.9
PtRh/C �0.54 �0.130 0.145 2.5
PtRh2/C �0.50 �0.160 0.105 3.2
Rh/C �0.52 �0.280 0.019 1.0
Pt2Rh/C catalyst shows the highest ethanol oxidation kinetics
in alkalinemedia; the E�onset on the Pt2Rh/C catalyst is�0.53 V,
which is about 40 mV lower than that on Pt/C; the j�peak on
Pt2Rh/C is 0.068 A cm�2, about 2.4 times of that on Pt/C. Fig. 7
shows the Tafel plots of the EOR on the Pt/C, Rh/C and Pt2Rh/C
catalysts at lower overpotentials, calculated from the quasi-
steady-state curves in Fig. 6. Being determined from the linear
regions, the Tafel slopes at lower overpotentials for the the Pt/
C, Rh/C and Pt2Rh/C catalysts are 112 mV dec�1, 77 mV dec�1
and 102 mV dec�1, respectively. The slope for the Pt/C catalyst
is close to 120 mV dec�1 as reported elsewhere [32,33], and the
different slope values for Pt2Rh/C and Rh/C may indicate
a different reaction mechanism caused by the different
adsorption types of ethanol on Pt and Rh [13e15]. By extrap-
olating the linear regions of the Tafel plots, the exchange
current density on these catalysts can be obtained. The
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
0.00
0.01
0.02
0.03
0.04
ytisnedtnerruC
(mc
A-
Potential (V) vs. MMO
Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
Fig. 6 e LSVs of the EOR on the Pt/C, Rh/C and PtRh/C
catalysts in 1.0 M KOH D 1.0 M ethanol.
Table 3 e Onset potentials, peak current densities andcurrent densities at L0.4 V and L0.2 V of the Pt/C, Rh/Cand PtRh/C catalystswith different Pt/Rh ratios during theLSV tests.
Nominalcomposition
E�onset
(V)j�peak
(A cm�2)j at �0.4 V(A cm�2)
j at �0.2 V(A cm�2)
Pt/C �0.49 0.029 0.006 0.025
Pt3Rh/C �0.52 0.060 0.024 0.057
Pt2Rh/C �0.53 0.068 0.026 0.065
PtRh/C �0.52 0.064 0.024 0.058
PtRh2/C �0.51 0.062 0.019 0.057
Rh/C �0.48 0.023 0.010 0.008
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exchange current density on the Pt2Rh/C catalyst is
1.5 � 10�6 A cm�2, which is higher than that on both Pt/C and
Rh/C (8.0 � 10�7 A cm�2 for Pt/C and 2.0 � 10�8 A cm�2 for Rh/
C), further indicating that the Pt2Rh/C catalyst has a higher
catalytic activity towards the EOR in alkaline media than both
Pt/C and Rh/C.
According to XRD and TEM, the Pt/C and Pt2Rh/C catalysts
have the same metal particle size distribution and the differ-
ence between their average metal particle sizes is rather
small. Hence, the catalytic activity difference between Pt/C
and Pt2Rh/C due to the particle size contribution can be
neglected. In Fig. 3b, it can be observed that the shift in the Pt4f
binding energies for all the PtRh/C samples relative to that of
Pt/C is less than 0.1 eV, negligible small; this fact suggests that
the change in the electronic structure of Pt due to the addition
of Rh contributes little to the higher catalytic activity of the
Pt2Rh/C catalyst. As shown in Table 3, the onset potential of
ethanol oxidation on the Pt2Rh/C catalyst is �0.53 V, only
40 mV lower than that on Pt/C; besides, extended investiga-
tions indicated that Rh is more difficult for water dissociation
than Pt [34]. It can be assumed that the bi-functional mecha-
nism role of the Pt2Rh/C catalyst plays only a small part for its
higher catalytic activity toward the EOR in alkalinemedia [18].
It is confessedly proved that the addition of Rh to Pt will
increase the CO2 yield during the EOR; however, in an acidic
-3.2 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.40.34
0.36
0.38
0.40
0.42
0.44
0.46
112 mV dec-1
77 mV dec-1
102 mV dec-1
log id (A cm-2)
)V(laitnetoprevO
Pt/C Pt2Rh/C Rh/C
Fig. 7 e Tafel plots of the EOR on the Pt/C, Rh/C and Pt2Rh/C
catalysts in 1.0 M KOH D 1.0 M ethanol.
medium the oxidation kinetics of COads to CO2 is a rate-limit
factor, still limiting the overall reaction rate [17e23]. In our
work, the EOR on the PtRh/C catalysts were studied in an
alkaline medium, and the overall reaction rate was indeed
increased due to the addition of Rh; it is suggested that not
only the CeC bond cleavage rate can be improved in alkaline
media but also the poisoning effect of both carbonyl species
and COads will be much weaker in alkaline media than in
acidic media [35]. Hence, we conclude that the enhanced
electro-catalytic activity of the Pt2Rh/C catalyst can be
ascribed not only to the improvement of the CeC bond
cleavage in the presence of Rh, but also to the accelerated
oxidation kinetics of COads to CO2 in alkaline media.
4. Conclusions
In this work, carbon supported PtRh catalysts were synthe-
sized by the microwave-polyol method and investigated for
the EOR in alkaline media. The CV results demonstrated that
in alkaline media the Pt2Rh/C catalyst had a higher catalytic
activity, in terms of both the onset potential and the peak
current density, for the EOR than Pt/C did. The LSV results
showed that the peak current density of the EOR on Pt2Rh/C
was 0.068 A cm�2, about 2.4 times of that on Pt/C and 3 time on
Rh/C. According to the Tafel plots analyses, the exchange
current density on Pt2Rh/Cwas 1.5� 10�6 A cm�2 and the Tafel
slope on Pt2Rh/C was 102 mV dec�1. The enhanced electro-
catalytic activity of the Pt2Rh/C catalyst can be ascribed not
only to the improvement of the CeC bond cleavage in the
presence of Rh, but also to the accelerated oxidation kinetics
of COads to CO2 in an alkaline medium.
Acknowledgements
The work described in this paper was fully supported by
a grant from the Research Grants Council of the Hong Kong
Special Administrative Region, China (Project No. 623008).
r e f e r e n c e s
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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 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 7 12917
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