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Carbon supported PtRh catalysts for ethanol oxidation in alkaline direct ethanol fuel cell
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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
abstract
Article history:
Owing to the formation of an oxametallacyclic conformation, the CeC bond cleavage is the
Received 26 May 2010
preferential channel for the ethanol dissociation on the Rh surface, the addition of Rh to Pt
Received in revised form
can increase the CO2 yield during the ethanol oxidation. However, in acidic media the slow
20 August 2010
oxidation kinetics of COads to CO2 limits the overall reaction rate. In this work, we prepare
Accepted 25 August 2010
carbon supported PtRh catalysts and compare their catalytic activities with that of Pt/C in
Available online 26 September 2010
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
Keywords:
show that the peak current density on Pt2Rh/C is about 2.4 times of that on Pt/C. The
Fuel cell
enhanced electro-activity can be ascribed not only to the improved CeC bond cleavage in
Ethanol oxidation reaction (EOR)
the presence of Rh, but also to the accelerated oxidation kinetics of COads to CO2 in alkaline
Alkaline direct ethanol fuel cell
media.
PtRh catalyst
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
The CeC bond cleavage
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 kg1 vs. 6.1 kWh kg1), 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 consecutive 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
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 CO2 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). 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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a
CH2 CH2
O Rh
b
CH3 H C
O Pd
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].
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 conformation (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 acidic media by in-situ FTIR technique, and found that the major product with the Ir electrode was acetic acid, but was CO2 with the Rh electrode. Since Rh is a far less active catalyst for the EOR, it is usually combined with 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 efficient dehydrogenation ability than Pt does, leading to a lower rate of the CeC bond cleavage to form COads, thus lowering the overall reaction rate. The other reason is related to the very high 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 microwavepolyol method [24,25], and investigated their catalytic activities for the EOR in alkaline media. The obtained PtRh/C catalysts with different Pt/Rh atomic ratios were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The EOR on the PtRh/C catalysts in alkaline media were examined by the CV and linear sweep voltammetry (LSV) methods.
2.
Experimental
2.1.
Synthesis of the PtRh/C catalysts
All the chemicals used were of analytical grade. Chloroplatinic acid hydrate (H2PtCl6$xH2O) and rhodium chloride hydrate
(RhCl3$xH2O) were purchased from Aldrich. Ethylene glycol (EG), potassium hydroxide (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 homogeneous 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 s1. 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 Xray 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 potentiostat (EG&G Princeton, model 273A) in a conventional threeelectrode 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 L1) (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 cm2. 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 s1, while 1 mV s1 for the LSV tests. The potentials in this paper all refer to the MMO, and the current densities were calculated according to the geometric area of the GCE (0.1256 cm2).
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3.
Results and discussion
Table 1 e Structural characteristics of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh ratios.
3.1.
Physicochemical characterizations
Nominal composition
Fig. 1 shows the XRD diffraction patterns of the PtRh/C catalysts, 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 referred to the graphite (002) facet of the carbon powder support, and the other four diffraction peaks are characteristics of the facecentered 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 catalysts 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 structure 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 the metal particles is calculated based on the broadening of the (220) diffraction peaks according to Scherrer’s equation [26]: d¼
0:9l B2q cos 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 spherical-like shape and are well dispersed on the carbon powder
Rh (200) graphite (002)
Rh (200)
Rh/C
Rh (220)
Rh (311)
Intensity (a.u.)
PtRh2/C PtRh/C Pt2Rh/C
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
Surface composition by XPS Pt4.4Rh/C Pt2.8Rh/C Pt1.5Rh/C Pt0.8Rh/C
d111 space ( A)
Particle size (nm) by XRD
2.277 2.271 2.252 2.231 2.223 2.199
1.7 1.7 1.8 2.0 2.4 2.6
support. The metal particles size distributions of the Pt/C, Rh/C, and Pt2Rh/C catalysts were, respectively, evaluated from an ensemble of 100 particles. Both the Pt/C and Pt2Rh/C catalysts 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 predicted from the XRD data via Bragg law. The XPS test was employed to analyze the surface composition and the oxidation state of the metals on the PtRh/ C catalysts. The surface composition analyses based on the intensities of XPS peaks for the PtRh/C catalysts are summarized 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-containing 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.
Pt3Rh/C graphite (002)
20
30
Pt (111) Pt (200)
40
50
3.2. Pt/C
60
Pt (220)
70
Electrochemical properties
Pt (311)
80
90
2θ (degree) Fig. 1 e XRD diffraction patterns of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh atomic ratios.
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 potential of ethanol oxidation (Eonset), the anodic peak current density in the forward scan ( jpeak), the potential corresponding to jpeak (Epeak), and the ratio of the forward anodic peak
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Fig. 2 e TEM images of Pt/C (a), Rh/C (b), Pt2Rh/C (c) and HRTEM image of Pt2Rh/C (d).
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,
b
Pt4f7/2
6000
Pt4f5/2
Pt/C Pt3Rh/C
5000
Pt2Rh/C
4000
c/s
Normalized intensity
a
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 alkaline media. 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 cm2, 0.027 A cm2 higher than that on Pt/C. Most attractively, the ratio of jf/jb on Pt2Rh/C is 1.9, twice as
PtRh/C PtRh2/C
3000
2000
1000
Pt4f 7/2
1.0
Pt4f5/2
0.8
0.6
0.4
Pt/C Pt3Rh/C Pt2Rh/C
0.2
PtRh/C PtRh 2/C
0
0.0 66
68
70
72
74
76
Binding Energy (eV)
78
80
82
70
72
74
76
Binding Energy (eV)
Fig. 3 e Pt4f XPS spectra of the Pt/C and PtRh/C catalyst with different Pt/Rh atomic ratios (a) and their normalized spectra (b).
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0.20
0.24
0.18
0.22
0.18
Current density ( A cm )
0.14 0.12
-2
Pt2Rh/C
-2
Current density ( A cm )
0.20
Pt/C Pt3Rh/C
0.16
PtRh/C PtRh2/C
0.10
Rh/C
0.08 0.06 0.04 0.02
0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02
0.00 -0.02 -1.0
5mV/s 10mV/s 25mV/s 50mV/s 100mV/s 200mV/s
0.00
-0.8
-0.6
-0.4
-0.2
0.0
0.2
-0.02 -1.0
0.4
-0.8
-0.6
0.0
0.2
0.4
Pt2Rh/C catalyst shows the highest ethanol oxidation kinetics in alkaline media; 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 cm2, 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 quasisteady-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 dec1, 77 mV dec1 and 102 mV dec1, respectively. The slope for the Pt/C catalyst is close to 120 mV dec1 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 extrapolating the linear regions of the Tafel plots, the exchange current density on these catalysts can be obtained. The
0.07 0.06 0.05
Pt/C Pt3Rh/C
-2
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 catalyst [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 quasisteady-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 1 mV s1. 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 alkaline 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 current densities and jf/jb ratios of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh ratios during the CV tests.
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
-0.2
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.
Current density ( A cm )
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.
Nominal composition
-0.4
Potential (V) vs. MMO
Potential (V) vs. MMO
Eonset (V)
Epeak (V)
jpeak (A cm2)
jf/jb ratio
0.50 0.54 0.55 0.54 0.50 0.52
0.060 0.075 0.080 0.130 0.160 0.280
0.145 0.154 0.172 0.145 0.105 0.019
0.9 1.6 1.9 2.5 3.2 1.0
0.04
Pt2Rh/C
0.03
PtRh/C PtRh2/C
0.02
Rh/C
0.01 0.00
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Potential (V) vs. MMO
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.
0.4
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Table 3 e Onset potentials, peak current densities and current densities at L0.4 V and L0.2 V of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh ratios during the LSV tests. Nominal composition
j peak E onset (V) (A cm2) 0.49 0.52 0.53 0.52 0.51 0.48
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
j at 0.4 V (A cm2)
j at 0.2 V (A cm2)
0.006 0.024 0.026 0.024 0.019 0.010
0.025 0.057 0.065 0.058 0.057 0.008
0.029 0.060 0.068 0.064 0.062 0.023
exchange current density on the Pt2Rh/C catalyst is 1.5 106 A cm2, which is higher than that on both Pt/C and Rh/C (8.0 107 A cm2 for Pt/C and 2.0 108 A cm2 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 difference 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 investigations indicated that Rh is more difficult for water dissociation than Pt [34]. It can be assumed that the bi-functional mechanism role of the Pt2Rh/C catalyst plays only a small part for its higher catalytic activity toward the EOR in alkaline media [18]. It is confessedly proved that the addition of Rh to Pt will increase the CO2 yield during the EOR; however, in an acidic
0.46
4.
Conclusions
In this work, carbon supported PtRh catalysts were synthesized 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 cm2, 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/C was 1.5 106 A cm2 and the Tafel slope on Pt2Rh/C was 102 mV dec1. The enhanced electrocatalytic 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).
references Pt/C -1 Pt2Rh/C 112 mV dec
0.44
Overpotential (V)
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.
Rh/C
0.42
-1
77 mV dec 0.40
0.38
-1
102 mV dec
0.36
0.34 -3.2
-3.0
-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
-1.6
-1.4
-2
log id ( A cm )
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.
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[7] Li HQ, Sun GQ, Cao L, Jiang LH, Xin Q. Comparison of different promotion effect of PtRu/C and PtSn/C electrocatalysts for ethanol electro-oxidation. Electrochim Acta 2007;52:6622e9. [8] Wang Q, Sun GQ, Jiang LH, Xin Q, Sun SG, Jiang YX, et al. Adsorption and oxidation of ethanol on colloid-based Pt/C, PtRu/C and Pt3Sn/C catalysts: in situ FTIR spectroscopy and on-line DEMS studies. Phys Chem Chem Phys 2007;9: 2686e96. [9] Xia XH, Liess HD, Iwasita T. Early stages in the oxidation of ethanol at low index single crystal platinnum electrodes. J Electroanal Chem 1997;437:233e40. [10] Wang H, Jusys Z, Behm RJ. Ethanol electro-oxidation on carbon-supported Pt, PtRu and Pt3Sn catalysts: a quantitative DEMS study. J Power Sources 2006;154:351e9. [11] Kowal A, Li M, Shao M, Sasaki K, Vukmirovic MB, Zhang J, et al. Ternary Pt/Rh/SnO2 electrocatalysts for oxidizing ethanol to CO2. Nature Materials 2009;8(4):325e30. SL, Lee KS, Olszewski P, Sung YE. [12] Kowal A, Gojkovic Synthesis, characterization and electrocatalytic activity for ethanol oxidation of carbon supported Pt. PteRh, PteRheSnO2 nanoclusters. Electrochem Commun 2009;11: 724e7. [13] Sheng PY, Yee A, Bowmaker GA, Idriss H. H2 production from ethanol over RhePt/CeO2 catalysts: the role of Rh for the efficient dissociation of the carbonecarbon bond. J Catal 2002;208:393e403. [14] Mavrikakis M, Doren DJ, Barteau MA. Density functional theory calculations for simple oxametallacycles: trends across the periodic table. J Phys Chem B 1998;102: 394e9. [15] Vesselli E, Baraldi A, Comelli G, Lizzit S, Rosei R. Ethanol decomposition: CeC cleavage selectivity on Rh(111). ChemPhysChem 2004;5:1133e40. [16] Tacconi NR, Lezna RO, Beden B, Hahn F, Lamy C. In-situ FTIR study of the electrocatalytic oxidation of ethanol at iridium and rhodium electrodes. J Electroanal Chem 1994;379: 329e37. [17] Souza JPI, Queiroz SL, Bergamaski K, Gonzalez ER, Nart FC. Electro-oxidation of ethanol on Pt, Rh, and PtRh electrodes. A study using DEMS and in-situ FTIR techniques. J Phys Chem B 2002;106:9825e30. [18] Gupta SS, Datta J. A comparative study on ethanol oxidation behavior at Pt and PtRh electrodeposites. J Electroanal Chem 2006;594:65e72. [19] Bergamaski K, Gonzalea ER, Nart FC. Ethanol oxidation on carbon supported platinumerhodium bimetallic catalysts. Electrochim Acta 2008;53:4396e406. [20] Lima FHB, Profeti D, Valbuena WHL, Ticianelli EA, Gonzalez ER. Carbon-dispersed PteRh nanoparticles for ethanol electro-oxidaiton. Effect of the crystallite size of temperature. J Electroanal Chem 2008;617:121e9.
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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
abstract
Article history:
Owing to the formation of an oxametallacyclic conformation, the CeC bond cleavage is the
Received 26 May 2010
preferential channel for the ethanol dissociation on the Rh surface, the addition of Rh to Pt
Received in revised form
can increase the CO2 yield during the ethanol oxidation. However, in acidic media the slow
20 August 2010
oxidation kinetics of COads to CO2 limits the overall reaction rate. In this work, we prepare
Accepted 25 August 2010
carbon supported PtRh catalysts and compare their catalytic activities with that of Pt/C in
Available online 26 September 2010
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
Keywords:
show that the peak current density on Pt2Rh/C is about 2.4 times of that on Pt/C. The
Fuel cell
enhanced electro-activity can be ascribed not only to the improved CeC bond cleavage in
Ethanol oxidation reaction (EOR)
the presence of Rh, but also to the accelerated oxidation kinetics of COads to CO2 in alkaline
Alkaline direct ethanol fuel cell
media.
PtRh catalyst
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
The CeC bond cleavage
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 kg1 vs. 6.1 kWh kg1), 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 consecutive 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
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 CO2 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). 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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a
CH2 CH2
O Rh
b
CH3 H C
O Pd
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].
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 conformation (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 acidic media by in-situ FTIR technique, and found that the major product with the Ir electrode was acetic acid, but was CO2 with the Rh electrode. Since Rh is a far less active catalyst for the EOR, it is usually combined with 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 efficient dehydrogenation ability than Pt does, leading to a lower rate of the CeC bond cleavage to form COads, thus lowering the overall reaction rate. The other reason is related to the very high 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 microwavepolyol method [24,25], and investigated their catalytic activities for the EOR in alkaline media. The obtained PtRh/C catalysts with different Pt/Rh atomic ratios were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The EOR on the PtRh/C catalysts in alkaline media were examined by the CV and linear sweep voltammetry (LSV) methods.
2.
Experimental
2.1.
Synthesis of the PtRh/C catalysts
All the chemicals used were of analytical grade. Chloroplatinic acid hydrate (H2PtCl6$xH2O) and rhodium chloride hydrate
(RhCl3$xH2O) were purchased from Aldrich. Ethylene glycol (EG), potassium hydroxide (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 homogeneous 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 s1. 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 Xray 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 potentiostat (EG&G Princeton, model 273A) in a conventional threeelectrode 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 L1) (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 cm2. 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 s1, while 1 mV s1 for the LSV tests. The potentials in this paper all refer to the MMO, and the current densities were calculated according to the geometric area of the GCE (0.1256 cm2).
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3.
Results and discussion
Table 1 e Structural characteristics of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh ratios.
3.1.
Physicochemical characterizations
Nominal composition
Fig. 1 shows the XRD diffraction patterns of the PtRh/C catalysts, 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 referred to the graphite (002) facet of the carbon powder support, and the other four diffraction peaks are characteristics of the facecentered 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 catalysts 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 structure 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 the metal particles is calculated based on the broadening of the (220) diffraction peaks according to Scherrer’s equation [26]: d¼
0:9l B2q cos 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 spherical-like shape and are well dispersed on the carbon powder
Rh (200) graphite (002)
Rh (200)
Rh/C
Rh (220)
Rh (311)
Intensity (a.u.)
PtRh2/C PtRh/C Pt2Rh/C
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
Surface composition by XPS Pt4.4Rh/C Pt2.8Rh/C Pt1.5Rh/C Pt0.8Rh/C
d111 space ( A)
Particle size (nm) by XRD
2.277 2.271 2.252 2.231 2.223 2.199
1.7 1.7 1.8 2.0 2.4 2.6
support. The metal particles size distributions of the Pt/C, Rh/C, and Pt2Rh/C catalysts were, respectively, evaluated from an ensemble of 100 particles. Both the Pt/C and Pt2Rh/C catalysts 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 predicted from the XRD data via Bragg law. The XPS test was employed to analyze the surface composition and the oxidation state of the metals on the PtRh/ C catalysts. The surface composition analyses based on the intensities of XPS peaks for the PtRh/C catalysts are summarized 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-containing 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.
Pt3Rh/C graphite (002)
20
30
Pt (111) Pt (200)
40
50
3.2. Pt/C
60
Pt (220)
70
Electrochemical properties
Pt (311)
80
90
2θ (degree) Fig. 1 e XRD diffraction patterns of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh atomic ratios.
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 potential of ethanol oxidation (Eonset), the anodic peak current density in the forward scan ( jpeak), the potential corresponding to jpeak (Epeak), and the ratio of the forward anodic peak
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Fig. 2 e TEM images of Pt/C (a), Rh/C (b), Pt2Rh/C (c) and HRTEM image of Pt2Rh/C (d).
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,
b
Pt4f7/2
6000
Pt4f5/2
Pt/C Pt3Rh/C
5000
Pt2Rh/C
4000
c/s
Normalized intensity
a
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 alkaline media. 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 cm2, 0.027 A cm2 higher than that on Pt/C. Most attractively, the ratio of jf/jb on Pt2Rh/C is 1.9, twice as
PtRh/C PtRh2/C
3000
2000
1000
Pt4f 7/2
1.0
Pt4f5/2
0.8
0.6
0.4
Pt/C Pt3Rh/C Pt2Rh/C
0.2
PtRh/C PtRh 2/C
0
0.0 66
68
70
72
74
76
Binding Energy (eV)
78
80
82
70
72
74
76
Binding Energy (eV)
Fig. 3 e Pt4f XPS spectra of the Pt/C and PtRh/C catalyst with different Pt/Rh atomic ratios (a) and their normalized spectra (b).
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0.20
0.24
0.18
0.22
0.18
Current density ( A cm )
0.14 0.12
-2
Pt2Rh/C
-2
Current density ( A cm )
0.20
Pt/C Pt3Rh/C
0.16
PtRh/C PtRh2/C
0.10
Rh/C
0.08 0.06 0.04 0.02
0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02
0.00 -0.02 -1.0
5mV/s 10mV/s 25mV/s 50mV/s 100mV/s 200mV/s
0.00
-0.8
-0.6
-0.4
-0.2
0.0
0.2
-0.02 -1.0
0.4
-0.8
-0.6
0.0
0.2
0.4
Pt2Rh/C catalyst shows the highest ethanol oxidation kinetics in alkaline media; 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 cm2, 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 quasisteady-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 dec1, 77 mV dec1 and 102 mV dec1, respectively. The slope for the Pt/C catalyst is close to 120 mV dec1 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 extrapolating the linear regions of the Tafel plots, the exchange current density on these catalysts can be obtained. The
0.07 0.06 0.05
Pt/C Pt3Rh/C
-2
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 catalyst [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 quasisteady-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 1 mV s1. 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 alkaline 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 current densities and jf/jb ratios of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh ratios during the CV tests.
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
-0.2
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.
Current density ( A cm )
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.
Nominal composition
-0.4
Potential (V) vs. MMO
Potential (V) vs. MMO
Eonset (V)
Epeak (V)
jpeak (A cm2)
jf/jb ratio
0.50 0.54 0.55 0.54 0.50 0.52
0.060 0.075 0.080 0.130 0.160 0.280
0.145 0.154 0.172 0.145 0.105 0.019
0.9 1.6 1.9 2.5 3.2 1.0
0.04
Pt2Rh/C
0.03
PtRh/C PtRh2/C
0.02
Rh/C
0.01 0.00
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Potential (V) vs. MMO
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.
0.4
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Table 3 e Onset potentials, peak current densities and current densities at L0.4 V and L0.2 V of the Pt/C, Rh/C and PtRh/C catalysts with different Pt/Rh ratios during the LSV tests. Nominal composition
j peak E onset (V) (A cm2) 0.49 0.52 0.53 0.52 0.51 0.48
Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C
j at 0.4 V (A cm2)
j at 0.2 V (A cm2)
0.006 0.024 0.026 0.024 0.019 0.010
0.025 0.057 0.065 0.058 0.057 0.008
0.029 0.060 0.068 0.064 0.062 0.023
exchange current density on the Pt2Rh/C catalyst is 1.5 106 A cm2, which is higher than that on both Pt/C and Rh/C (8.0 107 A cm2 for Pt/C and 2.0 108 A cm2 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 difference 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 investigations indicated that Rh is more difficult for water dissociation than Pt [34]. It can be assumed that the bi-functional mechanism role of the Pt2Rh/C catalyst plays only a small part for its higher catalytic activity toward the EOR in alkaline media [18]. It is confessedly proved that the addition of Rh to Pt will increase the CO2 yield during the EOR; however, in an acidic
0.46
4.
Conclusions
In this work, carbon supported PtRh catalysts were synthesized 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 cm2, 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/C was 1.5 106 A cm2 and the Tafel slope on Pt2Rh/C was 102 mV dec1. The enhanced electrocatalytic 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).
references Pt/C -1 Pt2Rh/C 112 mV dec
0.44
Overpotential (V)
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.
Rh/C
0.42
-1
77 mV dec 0.40
0.38
-1
102 mV dec
0.36
0.34 -3.2
-3.0
-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
-1.6
-1.4
-2
log id ( A cm )
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.
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