Carbon supported PtRh catalysts for ethanol oxidation in alkaline 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 article info 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 abstract 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 CO 2 yield during the ethanol oxidation. However, in acidic media the slow oxidation kinetics of CO ads to CO 2 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 Pt 2 Rh/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 Pt 2 Rh/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 CO ads to CO 2 in alkaline media. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction 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 CO 2 , 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 CO 2 requires the cleavage of the CeC bond, which is between two atoms with little electron affinity or ionization energy, making it difficult to break the CeC bond at low temperatures [4e6]. Up to now, 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 acidic media [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 CO 2 formation, and acetaldehyde and acetic acid were dominant products during the EOR. * Corresponding author. Tel.: þ852 2358 8647. E-mail address: [email protected](T.S. Zhao). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 35 (2010) 12911 e12917 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.107
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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
Avai lab le a t www.sc iencedi rec t .com
journa l homepage : www.e lsev ier . com/ loca te /he
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,
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
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 12915
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
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
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 712916
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
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).
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