- Email: [email protected]
C nanocatalyst with high performance for ethanol electro-oxidation in alkaline medium
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Facile synthesis of PdSbx/C nanocatalyst with high performance for ethanol electro-oxidation in alkaline medium Jindi Cai, Yiyin Huang, Yonglang Guo* College of Chemistry, Fuzhou University, Fuzhou 350116, PR China
article info
abstract
Article history:
The PdSbx/C (x ¼ 0, 0.5, 0.1, 0.15 and 0.2) nanocatalysts were synthesized by the microwave
Received 13 May 2014
treatment. The structure and morphology of the as-prepared catalysts were characterized
Received in revised form
by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelec-
31 August 2014
tron spectroscopy (XPS). It is found that the addition of antimony into the Pd/C catalyst
Accepted 1 September 2014
leads to formation of PdSb alloy and reduction in particle size. The electrochemical mea-
Available online 26 September 2014
surements indicate that the PdSb0.15/C catalyst has high electrocatalytic activity and excellent anti-poisoning ability towards ethanol oxidation in alkaline medium. The onset
Keywords:
potential of ethanol oxidation on PdSb0.15/C shifts in negative direction as compared with
Alkaline medium
Pd/C. The mass activity of PdSb0.15/C for ethanol oxidation reaches 3690 mA mg1 Pd,
Ethanol oxidation
which is ca. 1.7 times higher than that of Pd/C. The enhanced performance of PdSb0.15/C is
Microwave method
mainly ascribed to the bifunctional mechanism and electronic effect.
Palladiumeantimony
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Pd-based catalyst
Introduction Direct alcohol fuel cells (DAFCs) have obtained considerable attention because of their promising commercialization as the power source for portable electronic devices and fuel-cell vehicles [1]. In comparison with pure hydrogen, the use of liquid alcohols as fuel has several advantages, such as easy storage and transportation [2]. Ethanol is an attractive alternative fuel to methanol because it has high theoretical energy density (8.01 kWh kg1) and low toxicity. Besides, ethanol is a renewable fuel and can be easily gained from the fermentation of agricultural products [1,3,4]. However, the complete oxidation of ethanol is complicated and hard to achieve. The process involves 12 electron transfer and the cleavage of CeC
bond [5]. Furthermore, the kinetics of ethanol oxidation reaction (EOR) is still sluggish even on Pt-based catalyst, which is widely considered as the best catalyst for ethanol oxidation in acidic medium. In addition, the application of Pt-based catalyst is restrained by the limited availability and high cost of Pt [6]. Hence the design of highly active and cheap anodic catalyst is quite urgent in the development of direct ethanol fuel cell (DEFC). In recent years, with the development of anion-exchange membrane (AEM), more and more attention has been paid to electrocatalysis in alkaline medium [7]. The kinetics of both alcohol oxidation and oxygen reduction in alkaline medium is greatly enhanced as compared with that in acidic medium [8]. Pd is a prospective substitute for Pt because of its relatively abundant reserves and low price. Moreover, Pd presents high
* Corresponding author. Tel./fax: þ86 591 8807 3608. E-mail address: [email protected] (Y. Guo). http://dx.doi.org/10.1016/j.ijhydene.2014.09.011 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
electrocatalytic activity for ethanol oxidation in alkaline medium [9]. Thus Pd-based catalyst is an excellent candidate for the application in DEFCs. The combination of Pd with other metals or metal oxides can further improve its electrocatalytic activity for ethanol oxidation. Various bimetallic PdM (M ¼ Ru, Sn, Au, Ag, Ni, Cu, Bi, etc.) catalysts have been investigated and they present high electrocatalytic activity toward EOR in alkaline solution [10e16]. Besides, the presence of oxides on Pd-based catalysts such as CeO2, NiO, Co3O4, Mn3O4, MnO2 and In2O3 remarkably lowers the onset potential of ethanol oxidation and enhances their electrocatalytic activity and stability for EOR [17e19]. Antimony, as a co-catalyst, has been widely applied in fuel cells and shown to enhance the electrocatalytic property and poisoning tolerance of catalysts [20e24]. Yu et al. [20,21] prepared PtSb/C and PdSb/C catalysts via co-deposition method. The as-prepared catalysts showed superior performance in direct formic acid fuel cells (DFAFCs) and high resistance to poisoning in formic oxidation. Methanol electro-oxidation on PtSb ordered intermetallic compound had been studied by Zhang et al. [22]. The electrocatalytic activity of PtSb for methanol oxidation is higher than that of polycrystalline platinum electrode. Silva et al. [23] synthesized PtSb ordered intermetallic compound supported on Vulcan XC-72 active carbon (PtSb/C) and found that PtSb/C had excellent electrocatalytic activity for alcohol oxidation as compared with Pt/C. In this work, the PdSbx/C electrocatalysts with different atomic ratios of Pd:Sb were prepared via a facile microwave method. The as-prepared catalyst structures were characterized by XRD, TEM and XPS. The electrocatalytic activity and mechanism of the PdSbx/C catalysts toward EOR were investigated by the electrochemical measurements.
18257
Physical and electrochemical characterization The XRD patterns of the as-prepared catalysts were carried out by using a Philip X'Pert Pro MPP X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 Å) at the scan rate of 2 min1 with a step of 0.02 . The morphology and size distribution of the catalysts were measured by transmission electron microscope (TEM, JEOL JEM-1010) operating at 200 kV. The X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) was conducted using an Al Ka X-ray source of 1486.6 eV to characterize the chemical valence state and the surface composition of the catalysts. The electrochemical measurements were carried out with a CHI 660C electrochemical working station (CH Instrument Inc.). A three-electrode electrolytic cell with a piece of Pt foil and Hg/HgO/ 1 M KOH (MMO, 0.098 V vs. SHE) as the counter and reference electrodes, respectively, was used. To prepare the working electrode, the glassy carbon electrode (GCE, 0.1256 cm2) was burnished with Al2O3 paste, cleaned and then deposited with a thin film of the as-prepared catalyst. 5.0 mg as-prepared catalyst was dispersed in 1 mL solution of 985 mL isopropyl alcohol and 15 mL Nafion solution (15 wt%, DuPont) in ultrasonic bath for 30 min to form catalyst ink. Then, 4 mL ink was dispersed onto the GCE surface and subsequently dried under an infrared lamp. The Pd loading on GCE was maintained at 4 mg. The electrochemical measurements were performed at 30 ± 1 C. Before all measurements, the electrolyte solution was saturated with high purity nitrogen. Cyclic voltammograms (CV) were performed in the potential region of 0.95 ~ 0.5 V in 1 M KOH solution containing 1.0 M C2H5OH at a scan rate of 50 mV s1. Chronoamperometric measurements were carried out in 1 M KOH þ 1 M C2H5OH solution at 0.3 V for 1800 s. The CO-stripping experiments similar to our previous study were performed [16]. To obtain the stable data in all electrochemical measurements, the test electrode went through 20 cycles in advance in the N2-saturated blank KOH solution.
Experimental Synthesis of Pd-based nanocatalysts
Results and discussion
The active carbon black (Vulcan XC-72R, Cabot Corp.) was added to the concentrated HNO3 in a flask which was immersed in an oil bath at 80 C for 10 h. Then, the mixture was neutralized with KOH, filtered, washed with doubledistilled water and dried at 80 C overnight in a vacuum oven. The PdSbx/C catalysts were synthesized by a microwave polyol method. PdCl2 was dissolved in HCl solution to form H2PdCl4 solution, which was used as Pd precursor. 15.6 mg of Vulcan XC-72R carbon was dispersed in 50 mL of ethylene glycol (EG) in ultrasonic bath. Then, the appropriate amount of H2PdCl4 (37.8 mM) and antimony potassium tartrate (APT, 18.9 mM) were added to the suspension and the mixture was ultrasonically stirred for 30 min. The pH of the solution above was kept at 10 by dripping KOH solution (0.4 M). The welldispersed mixture in the flask with a reflux set was treated for 5 min under the microwave power of 800 W. The asprepared samples were filtrated, washed with double-distill water and dried in a vacuum oven overnight. The final catalysts were denoted as Pd/C and PdSbx/C (x ¼ 0.05, 0.1, 0.15 and 0.2). The weight percentage of Pd was maintained at 20 wt% in all catalysts.
Fig. 1 shows the XRD patterns of Pd/C and PdSbx/C catalysts. All samples display a broad diffraction peak at about 25 which is referred to graphite (002) facet of carbon black. The Pd/C catalyst presents four main diffraction peaks located at 2q values of about 39.9 , 46.4 , 68.0 and 81.8 which are attributed to the (111), (200), (220) and (311) facets of the Pd fcc crystal (JCPDS Card No. 05-0681), respectively [25]. As shown in Table 1, it is clear that the four main diffraction peaks of all PdSbx/C catalysts shift to lower 2q values with respect to those of Pd/C catalyst, suggesting the formation of PdSb alloying in the synthesis of PdSbx/C catalysts. Two peaks at 39.4 and 40.0 for the PdSb0.2/C catalyst can be ascribed to Pd (111) and Pd75Sb25 (220) facets (JCPDS Card No. 65-7876) [21]. The average size of the metal nanoparticles (d) for these catalysts is estimated from the broadening diffraction peak of Pd (220) by using the Scherrer's equation [26]: d¼
0:9l b cos q
(1)
where l is denoted as the wavelength of X-ray (1.5418 Å), q the angle of the Pd (220) peak, and b its half-peak width. According
18258
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
Fig. 1 e XRD patterns of Pd/C and PdSbx/C catalysts with different atomic ratios of Pd/Sb.
to Scherrer's equation, the average particle sizes of the Pd/C and PdSb0.15/C catalysts are 4.0 and 3.8 nm (see Table 1), respectively. The morphology and the particle-size distribution of Pd/C and PdSb0.15/C catalysts were analyzed by TEM images and shown in Fig. 2. The metal nanoparticles are well dispersed on the carbon support in both catalysts, but with a little agglomeration. This phenomenon was also reported in Refs. [16,27,28]. The rapid growth of Pd nuclei during microwave treatment inevitably induces some Pd aggregation. The insets in Fig. 2A and B show the high-resolution TEM (HRTEM) images of Pd/C and PdSb0.15/C catalysts, respectively. In Pd/C catalyst, there are two groups of typical lattice fringes with ca. 2.27 and 1.88 Å, which are ascribed to the (111) and (200) planes of Pd crystal, respectively. For PdSb0.15/C catalyst, besides the planes of Pd metal, the lattice stripe with interplanar spacing of 2.25 Å is also presented, which is corresponding to the Pd75Sb25 (220) plane (JCPDS card No. 65-7876). In addition, the particle size distribution histograms of the Pd/C and PdSb0.15/ C were evaluated by counting more than 500 casually selected nanoparticles in the corresponding HRTEM images, as shown in Fig. 2C and D. The average particle sizes of Pd/C and PdSb0.15/C catalysts are 4.4 and 4.0 nm, respectively. Therefore, the presence of Sb in Pd/C catalyst reduces the catalyst nanoparticle size, which is in agreement with the XRD results. The XPS spectra of Pd/C and PdSb0.15/C were used to determine the metal valence states on catalyst surfaces and shown in Fig. 3. The Pd 3d spectra of both catalysts display two
Table 1 e XRD data of Pd/C and PdSbx/C catalysts. Angles of Pd faces ( )
Samples
Half-peak Particle width of Pd sizes (nm) (111) (200) (220) (311) (220) (rad)
Pd/C PdSb0.05/C PdSb0.1/C PdSb0.15/C PdSb0.2/C
39.89 39.69 39.64 39.54 39.39
46.41 46.24 45.96 45.92 45.94
67.96 67.46 67.67 67.62 67.37
81.83 81.17 81.06 80.78 80.67
0.0418 0.0428 0.0438 0.0439 0.0451
4.0 3.9 3.8 3.8 3.7
asymmetrical peaks and consist of three pairs of doublets by deconvolving the Pd 3d signals (see Fig. 3A). The Pd binding energy of PdSb0.15/C shifts positively about 0.1 eV as compared with Pd/C, indicating that an electronic interaction exists between palladium and antimony [16]. The three pairs of deconvoluted peaks can be assigned to metallic Pd(0), PdOads and PdO, respectively [11,29]. The second peak of Pd 3d signal is due to the fact that oxygen atoms, water, or contamination from the supporting electrolyte generate chemisorbed oxygen which adsorbs on Pd surface [30]. The binding energy of each peak in Pd 3d3/2 region is about 5.3 eV higher than the corresponding one in Pd 3d5/2 region. As listed in Table 2, the amount of metallic Pd(0) calculated from the relative peak intensity in the PdSb0.15/C catalyst (49.6%) is approximately equal to that in the Pd/C catalyst (50.9%). In Fig. 3B, the deconvolution of Sb 3d level shows two pairs of doublets, corresponding to two chemical states of antimony. The doublet consisting of a low (Sb 3d5/2) and a high energy band (Sb 3d3/2) at 528.9 and 538.3 eV corresponds to metallic state Sb(0). The other doublet at 530.0 and 539.5 eV corresponds to Sb oxide species, mainly as Sb2O3 [20]. It is found that 65.7% Sb is alloying with Pd and 34.3% Sb exists in Sb2O3 (Table 2). As is well known, the oxides are beneficial to the oxidation of small organic molecules [31]. The O 1s signal with binding energy of about 531.7 and 533.2 eV can be attributed to metal oxide species (i.e., PdO, Sb2O3, etc.) and absorbed oxygen on the catalyst surface [29,32]. The CV curves on Pd/C and PdSbx/C catalysts were investigated in 1 M KOH solution, as presented in Fig. 4. It can be seen that hydrogen desorption/adsorption peaks located in the potential region of 0.95 ~ 0.6 V are clearly observed on all catalysts. And they gradually become low with the increase of Sb amount in PdSbx/C catalysts, suggesting that the peaks of hydrogen desorption/adsorption are restrained in the presence of Sb and its oxide [33]. The small anodic peak located at about 0.57 V is ascribed to the adsorption of OHads on the catalyst surface [34]:
Pd þ OH 4 Pd OHads þ e
(2)
This anodic peak slightly increases by adding Sb to Pd, which indicates the acceleration of hydroxyl adsorption on catalyst at lower potential. In addition, the reduction peak at around 0.28 V is associated with the reduction of palladium oxide in Pd/C catalyst [34]. It is obvious that the reduction peak shifts in the positive direction and becomes higher on PdSbx/C catalyst as compared with Pd/C catalyst, which is attributed to the simultaneous reduction of PdO and Sb oxide. The electrocatalytic activity of Pd/C and PdSbx/C catalysts for EOR was evaluated by the typical CV technique in 1 M KOH þ 1 M C2H5OH electrolyte as displayed in Fig. 5. It is clear that the addition of Sb enhances the electrocatalytic activity of Pd catalyst for ethanol oxidation in alkaline medium. However, too much Sb will lead to the decrease of the electrocatalytic activity. It is probably because the excessive Sb species will reduce the electronic conductivity of the catalyst and occupy the Pd active sites where EOR takes place [16,35]. The peak current density of ethanol oxidation on as-prepared catalysts is in the following order: PdSb0.15/C > PdSb0.1/
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
18259
Fig. 2 e TEM images and particle size distribution histograms of (A, C) Pd/C and (B, D) PdSb0.15/C.
C > PdSb0.2/C > PdSb0.05/C > Pd/C. It reaches 3690 mA mg1 Pd on PdSb0.15/C catalyst and only 2152 mA mg1 Pd on Pd/C catalyst. Besides, the onset potentials of EOR on PdSb0.15/C and Pd/C catalyst are ca. 0.63 and 0.59 V, respectively. The negative shift of onset potential on PdSb/C catalyst indicates the fast kinetics of ethanol oxidation. The Sb addition promotes the formation of hydroxyl species by dissociating water at a lower potential, in agreement with the results of CVs in Fig. 4. These results indicate that the Sb addition significantly enhances electrocatalytic activity of Pd/C toward EOR in alkaline medium, which is primarily due to the bifunctional mechanism and electronic effect [12,33,36]. To investigate the stability of the as-prepared catalysts for the EOR, chronoamperometric measurements were performed in 1 M KOH þ 1 M C2H5OH solution and shown in Fig. 6A. The current densityetime curves show that all catalysts present a rapid current decay at the initial stage of 100 s, which is attributed to the accumulation of strongly adsorbed intermediates on electrode surface. These poisonous species occupy the Pd active sites and inhibit the oxidation of ethanol. Subsequently, the current drops tardily and reaches a relatively steady state, in which the adsorption of poisonous intermediates and their electro-oxidation maintain a relative balance [16,35]. Finally, the current of ethanol oxidation at 10,000 s is in the following order: PdSb0.15/C > PdSb0.1/ C > PdSb0.2/C > PdSb0.05/C > Pd/C, which is in agreement with the results obtained on CV curves of Fig. 5. This indicates a
great enhancement in the durability and poisoning tolerance at PdSbx/C catalysts. The final current density on PdSb0.15/C (112.0 mA mg1 Pd) is the highest among these catalysts, which is 4.9 times higher than that obtained on Pd/C (22.8 mA mg1 Pd). Moreover, long-term stability test was used to further evaluate the catalytic activity of PdSb0.15/C catalyst and its stability, as shown in Fig. 6B. After 1000 cycles, the peak current on PdSb0.15/C still remains 74.3% and is higher than that on Pd/C catalyst (54.4%). In a word, the PdSb0.15/C catalyst has the best electrocatalytic durability and stability toward ethanol oxidation among these catalysts. Linear current sweep test is an effective approach to estimate the poisoning tolerance of the as-prepared catalysts [37]. The measurements were performed in 1 M KOH þ 1 M C2H5OH solution at the scan rate of 5 mA mg1 s1, as depicted in Fig. 7. In the current sweep, the electrode potentials increase gradually due to the poisonous intermediates accumulation on the electrode surface. To satisfy the applied currents, it is necessary for the adsorbed poisonous intermediates to be further oxidized by the sufficient oxygenated species. As the current increases continually, the electrode potentials need to be raised to produce abundant oxygenated species [16,35]. Finally, the potential skips to a very high value for oxygen evolution instead of the ethanol oxidation [38]. In comparison with the Pd/C catalyst, the polarization platform of Sbcontaining catalyst has a negative shift. The polarization time at the steep potential decreases in the following order:
18260
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
Fig. 3 e XPS spectra of (A) Pd/C and PdSb0.15/C in Pd 3d region and (B) PdSb0.15/C in Sb 3d region.
PdSb0.15/C > PdSb0.1/C > PdSb0.2/C > PdSb0.05/C > Pd/C. It indicates that the presence of Sb in Pd/C catalyst greatly promotes the formation of oxygenated species and the removal of poisonous intermediates. The polarization potential on PdSb0.15/C catalyst is the lowest among these catalysts. And the final polarization time on PdSb0.15/C catalyst is higher than that on the other catalysts. These results suggest that the PdSb0.15/C catalyst has the best anti-poisoning ability towards intermediate species among these catalysts. Fig. 8 shows the CO-stripping voltammograms on the Pd/C and PdSb0.15/C catalysts in 1.0 M KOH solution at a scan rate of 50 mV s1. It is well known that the CeC bond cleavage is
almost impossible for ethanol oxidation in alkaline medium. The oxidative removal of adsorbed intermediate (viz. ethoxi species) is identified as the rate-determining step [34]. The ethoxi species like CO are strongly bound to the catalyst surface. Thus, the CO stripping can be used to evaluate the capability of the catalyst to remove the adsorbed poisonous intermediates [39]. As shown in Fig. 8, the hydrogen desorption peaks of the two catalysts in the 1st cycle is absent in the lower potential region, indicating the saturated adsorption of COads species on the catalyst surface. The onset potential of COads oxidation is observed at 0.217 and 0.300 V on Pd/C and PdSb0.15/C, respectively. The peak potentials of COads oxidation on the PdSb0.15/C (0.134 V) is lower than that on Pd/ C catalyst (0.109 V). The lower onset potential and lower peak potential of COads oxidation on the PdSb0.15/C catalyst as compared with those on Pd/C catalyst indicate that the incorporation of Sb can weaken the adsorbed COads bond and facilitate oxidative removal of COads on catalyst surface [40]. Based on the bifunctional mechanism, the Sb addition can increase the hydroxyl group (OHads) concentration on Pd surface to promote the oxidation of the poisonous intermediates, and then result in the release of Pd active sites. This result accounts for the high activity of the PdSb0.15/C catalyst toward ethanol oxidation. The kinetics of ethanol oxidation on the Pd/C and PdSb0.15/C catalysts was investigated by Tafel polarization plots in 1 M KOH þ 1 M C2H5OH solution at a scan rate of 2 mV s1, as shown in Fig. 9. The curves of both catalysts are composed of two linear regions, indicating the different mechanism of ethanol oxidation or different dominant processes [41]. The Tafel slope values in these two regions are 171 and 240 mV dec1 for PdSb0.15/C catalyst, and 158 and 226 mV dec1 for Pd/C catalyst, respectively. The exchange current density of ethanol oxidation can be calculated by extrapolating the Tafel straight line in the low overpotential region. The exchange current densities are 4.1 105 and 7.7 106 A cm2 on the PdSb0.15/C and Pd/C catalysts, respectively. This result further indicates that the PdSb0.15/C has faster kinetics and higher catalytic activity for EOR in alkaline medium as compared with the Pd/C catalyst [41].
Conclusions Fast microwave polyol method was used to synthesize carbon-supported palladiumeantimony binary nanocatalysts (PdSbx/C) (x ¼ 0, 0.5, 0.1, 0.15 and 0.2). The physical characterizations demonstrate that the PdSbx/C catalysts have alloying nanoparticles which are smaller in size as compared with Pd/C catalyst. 65.7% Sb is alloying with Pd and 34.3% Sb
Table 2 e Surface composition of PdSb0.15/C and Pd/C obtained from XPS. Samples
Pd/C PdSb0.15/C
Surface concentration (at%) C
O
Pd
85.04 83.0
11.95 13.89
3.01 2.69
Relative atomic percentage (at%) Sb
Pd(0)/PdOads/PdO
Sb(0)/Sb2O3
0.42
50.9/28.8/20.3 49.6/36.4/14.0
65.7/34.3
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
18261
Fig. 4 e Cyclic voltammograms of Pd/C and PdSbx/C catalysts with different atomic ratios in 1 M KOH solution. Scan rate: 50 mV s¡1. The inset is the expansion of OH adsorption peak regions.
exists in Sb2O3. The presence of Sb facilitates the formation of the oxygenated species and enhances the catalytic activity of PdSbx/C catalysts for ethanol oxidation via electronic effect and bifunctional mechanism. Electrochemical results indicate that the PdSb0.15/C catalyst has the highest electrochemical performance among these catalysts. The onset potentials of ethanol oxidation and COads oxidation on PdSb0.15/C catalyst shift negatively by ca. 40 and 83 mV as compared with those on Pd/C catalyst, respectively. The peak current density of ethanol oxidation on as-prepared catalysts is in the following order: PdSb0.15/C > PdSb0.1/C > PdSb0.2/C > PdSb0.05/C > Pd/C. And it gets 3690 mA mg1 Pd on PdSb0.15/C, which is 1.71 times higher than that on Pd/C catalyst (2152 mA mg1 Pd). Moreover, the stability and anti-poisoning ability are improved on the PdSb0.15/C catalyst as well. The exchange current density on the PdSb0.15/C (4.1 105 A cm2) is much higher than that on Pd/C (7.7 106 A cm2), which indicates that the kinetics
Fig. 5 e Cyclic voltammograms of Pd/C and PdSbx/C catalysts with different atomic ratios in 1 M KOH þ 1 M C2H5OH solution.
Fig. 6 e (A) Currentetime curves of Pd/C and PdSbx/C catalysts with different atomic ratios at ¡0.3 V; (B) The 1st and 1000th CV curves on Pd/C and PdSb0.15/C catalysts. Solution: 1 M KOH þ 1 M C2H5OH.
Fig. 7 e Linear current sweep curves of Pd/C and PdSbx/C catalysts with different atomic ratios at the scan rate of 5 mA mg¡1 s¡1 in 1 M KOH þ 1 M C2H5OH solution.
18262
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
of ethanol oxidation in alkaline medium on PdSb0.15/C is greatly enhanced.
Acknowledgments The authors thank for the financial support of this work by the National Natural Science Foundation of China (No. 51072037) and Natural Science Foundation of Fujian Province (No. 2013J01039).
references
Fig. 8 e CO stripping curves on (A) Pd/C and (B) PdSb0.15/C in 1 M KOH solution.
Fig. 9 e Tafel plots on Pd/C and PdSb0.15/C catalysts in 1 M KOH þ 1 M C2H5OH solution.
[1] An L, Zhao TS. Performance of an alkaline-acid direct ethanol fuel cell. Int J Hydrogen Energy 2011;36:9994e9. [2] Zhou W, Zhou Z, Song S, Li W, Sun G, Tsiakaras P, et al. Pt based anode catalysts for direct ethanol fuel cells. Appl Catal B Environ 2003;46:273e85. [3] Du W, Wang Q, Saxner D, Deskins NA, Su D, Krzanowski JE, et al. Highly active iridium/iridium-tin/tin oxide heterogeneous nanoparticles as alternative electrocatalysts for the ethanol oxidation reaction. J Am Chem Soc 2011;133:15172e83. [4] Zignani SC, Baglio V, Linares JJ, Monforte G, Gonzalez ER, AS. Endurance study of a solid polymer electrolyte Arico direct ethanol fuel cell based on a Pt-Sn anode catalyst. Int J Hydrogen Energy 2013;38:11576e82. ger JM, Kadırgan F. Pronounced synergetic [5] Beyhana S, Le effect of the nano-sized PtSnNi/C catalyst for ethanol oxidation in direct ethanol fuel cell. Appl Catal B Environ 2013;130e131:305e13. [6] Antolini E. Catalysts for direct ethanol fuel cells. J Power Sources 2007;170:1e12. [7] Zhang Z, Xin L, Sun K, Li W. Pd-Ni electrocatalysts for efficient ethanol oxidation reactionin alkaline electrolyte. Int J Hydrogen Energy 2011;36:12686e97. [8] Antolini E, Gonzalez ER. Alkaline direct alcohol fuel cells. J Power Sources 2010;195:3431e50. [9] Bianchini C, Shen P. Palladium-based electrocatalysts for alcohol oxidation in half cells and in direct alcohol fuel cells. Chem Rev 2009;109:4183e206. [10] Modibedi RM, Masombuka T, Mathe MK. Carbon supported Pd-Sn and Pd-Ru-Sn nanocatalysts for ethanol electrooxidation in alkaline medium. Int J Hydrogen Energy 2011;36:4664e72. [11] Xu J, Zhao T, Shen S, Li Y. Stabilization of the palladium electrocatalyst with alloyed gold for ethanol oxidation. Int J Hydrogen Energy 2010;35:6490e500. [12] Wang Y, Sheng Z, Yang H, Jiang S, Li C. Electrocatalysis of carbon black- or activated carbon nanotubes-supported PdAg towards methanol oxidation in alkaline media. Int J Hydrogen Energy 2010;35:10087e93. [13] Maiyalagan T, Scott K. Performance of carbon nanofiber supported Pd-Ni catalysts for electro-oxidation of ethanol in alkaline medium. J Power Sources 2010;195:5246e51. [14] Kang W, Wei Y, Liu C, Wang K. Enhancement of electrochemical properties on Pd-Cu/C electrocatalysts toward ethanol oxidation by atmosphere induced surface and structural alteration. Electrochem Commun 2011;13:162e5. [15] Neto AO, Tusi MM, Polanco NSO, da Silva SG, dos Santos MC, EV. PdBi/C electrocatalysts for ethanol electroSpinace oxidation in alkaline medium. Int J Hydrogen Energy 2011;36:10522e6.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
[16] Cai J, Huang Y, Guo Y. Bi-modified Pd/C catalyst via irreversible adsorption and its catalytic activity for ethanol oxidation in alkaline medium. Electrochim Acta 2013;99:22e9. [17] Xu C, Tian Z, Shen P, Jiang SP. Oxide (CeO2, NiO, Co3O4 and Mn3O4)-promoted Pd/C electrocatalysts for alcohol electrooxidation in alkaline media. Electrochim Acta 2008;53:2610e8. [18] Zhao Y, Zhan L, Tian J, Nie S, Ning Z. MnO2 modified multiwalled carbon nanotubes supported Pd nanoparticles for methanol electro-oxidation in alkaline media. Int J Hydrogen Energy 2010;35:10522e6. [19] Chu D, Wang J, Wang S, Zha L, He J, Hou Y, et al. High activity of Pd-In2O3/CNTs electrocatalyst for electro-oxidation of ethanol. Catal Commun 2009;10:955e8. [20] Yu X, Pickup PG. Pb and Sb modified Pt/C catalysts for direct formic acid fuel cells. Electrochim Acta 2010;55:7354e61. [21] Yu X, Pickup PG. Deactivation resistant PdSb/C catalysts for direct formic acid fuel cells. Electrochem Commun 2010;12:800e3. [22] Zhang L, Xia D. Electrocatalytic activity of ordered intermetallic PtSb for methanol electro-oxidation. Appl Surf Sci 2006;252:2191e5. ^ [23] da Silva MR, Angelo ACD. Synthesis and characterization of ordered intermetallic nanostructured PtSn/C and PtSb/C and evaluationas electrodes for alcohol oxidation. Electrocatal 2010;1:95e103. [24] Casado-Rivera E, Volpe DJ, Alden L, Lind C, Downie C, zquez-Alvarez T, et al. Electrocatalytic activity of ordered Va intermetallic phases for fuel cell applications. J Am Chem Soc 2004;126:4043e9. [25] Zhou Y, Itoh H, Uemura T, Naka K, Chujo Y. Synthesis of novel stable nanometer-sized metal (M ¼ Pd, Au, Pt) colloids protected by a p-conjugated polymer. Langmuir 2002;18:277e83. [26] Sieben JM, Duart MME. Nanostructured Pt and Pt-Sn catalysts supported on oxidized carbon nanotubes for ethanol and ethylene glycol electro-oxidation. Int J Hydrogen Energy 2011;36:3313e21. [27] Ramulifho T, Ozoemena KI, Modibedi RM, Jafta CJ, Mathe MK. Fast microwave-assisted solvothermal synthesis of metal nanoparticles (Pd, Ni, Sn) supported on sulfonated MWCNTs: Pd-based bimetallic catalysts for ethanol oxidation in alkaline medium. Electrochim Acta 2012;59:310e20. [28] Hsieh C, Liu Y, Cheng Y, Chen W. Microwave-assisted synthesis and pulse electrodeposition of palladium
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
18263
nanocatalysts on carbon nanotube-based electrodes. Electrochim Acta 2011;56:6336e44. Casella IG, Contursi M. An electrochemical and XPS study of the electrodeposited binary Pd-Sn catalyst: the electroreduction of nitrate ions in acid medium. J Electroanal Chem 2006;588:147e54. Kim KS, Gossmann AF, Winograd N. X-Ray photoelectron spectroscopic studies of palladium oxides and the palladium-oxygen electrode. Anal Chem 1974;46:197e200. Huang Y, Guo Y, Wang Y, Yao J. Synthesis and performance of a novel PdCuPb/C nanocatalyst for ethanol electrooxidation in alkaline medium. Int J Hydrogen Energy 2014;39:4274e81. Casella IG, Gatta M. Anodic electrodeposition of copper oxide/hydroxide films by alkaline solutions containing cuprous cyanide ions. J Electroanal Chem 2000;494:12e20. Du W, Mackenzie KE, Milano DF, Deskins NA, Su D, Teng X. Palladium-tin alloyed catalysts for the ethanol oxidation reaction in an alkaline medium. ACS Catal 2012;2:287e97. Liang ZX, Zhao TS, Xu JB, Zhu LD. Mechanism study of the ethanol oxidation reaction on palladium in alkaline media. Electrochim Acta 2009;54:2203e8. Huang Y, Cai J, Guo Y. A high-efficiency microwave approach to synthesis of Bi-modified Pt nanoparticle catalysts for ethanol electro-oxidation in alkaline medium. Appl Catal B Environ 2013;129:549e55. Petry OA, Podlovchenko BI, Frumkin AN, Lal H. The behaviour of platinized-platinum and platinum-ruthenium electrodes in methanol solutions. J Electroanal Chem 1965;10:253e69. Lee J, Eickes C, Eiswirth M. Electrochemical oscillations in the methanol oxidation on Pt. Electrochim Acta 2002;47:2297e301. Hu F, Ding F, Song S, Shen P. Pd electrocatalyst supported on carbonized TiO2 nanotube for ethanol oxidation. J Power Sources 2006;163:415e9. Wang Y, Nguyen TS, Liu X, Wang X. Novel palladium-lead (Pd-Pb/C) bimetallic catalysts for electrooxidation of ethanol in alkaline media. J Power Sources 2010;195:2619e22. Jiang Q, Jiang L, Qi J, Wang S, Sun G. Experimental and density functional theory studies on PtPb/C bimetallic electrocatalysts for methanol electrooxidation reaction in alkaline media. Electrochim Acta 2011;56:6431e40. Shen SY, Zhao TS, Xu JB. Carbon-supported bimetallic PdIr catalysts for ethanol oxidation in alkaline media. Electrochim Acta 2010;55:9179e84.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Facile synthesis of PdSbx/C nanocatalyst with high performance for ethanol electro-oxidation in alkaline medium Jindi Cai, Yiyin Huang, Yonglang Guo* College of Chemistry, Fuzhou University, Fuzhou 350116, PR China
article info
abstract
Article history:
The PdSbx/C (x ¼ 0, 0.5, 0.1, 0.15 and 0.2) nanocatalysts were synthesized by the microwave
Received 13 May 2014
treatment. The structure and morphology of the as-prepared catalysts were characterized
Received in revised form
by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelec-
31 August 2014
tron spectroscopy (XPS). It is found that the addition of antimony into the Pd/C catalyst
Accepted 1 September 2014
leads to formation of PdSb alloy and reduction in particle size. The electrochemical mea-
Available online 26 September 2014
surements indicate that the PdSb0.15/C catalyst has high electrocatalytic activity and excellent anti-poisoning ability towards ethanol oxidation in alkaline medium. The onset
Keywords:
potential of ethanol oxidation on PdSb0.15/C shifts in negative direction as compared with
Alkaline medium
Pd/C. The mass activity of PdSb0.15/C for ethanol oxidation reaches 3690 mA mg1 Pd,
Ethanol oxidation
which is ca. 1.7 times higher than that of Pd/C. The enhanced performance of PdSb0.15/C is
Microwave method
mainly ascribed to the bifunctional mechanism and electronic effect.
Palladiumeantimony
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Pd-based catalyst
Introduction Direct alcohol fuel cells (DAFCs) have obtained considerable attention because of their promising commercialization as the power source for portable electronic devices and fuel-cell vehicles [1]. In comparison with pure hydrogen, the use of liquid alcohols as fuel has several advantages, such as easy storage and transportation [2]. Ethanol is an attractive alternative fuel to methanol because it has high theoretical energy density (8.01 kWh kg1) and low toxicity. Besides, ethanol is a renewable fuel and can be easily gained from the fermentation of agricultural products [1,3,4]. However, the complete oxidation of ethanol is complicated and hard to achieve. The process involves 12 electron transfer and the cleavage of CeC
bond [5]. Furthermore, the kinetics of ethanol oxidation reaction (EOR) is still sluggish even on Pt-based catalyst, which is widely considered as the best catalyst for ethanol oxidation in acidic medium. In addition, the application of Pt-based catalyst is restrained by the limited availability and high cost of Pt [6]. Hence the design of highly active and cheap anodic catalyst is quite urgent in the development of direct ethanol fuel cell (DEFC). In recent years, with the development of anion-exchange membrane (AEM), more and more attention has been paid to electrocatalysis in alkaline medium [7]. The kinetics of both alcohol oxidation and oxygen reduction in alkaline medium is greatly enhanced as compared with that in acidic medium [8]. Pd is a prospective substitute for Pt because of its relatively abundant reserves and low price. Moreover, Pd presents high
* Corresponding author. Tel./fax: þ86 591 8807 3608. E-mail address: [email protected] (Y. Guo). http://dx.doi.org/10.1016/j.ijhydene.2014.09.011 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
electrocatalytic activity for ethanol oxidation in alkaline medium [9]. Thus Pd-based catalyst is an excellent candidate for the application in DEFCs. The combination of Pd with other metals or metal oxides can further improve its electrocatalytic activity for ethanol oxidation. Various bimetallic PdM (M ¼ Ru, Sn, Au, Ag, Ni, Cu, Bi, etc.) catalysts have been investigated and they present high electrocatalytic activity toward EOR in alkaline solution [10e16]. Besides, the presence of oxides on Pd-based catalysts such as CeO2, NiO, Co3O4, Mn3O4, MnO2 and In2O3 remarkably lowers the onset potential of ethanol oxidation and enhances their electrocatalytic activity and stability for EOR [17e19]. Antimony, as a co-catalyst, has been widely applied in fuel cells and shown to enhance the electrocatalytic property and poisoning tolerance of catalysts [20e24]. Yu et al. [20,21] prepared PtSb/C and PdSb/C catalysts via co-deposition method. The as-prepared catalysts showed superior performance in direct formic acid fuel cells (DFAFCs) and high resistance to poisoning in formic oxidation. Methanol electro-oxidation on PtSb ordered intermetallic compound had been studied by Zhang et al. [22]. The electrocatalytic activity of PtSb for methanol oxidation is higher than that of polycrystalline platinum electrode. Silva et al. [23] synthesized PtSb ordered intermetallic compound supported on Vulcan XC-72 active carbon (PtSb/C) and found that PtSb/C had excellent electrocatalytic activity for alcohol oxidation as compared with Pt/C. In this work, the PdSbx/C electrocatalysts with different atomic ratios of Pd:Sb were prepared via a facile microwave method. The as-prepared catalyst structures were characterized by XRD, TEM and XPS. The electrocatalytic activity and mechanism of the PdSbx/C catalysts toward EOR were investigated by the electrochemical measurements.
18257
Physical and electrochemical characterization The XRD patterns of the as-prepared catalysts were carried out by using a Philip X'Pert Pro MPP X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 Å) at the scan rate of 2 min1 with a step of 0.02 . The morphology and size distribution of the catalysts were measured by transmission electron microscope (TEM, JEOL JEM-1010) operating at 200 kV. The X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) was conducted using an Al Ka X-ray source of 1486.6 eV to characterize the chemical valence state and the surface composition of the catalysts. The electrochemical measurements were carried out with a CHI 660C electrochemical working station (CH Instrument Inc.). A three-electrode electrolytic cell with a piece of Pt foil and Hg/HgO/ 1 M KOH (MMO, 0.098 V vs. SHE) as the counter and reference electrodes, respectively, was used. To prepare the working electrode, the glassy carbon electrode (GCE, 0.1256 cm2) was burnished with Al2O3 paste, cleaned and then deposited with a thin film of the as-prepared catalyst. 5.0 mg as-prepared catalyst was dispersed in 1 mL solution of 985 mL isopropyl alcohol and 15 mL Nafion solution (15 wt%, DuPont) in ultrasonic bath for 30 min to form catalyst ink. Then, 4 mL ink was dispersed onto the GCE surface and subsequently dried under an infrared lamp. The Pd loading on GCE was maintained at 4 mg. The electrochemical measurements were performed at 30 ± 1 C. Before all measurements, the electrolyte solution was saturated with high purity nitrogen. Cyclic voltammograms (CV) were performed in the potential region of 0.95 ~ 0.5 V in 1 M KOH solution containing 1.0 M C2H5OH at a scan rate of 50 mV s1. Chronoamperometric measurements were carried out in 1 M KOH þ 1 M C2H5OH solution at 0.3 V for 1800 s. The CO-stripping experiments similar to our previous study were performed [16]. To obtain the stable data in all electrochemical measurements, the test electrode went through 20 cycles in advance in the N2-saturated blank KOH solution.
Experimental Synthesis of Pd-based nanocatalysts
Results and discussion
The active carbon black (Vulcan XC-72R, Cabot Corp.) was added to the concentrated HNO3 in a flask which was immersed in an oil bath at 80 C for 10 h. Then, the mixture was neutralized with KOH, filtered, washed with doubledistilled water and dried at 80 C overnight in a vacuum oven. The PdSbx/C catalysts were synthesized by a microwave polyol method. PdCl2 was dissolved in HCl solution to form H2PdCl4 solution, which was used as Pd precursor. 15.6 mg of Vulcan XC-72R carbon was dispersed in 50 mL of ethylene glycol (EG) in ultrasonic bath. Then, the appropriate amount of H2PdCl4 (37.8 mM) and antimony potassium tartrate (APT, 18.9 mM) were added to the suspension and the mixture was ultrasonically stirred for 30 min. The pH of the solution above was kept at 10 by dripping KOH solution (0.4 M). The welldispersed mixture in the flask with a reflux set was treated for 5 min under the microwave power of 800 W. The asprepared samples were filtrated, washed with double-distill water and dried in a vacuum oven overnight. The final catalysts were denoted as Pd/C and PdSbx/C (x ¼ 0.05, 0.1, 0.15 and 0.2). The weight percentage of Pd was maintained at 20 wt% in all catalysts.
Fig. 1 shows the XRD patterns of Pd/C and PdSbx/C catalysts. All samples display a broad diffraction peak at about 25 which is referred to graphite (002) facet of carbon black. The Pd/C catalyst presents four main diffraction peaks located at 2q values of about 39.9 , 46.4 , 68.0 and 81.8 which are attributed to the (111), (200), (220) and (311) facets of the Pd fcc crystal (JCPDS Card No. 05-0681), respectively [25]. As shown in Table 1, it is clear that the four main diffraction peaks of all PdSbx/C catalysts shift to lower 2q values with respect to those of Pd/C catalyst, suggesting the formation of PdSb alloying in the synthesis of PdSbx/C catalysts. Two peaks at 39.4 and 40.0 for the PdSb0.2/C catalyst can be ascribed to Pd (111) and Pd75Sb25 (220) facets (JCPDS Card No. 65-7876) [21]. The average size of the metal nanoparticles (d) for these catalysts is estimated from the broadening diffraction peak of Pd (220) by using the Scherrer's equation [26]: d¼
0:9l b cos q
(1)
where l is denoted as the wavelength of X-ray (1.5418 Å), q the angle of the Pd (220) peak, and b its half-peak width. According
18258
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
Fig. 1 e XRD patterns of Pd/C and PdSbx/C catalysts with different atomic ratios of Pd/Sb.
to Scherrer's equation, the average particle sizes of the Pd/C and PdSb0.15/C catalysts are 4.0 and 3.8 nm (see Table 1), respectively. The morphology and the particle-size distribution of Pd/C and PdSb0.15/C catalysts were analyzed by TEM images and shown in Fig. 2. The metal nanoparticles are well dispersed on the carbon support in both catalysts, but with a little agglomeration. This phenomenon was also reported in Refs. [16,27,28]. The rapid growth of Pd nuclei during microwave treatment inevitably induces some Pd aggregation. The insets in Fig. 2A and B show the high-resolution TEM (HRTEM) images of Pd/C and PdSb0.15/C catalysts, respectively. In Pd/C catalyst, there are two groups of typical lattice fringes with ca. 2.27 and 1.88 Å, which are ascribed to the (111) and (200) planes of Pd crystal, respectively. For PdSb0.15/C catalyst, besides the planes of Pd metal, the lattice stripe with interplanar spacing of 2.25 Å is also presented, which is corresponding to the Pd75Sb25 (220) plane (JCPDS card No. 65-7876). In addition, the particle size distribution histograms of the Pd/C and PdSb0.15/ C were evaluated by counting more than 500 casually selected nanoparticles in the corresponding HRTEM images, as shown in Fig. 2C and D. The average particle sizes of Pd/C and PdSb0.15/C catalysts are 4.4 and 4.0 nm, respectively. Therefore, the presence of Sb in Pd/C catalyst reduces the catalyst nanoparticle size, which is in agreement with the XRD results. The XPS spectra of Pd/C and PdSb0.15/C were used to determine the metal valence states on catalyst surfaces and shown in Fig. 3. The Pd 3d spectra of both catalysts display two
Table 1 e XRD data of Pd/C and PdSbx/C catalysts. Angles of Pd faces ( )
Samples
Half-peak Particle width of Pd sizes (nm) (111) (200) (220) (311) (220) (rad)
Pd/C PdSb0.05/C PdSb0.1/C PdSb0.15/C PdSb0.2/C
39.89 39.69 39.64 39.54 39.39
46.41 46.24 45.96 45.92 45.94
67.96 67.46 67.67 67.62 67.37
81.83 81.17 81.06 80.78 80.67
0.0418 0.0428 0.0438 0.0439 0.0451
4.0 3.9 3.8 3.8 3.7
asymmetrical peaks and consist of three pairs of doublets by deconvolving the Pd 3d signals (see Fig. 3A). The Pd binding energy of PdSb0.15/C shifts positively about 0.1 eV as compared with Pd/C, indicating that an electronic interaction exists between palladium and antimony [16]. The three pairs of deconvoluted peaks can be assigned to metallic Pd(0), PdOads and PdO, respectively [11,29]. The second peak of Pd 3d signal is due to the fact that oxygen atoms, water, or contamination from the supporting electrolyte generate chemisorbed oxygen which adsorbs on Pd surface [30]. The binding energy of each peak in Pd 3d3/2 region is about 5.3 eV higher than the corresponding one in Pd 3d5/2 region. As listed in Table 2, the amount of metallic Pd(0) calculated from the relative peak intensity in the PdSb0.15/C catalyst (49.6%) is approximately equal to that in the Pd/C catalyst (50.9%). In Fig. 3B, the deconvolution of Sb 3d level shows two pairs of doublets, corresponding to two chemical states of antimony. The doublet consisting of a low (Sb 3d5/2) and a high energy band (Sb 3d3/2) at 528.9 and 538.3 eV corresponds to metallic state Sb(0). The other doublet at 530.0 and 539.5 eV corresponds to Sb oxide species, mainly as Sb2O3 [20]. It is found that 65.7% Sb is alloying with Pd and 34.3% Sb exists in Sb2O3 (Table 2). As is well known, the oxides are beneficial to the oxidation of small organic molecules [31]. The O 1s signal with binding energy of about 531.7 and 533.2 eV can be attributed to metal oxide species (i.e., PdO, Sb2O3, etc.) and absorbed oxygen on the catalyst surface [29,32]. The CV curves on Pd/C and PdSbx/C catalysts were investigated in 1 M KOH solution, as presented in Fig. 4. It can be seen that hydrogen desorption/adsorption peaks located in the potential region of 0.95 ~ 0.6 V are clearly observed on all catalysts. And they gradually become low with the increase of Sb amount in PdSbx/C catalysts, suggesting that the peaks of hydrogen desorption/adsorption are restrained in the presence of Sb and its oxide [33]. The small anodic peak located at about 0.57 V is ascribed to the adsorption of OHads on the catalyst surface [34]:
Pd þ OH 4 Pd OHads þ e
(2)
This anodic peak slightly increases by adding Sb to Pd, which indicates the acceleration of hydroxyl adsorption on catalyst at lower potential. In addition, the reduction peak at around 0.28 V is associated with the reduction of palladium oxide in Pd/C catalyst [34]. It is obvious that the reduction peak shifts in the positive direction and becomes higher on PdSbx/C catalyst as compared with Pd/C catalyst, which is attributed to the simultaneous reduction of PdO and Sb oxide. The electrocatalytic activity of Pd/C and PdSbx/C catalysts for EOR was evaluated by the typical CV technique in 1 M KOH þ 1 M C2H5OH electrolyte as displayed in Fig. 5. It is clear that the addition of Sb enhances the electrocatalytic activity of Pd catalyst for ethanol oxidation in alkaline medium. However, too much Sb will lead to the decrease of the electrocatalytic activity. It is probably because the excessive Sb species will reduce the electronic conductivity of the catalyst and occupy the Pd active sites where EOR takes place [16,35]. The peak current density of ethanol oxidation on as-prepared catalysts is in the following order: PdSb0.15/C > PdSb0.1/
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
18259
Fig. 2 e TEM images and particle size distribution histograms of (A, C) Pd/C and (B, D) PdSb0.15/C.
C > PdSb0.2/C > PdSb0.05/C > Pd/C. It reaches 3690 mA mg1 Pd on PdSb0.15/C catalyst and only 2152 mA mg1 Pd on Pd/C catalyst. Besides, the onset potentials of EOR on PdSb0.15/C and Pd/C catalyst are ca. 0.63 and 0.59 V, respectively. The negative shift of onset potential on PdSb/C catalyst indicates the fast kinetics of ethanol oxidation. The Sb addition promotes the formation of hydroxyl species by dissociating water at a lower potential, in agreement with the results of CVs in Fig. 4. These results indicate that the Sb addition significantly enhances electrocatalytic activity of Pd/C toward EOR in alkaline medium, which is primarily due to the bifunctional mechanism and electronic effect [12,33,36]. To investigate the stability of the as-prepared catalysts for the EOR, chronoamperometric measurements were performed in 1 M KOH þ 1 M C2H5OH solution and shown in Fig. 6A. The current densityetime curves show that all catalysts present a rapid current decay at the initial stage of 100 s, which is attributed to the accumulation of strongly adsorbed intermediates on electrode surface. These poisonous species occupy the Pd active sites and inhibit the oxidation of ethanol. Subsequently, the current drops tardily and reaches a relatively steady state, in which the adsorption of poisonous intermediates and their electro-oxidation maintain a relative balance [16,35]. Finally, the current of ethanol oxidation at 10,000 s is in the following order: PdSb0.15/C > PdSb0.1/ C > PdSb0.2/C > PdSb0.05/C > Pd/C, which is in agreement with the results obtained on CV curves of Fig. 5. This indicates a
great enhancement in the durability and poisoning tolerance at PdSbx/C catalysts. The final current density on PdSb0.15/C (112.0 mA mg1 Pd) is the highest among these catalysts, which is 4.9 times higher than that obtained on Pd/C (22.8 mA mg1 Pd). Moreover, long-term stability test was used to further evaluate the catalytic activity of PdSb0.15/C catalyst and its stability, as shown in Fig. 6B. After 1000 cycles, the peak current on PdSb0.15/C still remains 74.3% and is higher than that on Pd/C catalyst (54.4%). In a word, the PdSb0.15/C catalyst has the best electrocatalytic durability and stability toward ethanol oxidation among these catalysts. Linear current sweep test is an effective approach to estimate the poisoning tolerance of the as-prepared catalysts [37]. The measurements were performed in 1 M KOH þ 1 M C2H5OH solution at the scan rate of 5 mA mg1 s1, as depicted in Fig. 7. In the current sweep, the electrode potentials increase gradually due to the poisonous intermediates accumulation on the electrode surface. To satisfy the applied currents, it is necessary for the adsorbed poisonous intermediates to be further oxidized by the sufficient oxygenated species. As the current increases continually, the electrode potentials need to be raised to produce abundant oxygenated species [16,35]. Finally, the potential skips to a very high value for oxygen evolution instead of the ethanol oxidation [38]. In comparison with the Pd/C catalyst, the polarization platform of Sbcontaining catalyst has a negative shift. The polarization time at the steep potential decreases in the following order:
18260
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
Fig. 3 e XPS spectra of (A) Pd/C and PdSb0.15/C in Pd 3d region and (B) PdSb0.15/C in Sb 3d region.
PdSb0.15/C > PdSb0.1/C > PdSb0.2/C > PdSb0.05/C > Pd/C. It indicates that the presence of Sb in Pd/C catalyst greatly promotes the formation of oxygenated species and the removal of poisonous intermediates. The polarization potential on PdSb0.15/C catalyst is the lowest among these catalysts. And the final polarization time on PdSb0.15/C catalyst is higher than that on the other catalysts. These results suggest that the PdSb0.15/C catalyst has the best anti-poisoning ability towards intermediate species among these catalysts. Fig. 8 shows the CO-stripping voltammograms on the Pd/C and PdSb0.15/C catalysts in 1.0 M KOH solution at a scan rate of 50 mV s1. It is well known that the CeC bond cleavage is
almost impossible for ethanol oxidation in alkaline medium. The oxidative removal of adsorbed intermediate (viz. ethoxi species) is identified as the rate-determining step [34]. The ethoxi species like CO are strongly bound to the catalyst surface. Thus, the CO stripping can be used to evaluate the capability of the catalyst to remove the adsorbed poisonous intermediates [39]. As shown in Fig. 8, the hydrogen desorption peaks of the two catalysts in the 1st cycle is absent in the lower potential region, indicating the saturated adsorption of COads species on the catalyst surface. The onset potential of COads oxidation is observed at 0.217 and 0.300 V on Pd/C and PdSb0.15/C, respectively. The peak potentials of COads oxidation on the PdSb0.15/C (0.134 V) is lower than that on Pd/ C catalyst (0.109 V). The lower onset potential and lower peak potential of COads oxidation on the PdSb0.15/C catalyst as compared with those on Pd/C catalyst indicate that the incorporation of Sb can weaken the adsorbed COads bond and facilitate oxidative removal of COads on catalyst surface [40]. Based on the bifunctional mechanism, the Sb addition can increase the hydroxyl group (OHads) concentration on Pd surface to promote the oxidation of the poisonous intermediates, and then result in the release of Pd active sites. This result accounts for the high activity of the PdSb0.15/C catalyst toward ethanol oxidation. The kinetics of ethanol oxidation on the Pd/C and PdSb0.15/C catalysts was investigated by Tafel polarization plots in 1 M KOH þ 1 M C2H5OH solution at a scan rate of 2 mV s1, as shown in Fig. 9. The curves of both catalysts are composed of two linear regions, indicating the different mechanism of ethanol oxidation or different dominant processes [41]. The Tafel slope values in these two regions are 171 and 240 mV dec1 for PdSb0.15/C catalyst, and 158 and 226 mV dec1 for Pd/C catalyst, respectively. The exchange current density of ethanol oxidation can be calculated by extrapolating the Tafel straight line in the low overpotential region. The exchange current densities are 4.1 105 and 7.7 106 A cm2 on the PdSb0.15/C and Pd/C catalysts, respectively. This result further indicates that the PdSb0.15/C has faster kinetics and higher catalytic activity for EOR in alkaline medium as compared with the Pd/C catalyst [41].
Conclusions Fast microwave polyol method was used to synthesize carbon-supported palladiumeantimony binary nanocatalysts (PdSbx/C) (x ¼ 0, 0.5, 0.1, 0.15 and 0.2). The physical characterizations demonstrate that the PdSbx/C catalysts have alloying nanoparticles which are smaller in size as compared with Pd/C catalyst. 65.7% Sb is alloying with Pd and 34.3% Sb
Table 2 e Surface composition of PdSb0.15/C and Pd/C obtained from XPS. Samples
Pd/C PdSb0.15/C
Surface concentration (at%) C
O
Pd
85.04 83.0
11.95 13.89
3.01 2.69
Relative atomic percentage (at%) Sb
Pd(0)/PdOads/PdO
Sb(0)/Sb2O3
0.42
50.9/28.8/20.3 49.6/36.4/14.0
65.7/34.3
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
18261
Fig. 4 e Cyclic voltammograms of Pd/C and PdSbx/C catalysts with different atomic ratios in 1 M KOH solution. Scan rate: 50 mV s¡1. The inset is the expansion of OH adsorption peak regions.
exists in Sb2O3. The presence of Sb facilitates the formation of the oxygenated species and enhances the catalytic activity of PdSbx/C catalysts for ethanol oxidation via electronic effect and bifunctional mechanism. Electrochemical results indicate that the PdSb0.15/C catalyst has the highest electrochemical performance among these catalysts. The onset potentials of ethanol oxidation and COads oxidation on PdSb0.15/C catalyst shift negatively by ca. 40 and 83 mV as compared with those on Pd/C catalyst, respectively. The peak current density of ethanol oxidation on as-prepared catalysts is in the following order: PdSb0.15/C > PdSb0.1/C > PdSb0.2/C > PdSb0.05/C > Pd/C. And it gets 3690 mA mg1 Pd on PdSb0.15/C, which is 1.71 times higher than that on Pd/C catalyst (2152 mA mg1 Pd). Moreover, the stability and anti-poisoning ability are improved on the PdSb0.15/C catalyst as well. The exchange current density on the PdSb0.15/C (4.1 105 A cm2) is much higher than that on Pd/C (7.7 106 A cm2), which indicates that the kinetics
Fig. 5 e Cyclic voltammograms of Pd/C and PdSbx/C catalysts with different atomic ratios in 1 M KOH þ 1 M C2H5OH solution.
Fig. 6 e (A) Currentetime curves of Pd/C and PdSbx/C catalysts with different atomic ratios at ¡0.3 V; (B) The 1st and 1000th CV curves on Pd/C and PdSb0.15/C catalysts. Solution: 1 M KOH þ 1 M C2H5OH.
Fig. 7 e Linear current sweep curves of Pd/C and PdSbx/C catalysts with different atomic ratios at the scan rate of 5 mA mg¡1 s¡1 in 1 M KOH þ 1 M C2H5OH solution.
18262
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
of ethanol oxidation in alkaline medium on PdSb0.15/C is greatly enhanced.
Acknowledgments The authors thank for the financial support of this work by the National Natural Science Foundation of China (No. 51072037) and Natural Science Foundation of Fujian Province (No. 2013J01039).
references
Fig. 8 e CO stripping curves on (A) Pd/C and (B) PdSb0.15/C in 1 M KOH solution.
Fig. 9 e Tafel plots on Pd/C and PdSb0.15/C catalysts in 1 M KOH þ 1 M C2H5OH solution.
[1] An L, Zhao TS. Performance of an alkaline-acid direct ethanol fuel cell. Int J Hydrogen Energy 2011;36:9994e9. [2] Zhou W, Zhou Z, Song S, Li W, Sun G, Tsiakaras P, et al. Pt based anode catalysts for direct ethanol fuel cells. Appl Catal B Environ 2003;46:273e85. [3] Du W, Wang Q, Saxner D, Deskins NA, Su D, Krzanowski JE, et al. Highly active iridium/iridium-tin/tin oxide heterogeneous nanoparticles as alternative electrocatalysts for the ethanol oxidation reaction. J Am Chem Soc 2011;133:15172e83. [4] Zignani SC, Baglio V, Linares JJ, Monforte G, Gonzalez ER, AS. Endurance study of a solid polymer electrolyte Arico direct ethanol fuel cell based on a Pt-Sn anode catalyst. Int J Hydrogen Energy 2013;38:11576e82. ger JM, Kadırgan F. Pronounced synergetic [5] Beyhana S, Le effect of the nano-sized PtSnNi/C catalyst for ethanol oxidation in direct ethanol fuel cell. Appl Catal B Environ 2013;130e131:305e13. [6] Antolini E. Catalysts for direct ethanol fuel cells. J Power Sources 2007;170:1e12. [7] Zhang Z, Xin L, Sun K, Li W. Pd-Ni electrocatalysts for efficient ethanol oxidation reactionin alkaline electrolyte. Int J Hydrogen Energy 2011;36:12686e97. [8] Antolini E, Gonzalez ER. Alkaline direct alcohol fuel cells. J Power Sources 2010;195:3431e50. [9] Bianchini C, Shen P. Palladium-based electrocatalysts for alcohol oxidation in half cells and in direct alcohol fuel cells. Chem Rev 2009;109:4183e206. [10] Modibedi RM, Masombuka T, Mathe MK. Carbon supported Pd-Sn and Pd-Ru-Sn nanocatalysts for ethanol electrooxidation in alkaline medium. Int J Hydrogen Energy 2011;36:4664e72. [11] Xu J, Zhao T, Shen S, Li Y. Stabilization of the palladium electrocatalyst with alloyed gold for ethanol oxidation. Int J Hydrogen Energy 2010;35:6490e500. [12] Wang Y, Sheng Z, Yang H, Jiang S, Li C. Electrocatalysis of carbon black- or activated carbon nanotubes-supported PdAg towards methanol oxidation in alkaline media. Int J Hydrogen Energy 2010;35:10087e93. [13] Maiyalagan T, Scott K. Performance of carbon nanofiber supported Pd-Ni catalysts for electro-oxidation of ethanol in alkaline medium. J Power Sources 2010;195:5246e51. [14] Kang W, Wei Y, Liu C, Wang K. Enhancement of electrochemical properties on Pd-Cu/C electrocatalysts toward ethanol oxidation by atmosphere induced surface and structural alteration. Electrochem Commun 2011;13:162e5. [15] Neto AO, Tusi MM, Polanco NSO, da Silva SG, dos Santos MC, EV. PdBi/C electrocatalysts for ethanol electroSpinace oxidation in alkaline medium. Int J Hydrogen Energy 2011;36:10522e6.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 8 2 5 6 e1 8 2 6 3
[16] Cai J, Huang Y, Guo Y. Bi-modified Pd/C catalyst via irreversible adsorption and its catalytic activity for ethanol oxidation in alkaline medium. Electrochim Acta 2013;99:22e9. [17] Xu C, Tian Z, Shen P, Jiang SP. Oxide (CeO2, NiO, Co3O4 and Mn3O4)-promoted Pd/C electrocatalysts for alcohol electrooxidation in alkaline media. Electrochim Acta 2008;53:2610e8. [18] Zhao Y, Zhan L, Tian J, Nie S, Ning Z. MnO2 modified multiwalled carbon nanotubes supported Pd nanoparticles for methanol electro-oxidation in alkaline media. Int J Hydrogen Energy 2010;35:10522e6. [19] Chu D, Wang J, Wang S, Zha L, He J, Hou Y, et al. High activity of Pd-In2O3/CNTs electrocatalyst for electro-oxidation of ethanol. Catal Commun 2009;10:955e8. [20] Yu X, Pickup PG. Pb and Sb modified Pt/C catalysts for direct formic acid fuel cells. Electrochim Acta 2010;55:7354e61. [21] Yu X, Pickup PG. Deactivation resistant PdSb/C catalysts for direct formic acid fuel cells. Electrochem Commun 2010;12:800e3. [22] Zhang L, Xia D. Electrocatalytic activity of ordered intermetallic PtSb for methanol electro-oxidation. Appl Surf Sci 2006;252:2191e5. ^ [23] da Silva MR, Angelo ACD. Synthesis and characterization of ordered intermetallic nanostructured PtSn/C and PtSb/C and evaluationas electrodes for alcohol oxidation. Electrocatal 2010;1:95e103. [24] Casado-Rivera E, Volpe DJ, Alden L, Lind C, Downie C, zquez-Alvarez T, et al. Electrocatalytic activity of ordered Va intermetallic phases for fuel cell applications. J Am Chem Soc 2004;126:4043e9. [25] Zhou Y, Itoh H, Uemura T, Naka K, Chujo Y. Synthesis of novel stable nanometer-sized metal (M ¼ Pd, Au, Pt) colloids protected by a p-conjugated polymer. Langmuir 2002;18:277e83. [26] Sieben JM, Duart MME. Nanostructured Pt and Pt-Sn catalysts supported on oxidized carbon nanotubes for ethanol and ethylene glycol electro-oxidation. Int J Hydrogen Energy 2011;36:3313e21. [27] Ramulifho T, Ozoemena KI, Modibedi RM, Jafta CJ, Mathe MK. Fast microwave-assisted solvothermal synthesis of metal nanoparticles (Pd, Ni, Sn) supported on sulfonated MWCNTs: Pd-based bimetallic catalysts for ethanol oxidation in alkaline medium. Electrochim Acta 2012;59:310e20. [28] Hsieh C, Liu Y, Cheng Y, Chen W. Microwave-assisted synthesis and pulse electrodeposition of palladium
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
18263
nanocatalysts on carbon nanotube-based electrodes. Electrochim Acta 2011;56:6336e44. Casella IG, Contursi M. An electrochemical and XPS study of the electrodeposited binary Pd-Sn catalyst: the electroreduction of nitrate ions in acid medium. J Electroanal Chem 2006;588:147e54. Kim KS, Gossmann AF, Winograd N. X-Ray photoelectron spectroscopic studies of palladium oxides and the palladium-oxygen electrode. Anal Chem 1974;46:197e200. Huang Y, Guo Y, Wang Y, Yao J. Synthesis and performance of a novel PdCuPb/C nanocatalyst for ethanol electrooxidation in alkaline medium. Int J Hydrogen Energy 2014;39:4274e81. Casella IG, Gatta M. Anodic electrodeposition of copper oxide/hydroxide films by alkaline solutions containing cuprous cyanide ions. J Electroanal Chem 2000;494:12e20. Du W, Mackenzie KE, Milano DF, Deskins NA, Su D, Teng X. Palladium-tin alloyed catalysts for the ethanol oxidation reaction in an alkaline medium. ACS Catal 2012;2:287e97. Liang ZX, Zhao TS, Xu JB, Zhu LD. Mechanism study of the ethanol oxidation reaction on palladium in alkaline media. Electrochim Acta 2009;54:2203e8. Huang Y, Cai J, Guo Y. A high-efficiency microwave approach to synthesis of Bi-modified Pt nanoparticle catalysts for ethanol electro-oxidation in alkaline medium. Appl Catal B Environ 2013;129:549e55. Petry OA, Podlovchenko BI, Frumkin AN, Lal H. The behaviour of platinized-platinum and platinum-ruthenium electrodes in methanol solutions. J Electroanal Chem 1965;10:253e69. Lee J, Eickes C, Eiswirth M. Electrochemical oscillations in the methanol oxidation on Pt. Electrochim Acta 2002;47:2297e301. Hu F, Ding F, Song S, Shen P. Pd electrocatalyst supported on carbonized TiO2 nanotube for ethanol oxidation. J Power Sources 2006;163:415e9. Wang Y, Nguyen TS, Liu X, Wang X. Novel palladium-lead (Pd-Pb/C) bimetallic catalysts for electrooxidation of ethanol in alkaline media. J Power Sources 2010;195:2619e22. Jiang Q, Jiang L, Qi J, Wang S, Sun G. Experimental and density functional theory studies on PtPb/C bimetallic electrocatalysts for methanol electrooxidation reaction in alkaline media. Electrochim Acta 2011;56:6431e40. Shen SY, Zhao TS, Xu JB. Carbon-supported bimetallic PdIr catalysts for ethanol oxidation in alkaline media. Electrochim Acta 2010;55:9179e84.