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graphene nanosheets towards ethanol oxidation by tin oxide
Electrochimica Acta 56 (2010) 139–144
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The improved electrocatalytic activity of palladium/graphene nanosheets towards ethanol oxidation by tin oxide Zhuliang Wen a , Sudong Yang b , Yanyu Liang b , Wei He b , Hao Tong b , Liang Hao b , Xiaogang Zhang b,∗ , Qijun Song a a b
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 8 September 2010 Accepted 9 September 2010 Available online 17 September 2010 Keywords: SnO2 -GNS Catalyst support Microwave-assisted reduction Electrocatalytic performances Ethanol oxidation
a b s t r a c t Tin oxide (SnO2 )/graphene nanosheets (GNS) composite was prepared by a simple chemical-solution method as the catalyst support for direct ethanol fuel cells. Then the SnO2 -GNS composites supporting Pd (Pd/SnO2 -GNS) catalysts were synthesized by a microwave-assisted reduction process. The Pd/SnO2 -GNS catalysts were characterized by using X-ray diffraction, transmission electron microscopy and energydispersive spectroscopy techniques. The electrocatalytic performances of Pd/SnO2 -GNS catalysts for ethanol oxidation were studied by cyclic voltammetric and chronoamperometric measurements. It was found that compared with Pd/GNS, the Pd/SnO2 -GNS catalyst showed superior electrocatalytic activity for ethanol oxidation when the mass ratio of SnCl2 ·2H2 O precursor salt to graphite oxide was about 1:2. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, electrooxidation of small organic molecules has attracted great attention due to the development of direct liquid fuel cells, which requires highly reactive fuels with high energy density [1–3]. Since methanol is the simplest alcohol and its electrochemical reaction kinetics is faster than those of other alcohol fuels, direct methanol fuel cell has been extensively studied over the past decade. The toxicity of methanol, however, limits the widespread use by consumers. By contrast, ethanol is a promising source since ethanol has no toxicity and can be massively produced by biomass feedstocks from agriculture. For these reasons, direct ethanol fuel cell (DEFC) has recently been receiving increased attention [4,5]. A significant challenge in the development of DEFC technology is the need for highly active anode catalysts for the ethanol oxidation reaction. Pt based catalysts are generally identified as the best catalysts for low temperature fuel cells [6]. Oxides promoted Pt have been used as catalysts for alcohol oxidation which significantly improved the electrode performance by enhancing activity and poison resistance [7,8]. However, the high cost and limited supply of Pt remain a challenge that demands its efficient commercial usage. Therefore, extensive research efforts have also been devoted to the development of low Pt loading or Pt-free catalysts for fuel cell
∗ Corresponding author. Tel.: +86 025 52112902; fax: +86 025 52112626. E-mail address: [email protected] (X. Zhang). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.032
electrochemistry recently. As the replacement of Pt based catalysts, Pd and Pd based catalysts are emerging as a good choice due to their good electrocatalytic activity for alcohol and especially for the ethanol oxidation in alkaline media [9,10]. More important is that Pd is more abundant in nature and less expensive than Pt. Pd based catalysts decorated with oxides (CeO2 , NiO, Co3 O4 , Mn3 O4 ) have significant promotion effect on the catalytic activity and stability for the ethanol oxidation [11,12]. Tin oxide (SnO2 ) has been extensively used as anode in lithium ion batteries [13] and gas sensors [14]. Qiu and co-workers [15] and Kowal et al. [16] have reported that the presence of SnO2 in Pt/SnO2 catalyst led to higher current densities in acidic media for methanol and ethanol oxidation in comparison with the case of a pure Pt catalyst. Graphene nanosheets (GNS), a two-dimensional (2D) carbon material with single (or a few) atomic layer, has attracted great attention for both fundamental science and applied research [17–19]. The combination of high specific surface area (theoretical value of 2600 m2 g−1 ) [20], excellent electronic conductivity [21], high chemical stability, unique graphitized basal plane structure and potentially low manufacturing cost [22,23], GNS can thus be exploited as an alternative material for catalyst support in fuel cells. Recently, graphene has received great attention as the catalyst support for fuel cell application [24–26]. In particular, Li et al. [24] showed Pt/graphene exhibited better catalytic performance and stability compared to the Pt/Vulcan XC-72 catalyst. Seger and Kamat [25] reported about the deposition of Pt on graphene and their electrochemical activity in the proton exchange membrane
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fuel cells was studied. However, dispersion of Pd on a metal oxide coated GNS composite has not been reported so far. In this report, we describe a composite catalyst combining the properties of tin oxide in contact with a Pd phase and a new type of carbon support. Electrochemical measurements show that the as-prepared Pd/SnO2 -GNS catalysts have higher catalytic activity than Pd/GNS catalyst for ethanol oxidation. 2. Experimental 2.1. Synthesis of the Pd/SnO2 -GNS and Pd/GNS catalysts The graphite oxide (GO) was prepared according to modified Hummer’s method [27]. SnO2 -GNS composite used in this study was synthesized by a simple chemical-solution method [28]. In brief, dried GO (50 mg) was exfoliated in distilled water (100 mL) with ultrasonic treatment to form a colloidal suspension. Then 0.15 mL of concentrated HCl, required amount of SnCl2 ·2H2 O and 0.10 g of urea were added, subsequently, the mixture was continually stirred at 60 ◦ C for 6 h. The mass ratio of SnCl2 ·2H2 O to GO for each of the composite supports was 1:4, 1:2, and 1:1, denoted in this report as SnO2 -GNS-1, SnO2 -GNS-2, SnO2 -GNS-3, respectively. The products were rinsed completely with distilled water and dried at room temperature under vacuum. The GNS was prepared by reduction of GO with hydrazine hydrate as described elsewhere [29]. Pd/SnO2 -GNS catalysts with the same Pd loading were prepared by a microwave-assisted reduction process, where ethylene glycol (EG) served as both the reducing agent and solvent. In a typical procedure, 20 mg SnO2 -GNS powder was dispersed in 30 mL EG solvent by ultrasonic treatment for approximately 2 h and then 0.71 mL of 7 mg mL−1 PdCl2 solution was added under magnetic stirring. The pH value of this mixture was adjusted to 10.0 by adding 1 mol L−1 NaOH aqueous solution. Subsequently, the solution was put into a microwave oven (1000 W, 2.45 GHz) and then was alternatively heated for 2 min and paused for 30 s for eight times at a maximum temperature of 140 ◦ C. The resulting slurry was centrifuged, washed thoroughly with deionized water and then dried in a vacuum oven (Fig. 1). The metal loading on SnO2 -GNS was determined to be 20 wt%. For comparative purposes, a sample of Pd-loaded GNS (Pd/GNS) was also prepared under identical conditions. 2.2. Preparation of electrode Glassy carbon (GC) electrode, 5 mm in diameter (electrode area 0.2 cm2 ), polished with 0.05 m alumina to a mirror-finish before each experiment, was used as substrates for supported catalysts. For the electrode preparation, typically, 3 mg catalyst was added into 0.5 mL of 0.05 wt% Nafion solution, and then the mixture was treated for 1 h with ultrasonication for uniform dispersion. A measured volume (30 L) of this mixture was dropped by a microsyringe onto the top surface of the GC electrode. The asobtained catalyst modified GC electrode was employed as the working electrode in our experiments. 2.3. Instrument and measurement X-ray diffraction (XRD) analyses were carried out on a Bruker D8-ADVANCE diffractometer with Cu K␣ radiation of wavelength = 0.15418 nm. The morphology of the samples was observed by transmission electron microscopy (TEM, JEOL JEM-2100) and the Pd/SnO2 -GNS-2 catalyst was analyzed by energy-dispersive spectroscopy (EDS) attached to the field emission scanning electron microscope (FESEM, LEO-1550). All electrochemical measurements were carried out with a three-electrode test cell using a CHI 660c electrochemical work-
Fig. 1. Schematic representation of the synthesis of Pd/SnO2 -GNS catalysts.
station (Shanghai, China). The conventional three-electrode system was used with a modified GC electrode as working electrode, a Pt wire as counter electrode and a saturated Ag/AgCl electrode as reference electrode. To determine the real electrochemical surface area (ECSA) of the as-prepared catalysts, CO stripping voltammetry was conducted. CO was pre-adsorbed at a potential of 0.05 V for 30 min by bubbling CO into 0.5 mol L−1 H2 SO4 solution, and CO dissolved in solution was subsequently removed by purging highpurity N2 for 45 min. The ECSA was derived by integrating the COad oxidation area, after subtracting the background current in the CO stripping voltammograms, and assuming 420 C cm−2 as the oxidation charge of monolayer CO on a smooth Pd surface [30]. All electrolytes were deaerated by bubbling N2 for 20 min and protected with a nitrogen atmosphere during the entire experimental procedure. All experiments were carried out at a temperature of 25 ± 1 ◦ C. 3. Results and discussion 3.1. Physical characterization of catalysts Structural features of the obtained composite samples were confirmed by the XRD patterns (shown in Fig. 2). As shown in SnO2 -GNS-2, the major diffraction line can be indexed to the tetragonal SnO2 phase. The typical diffraction peak of GO (2 = 9.7◦ ) disappeared after deposition of SnO2 nanoparticles, which could be attributed to fact that GO nanosheets are partially reduced to graphene [2,28]. Further, the broad diffraction peaks of SnO2 indicate the formation of small crystalline particles. For the pattern of Pd/GNS, the catalyst has crystalline structure with peaks emerging at 39.8◦ , 46◦ , 67.8◦ and 81.6◦ , corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face-centered cubic structure of Pd, respectively. In contrast, the characteristic diffraction peaks of Pd slightly shift to a lower angle in the case of Pd/SnO2 -GNS-2, which can be attributed to the effect from crystallined SnO2 . It is worthwhile to note that a characteristic peak at 51.8◦ corresponding to the SnO2 (2 1 1) plane is detectable, which further indicating the coexistence of Pd and SnO2 in the Pd/SnO2 -GNS-2 catalyst. On the
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Fig. 2. XRD patterns of SnO2 -GNS-2, Pd/GNS and Pd/SnO2 -GNS-2. The inset is the XRD pattern of GO.
Fig. 4. EDS spectrum of Pd/SnO2 -GNS-2 catalyst.
basis of Sherrer’s equation through line broadening of the Pd (2 2 0) peak, the average size of Pd nanoparticles for Pd/SnO2 -GNS-2 and Pd/GNS was calculated as 5.3 and 6.4 nm, respectively. Fig. 3 shows the typical TEM images of as-synthesized samples. Fig. 3a shows a clean GO sheet with a smooth finish and plenty of wrinkles owing to the thin structure of the sheet. Fig. 3b presents a TEM image of the SnO2 -GNS-2 composite. Clearly, SnO2 nanoparticles are uniformly coated on 2D graphene nanosheets with high particle densities. The average particle size is determined from the high-resolution TEM image (inset in Fig. 3b) to be about 1.5 nm. In Fig. 3c, it can be seen that Pd nanoparticles are deposited on GNS,
however, their dispersion is not uniform and some Pd nanoparticles agglomerated together. By comparison, it is clearly seen in Fig. 3d that SnO2 -GNS composite is uniformly decorated by the nanosized Pd particles with much few aggregations. It could be assumed that SnO2 nanoparticles on the GNS surface act as anchoring sites for Pd nanoparticles and be beneficial to restricting the Pd migration. This unique structure appears to provide a suitable support for many uniform Pd nanoparticles, thereby favoring the high performance of ethanol oxidation. Furthermore, an EDS spectrum of the Pd/SnO2 -GNS-2 catalyst is shown in Fig. 4. The EDS analysis confirms the presence of Pd, Sn, O and C. Combing the facts of the
Fig. 3. TEM images of (a) GO, (b) SnO2 -GNS-2, (c) Pd/GNS and (d) Pd/SnO2 -GNS-2. The inset image of (b) is the high-resolution TEM image of SnO2 -GNS-2.
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Fig. 5. CO stripping voltammograms of the as-prepared catalysts in 0.5 mol L−1 H2 SO4 solution after 30 min CO adsorption.
crystal plane formation and a large amount of Pd and Sn detected, it can be inferred that the nanoparticles mainly consist of Pd and SnO2 . 3.2. Electrochemical properties of catalysts Fig. 5 shows the CO stripping voltammograms of Pd/SnO2 -GNS and Pd/GNS catalysts recorded in 0.5 mol L−1 H2 SO4 at a scan rate of 25 mV s−1 . The differences in the profile of these curves for four catalysts can clearly be seen. The Pd/GNS catalyst shows a single CO stripping peak centered at around 0.77 V, which is derived from the electrooxidation of adsorbed CO monolayer on pure Pd surface [30,31]. For the Pd/SnO2 -GNS catalysts, the CO stripping peaks become broadened and the peak potentials for CO stripping shift to lower potentials, which suggests the presence of SnO2 that favor CO oxidation at a lower potential. Moreover, a lower onset potential of CO oxidation on all Pd/SnO2 -GNS catalysts than on Pd/GNS is due to the addition of SnO2 . These results show that the Pd/SnO2 GNS catalysts possess much higher activity toward CO oxidation than Pd/GNS. By integrating the COad oxidation area, the ECSA for each sample can be obtained and is listed in Table 1. It is interesting to note that Pd/SnO2 -GNS catalysts exhibit larger ECSA to a different degree than Pd/GNS. It is due to the fact that Pd nanoparticles are smaller and more uniformly dispersed on the surfaces of SnO2 -GNS, when compared with GNS. It is well known that more uniformly dispersed of catalyst particles show higher mass activity. Furthermore, SnO2 nanoparticles coated onto the GNS though van der Waals interaction prevented the aggregation and restacking of the reduced GO during the reduction process [32]. Thereby, the SnO2 -GNS composites may possess larger surface areas, which pro-
Table 1 Comparison of different parameters between Pd/GNS and Pd/SnO2 -GNS catalysts. Sample
ECSA (m2 g−1 )
Onset potentiala of ethanol oxidation (V)
Peak current density of ethanol oxidation (mA cm−2 )
Pd/GNS Pd/SnO2 -GNS-1 Pd/SnO2 -GNS-2 Pd/SnO2 -GNS-3
58.3 209.4 215.8 112.9
−0.5 −0.59 −0.6 −0.58
29.1 43.2 46.1 34.6
a
vs. Ag/AgCl.
Fig. 6. Cyclic voltammograms for ethanol oxidation on Pd/SnO2 -GNS and Pd/GNS catalysts in 0.25 mol L−1 NaOH and 0.25 mol L−1 C2 H5 OH solution at a scan rate of 50 mV s−1 . Current values are reported for per unit ECSA.
duce much more accessible Pd sites for efficient catalytic activity in comparison with GNS used as catalyst support. Now, it can be concluded that the SnO2 in the Pd/SnO2 -GNS catalysts can increase the ECSA of Pd and enhance its CO oxidation ability. In order to evaluate the effect of SnO2 on the Pd activity, the electrocatalytic activities of all prepared catalysts for ethanol oxidation were estimated by cyclic voltammetry. Fig. 6 shows the cyclic voltammograms of the Pd/SnO2 -GNS and Pd/GNS catalysts in 0.25 mol L−1 NaOH solution containing 0.25 mol L−1 C2 H5 OH. All of the currents are reported for per unit ECSA. The obtained CV curves exhibit very prominent characteristic peaks for the ethanol oxidation at about 30 mV. It can be seen from the data listed in Table 1 that the peak current densities of ethanol oxidation on Pd/SnO2 -GNS catalysts increase with different degrees towards Pd/GNS. They are 43.2, 46.1 and 34.6 mA cm−2 for Pd/SnO2 -GNS-1, Pd/SnO2 -GNS-2 and Pd/SnO2 -GNS-3, respectively. All those values are higher than the 29.1 mA cm−2 obtained for the Pd/GNS catalyst. When bare SnO2 -GNS without Pd nanoparticles was used as electrode, there was no current response for ethanol oxidation, indicating that SnO2 nanoparticles have no catalytic activity for ethanol oxidation (not shown). Further, it can be observed from the inset in Fig. 6 that the onset potentials of ethanol oxidation on Pd/SnO2 -GNS catalysts are more negative than that on Pd/GNS. The change in the onset potential demonstrates the improved reaction kinetics of ethanol oxidation reaction on Pd/SnO2 -GNS catalysts due to synergistic effect between Pd and SnO2 [33]. The results reveal that Pd supported on SnO2 -GNS composite significantly improves the catalytic activity for ethanol oxidation in comparison with GNS used as catalyst support. As shown in Fig. 7, the content of SnO2 in the Pd/SnO2 -GNS catalysts affects the catalytic activity for ethanol oxidation. With the SnO2 content increasing, the peak current density increases at first and then decreases with the excess amount of SnO2 in the catalysts. It can be seen that the best performance of the Pd/SnO2 -GNS catalysts is obtained when the mass ratio of SnCl2 ·2H2 O precursor salt to GO is 1:2; too high and too low SnO2 contents both cause a decrease in the activity of catalyst. All the results indicate that SnO2 -GNS used as catalyst support improves the catalytic activity for ethanol oxidation in comparison with GNS and the best activity comes from the appropriate amount of SnO2 in the catalysts. The chronoamperometric technique is an effective method to evaluate the catalytic activity and stability of catalyst materials. Fig. 8 shows the typical current density–time responses of Pd/SnO2 GNS and Pd/GNS catalysts for ethanol oxidation measured at a
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dissociation–adsorption of ethanol proceeds quickly [Eqs. (1)–(3)].
Fig. 7. Variation of current density for ethanol oxidation with a Pd/SnO2 -GNS catalyst having a SnCl2 ·2H2 O to GO mass ratio of without SnCl2 ·2H2 O, 1:4, 1:2 and 1:1.
fixed potential of −0.3 V. In principle, all of them present a gradual current decay before a steady current status is attained, which is attributed to the formation of some Pd oxides and/or adsorbed intermediates in ethanol oxidation reaction. Gradually, the current decayed and a pseudo-steady state is achieved. The current density measured for the Pd/SnO2 -GNS catalysts are higher than that of Pd/GNS during the whole testing time. For different Pd/SnO2 -GNS catalysts, the current density of Pd/SnO2 -GNS-2 is higher than other samples. These results are consistent with the CV measurements mentioned above. For Pt based catalysts, the incorporation of metal oxides into the anode has been shown to minimize the CO poisoning effects in a methanol fuel cell [34,35], but very few mechanistic studied on the oxidation of ethanol on Pd-metal oxides catalysts have been reported. The beneficial effect of the added metal oxide was attributed to the fact that OHads could easily form on the introduced metal oxides, which is similar with the well-known bi-functional mechanism between Pt and Ru [36]. Referring to the depiction of Pt-metal oxides and the generally accepted oxidation sequence for ethanol oxidation on Pd in alkaline media [37], we propose a tentative reaction mechanism for the ethanol oxidation on Pd/SnO2 -GNS catalysts as follows. The surface adsorbed hydroxyl of SnO2 may remove the adsorbed acyl on the surface of Pd, and then the
SnO2 + OH− → SnO2 -OHads + e−
(1)
Pd-(CH3 CH2 OH)ads + 3OH− → Pd-(COCH3 )ads + 3H2 O + 3e−
(2)
Pd-(COCH3 )ads + SnO2 -OHads → Pd-CH3 COOH + SnO2
(3)
SnO2 would increase the concentration of OHads species on the catalyst surface, favoring the oxidation of the acetaldehyde into acetic acid via acylads -OHads coupling, thereby releasing the active sites on Pd for further electrochemical reaction [38]. It is worth noting that the Pd/SnO2 -GNS catalysts are more effective for ethanol oxidation than Pd/GNS catalyst by cyclic voltammetric studies and chronoamperometric determination. The amount of SnO2 in the Pd/SnO2 -GNS catalysts also affects the catalytic activity. Thus, a high content of SnO2 has the following consequences: (a) higher content of active Pd/SnO2 interface; (b) more OH species for removing reaction intermediates; (c) partial blocking of Pd active sites; (d) higher resistance of the catalysts due to the semiconducting nature of SnO2 . Among these, the first two factors lead to higher catalytic activity, while the latter two lead to lower catalytic activity [39]. Hence, it is needed to balance all the factors so as to obtain the best catalytic activity. 4. Conclusions In summary, Pd/SnO2 -GNS catalysts were successfully prepared and their electrochemical properties have been investigated by cyclic voltammetry and chronoamperometry. Because of the high ECSA and good scavenging capacity towards the intermediate species formed during the ethanol oxidation, the Pd/SnO2 -GNS catalysts show much more enhanced catalytic activity when compared with Pd/GNS catalyst and the optimal mass ratio of SnCl2 ·2H2 O precursor salt to GO is about 1:2. These show that the SnO2 -GNS composite may have the splendid future as the catalyst support for fuel cell. Acknowledgements The authors gratefully acknowledge the support by National Basic Research Program of China (973 Program) (no. 2007CB209700), National Natural Science Foundation of China (no. 50701023), Graduated Student Innovation Foundation of Jiangsu Province (CX09B 075Z and CX09B 076Z) and NUAA Research Funding (no. NS2010165). References
Fig. 8. Chronoamperometric curves for ethanol oxidation on Pd/SnO2 -GNS and Pd/GNS catalysts in 0.25 mol L−1 NaOH and 0.25 mol L−1 C2 H5 OH solution at −0.3 V.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The improved electrocatalytic activity of palladium/graphene nanosheets towards ethanol oxidation by tin oxide Zhuliang Wen a , Sudong Yang b , Yanyu Liang b , Wei He b , Hao Tong b , Liang Hao b , Xiaogang Zhang b,∗ , Qijun Song a a b
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 8 September 2010 Accepted 9 September 2010 Available online 17 September 2010 Keywords: SnO2 -GNS Catalyst support Microwave-assisted reduction Electrocatalytic performances Ethanol oxidation
a b s t r a c t Tin oxide (SnO2 )/graphene nanosheets (GNS) composite was prepared by a simple chemical-solution method as the catalyst support for direct ethanol fuel cells. Then the SnO2 -GNS composites supporting Pd (Pd/SnO2 -GNS) catalysts were synthesized by a microwave-assisted reduction process. The Pd/SnO2 -GNS catalysts were characterized by using X-ray diffraction, transmission electron microscopy and energydispersive spectroscopy techniques. The electrocatalytic performances of Pd/SnO2 -GNS catalysts for ethanol oxidation were studied by cyclic voltammetric and chronoamperometric measurements. It was found that compared with Pd/GNS, the Pd/SnO2 -GNS catalyst showed superior electrocatalytic activity for ethanol oxidation when the mass ratio of SnCl2 ·2H2 O precursor salt to graphite oxide was about 1:2. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, electrooxidation of small organic molecules has attracted great attention due to the development of direct liquid fuel cells, which requires highly reactive fuels with high energy density [1–3]. Since methanol is the simplest alcohol and its electrochemical reaction kinetics is faster than those of other alcohol fuels, direct methanol fuel cell has been extensively studied over the past decade. The toxicity of methanol, however, limits the widespread use by consumers. By contrast, ethanol is a promising source since ethanol has no toxicity and can be massively produced by biomass feedstocks from agriculture. For these reasons, direct ethanol fuel cell (DEFC) has recently been receiving increased attention [4,5]. A significant challenge in the development of DEFC technology is the need for highly active anode catalysts for the ethanol oxidation reaction. Pt based catalysts are generally identified as the best catalysts for low temperature fuel cells [6]. Oxides promoted Pt have been used as catalysts for alcohol oxidation which significantly improved the electrode performance by enhancing activity and poison resistance [7,8]. However, the high cost and limited supply of Pt remain a challenge that demands its efficient commercial usage. Therefore, extensive research efforts have also been devoted to the development of low Pt loading or Pt-free catalysts for fuel cell
∗ Corresponding author. Tel.: +86 025 52112902; fax: +86 025 52112626. E-mail address: [email protected] (X. Zhang). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.032
electrochemistry recently. As the replacement of Pt based catalysts, Pd and Pd based catalysts are emerging as a good choice due to their good electrocatalytic activity for alcohol and especially for the ethanol oxidation in alkaline media [9,10]. More important is that Pd is more abundant in nature and less expensive than Pt. Pd based catalysts decorated with oxides (CeO2 , NiO, Co3 O4 , Mn3 O4 ) have significant promotion effect on the catalytic activity and stability for the ethanol oxidation [11,12]. Tin oxide (SnO2 ) has been extensively used as anode in lithium ion batteries [13] and gas sensors [14]. Qiu and co-workers [15] and Kowal et al. [16] have reported that the presence of SnO2 in Pt/SnO2 catalyst led to higher current densities in acidic media for methanol and ethanol oxidation in comparison with the case of a pure Pt catalyst. Graphene nanosheets (GNS), a two-dimensional (2D) carbon material with single (or a few) atomic layer, has attracted great attention for both fundamental science and applied research [17–19]. The combination of high specific surface area (theoretical value of 2600 m2 g−1 ) [20], excellent electronic conductivity [21], high chemical stability, unique graphitized basal plane structure and potentially low manufacturing cost [22,23], GNS can thus be exploited as an alternative material for catalyst support in fuel cells. Recently, graphene has received great attention as the catalyst support for fuel cell application [24–26]. In particular, Li et al. [24] showed Pt/graphene exhibited better catalytic performance and stability compared to the Pt/Vulcan XC-72 catalyst. Seger and Kamat [25] reported about the deposition of Pt on graphene and their electrochemical activity in the proton exchange membrane
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fuel cells was studied. However, dispersion of Pd on a metal oxide coated GNS composite has not been reported so far. In this report, we describe a composite catalyst combining the properties of tin oxide in contact with a Pd phase and a new type of carbon support. Electrochemical measurements show that the as-prepared Pd/SnO2 -GNS catalysts have higher catalytic activity than Pd/GNS catalyst for ethanol oxidation. 2. Experimental 2.1. Synthesis of the Pd/SnO2 -GNS and Pd/GNS catalysts The graphite oxide (GO) was prepared according to modified Hummer’s method [27]. SnO2 -GNS composite used in this study was synthesized by a simple chemical-solution method [28]. In brief, dried GO (50 mg) was exfoliated in distilled water (100 mL) with ultrasonic treatment to form a colloidal suspension. Then 0.15 mL of concentrated HCl, required amount of SnCl2 ·2H2 O and 0.10 g of urea were added, subsequently, the mixture was continually stirred at 60 ◦ C for 6 h. The mass ratio of SnCl2 ·2H2 O to GO for each of the composite supports was 1:4, 1:2, and 1:1, denoted in this report as SnO2 -GNS-1, SnO2 -GNS-2, SnO2 -GNS-3, respectively. The products were rinsed completely with distilled water and dried at room temperature under vacuum. The GNS was prepared by reduction of GO with hydrazine hydrate as described elsewhere [29]. Pd/SnO2 -GNS catalysts with the same Pd loading were prepared by a microwave-assisted reduction process, where ethylene glycol (EG) served as both the reducing agent and solvent. In a typical procedure, 20 mg SnO2 -GNS powder was dispersed in 30 mL EG solvent by ultrasonic treatment for approximately 2 h and then 0.71 mL of 7 mg mL−1 PdCl2 solution was added under magnetic stirring. The pH value of this mixture was adjusted to 10.0 by adding 1 mol L−1 NaOH aqueous solution. Subsequently, the solution was put into a microwave oven (1000 W, 2.45 GHz) and then was alternatively heated for 2 min and paused for 30 s for eight times at a maximum temperature of 140 ◦ C. The resulting slurry was centrifuged, washed thoroughly with deionized water and then dried in a vacuum oven (Fig. 1). The metal loading on SnO2 -GNS was determined to be 20 wt%. For comparative purposes, a sample of Pd-loaded GNS (Pd/GNS) was also prepared under identical conditions. 2.2. Preparation of electrode Glassy carbon (GC) electrode, 5 mm in diameter (electrode area 0.2 cm2 ), polished with 0.05 m alumina to a mirror-finish before each experiment, was used as substrates for supported catalysts. For the electrode preparation, typically, 3 mg catalyst was added into 0.5 mL of 0.05 wt% Nafion solution, and then the mixture was treated for 1 h with ultrasonication for uniform dispersion. A measured volume (30 L) of this mixture was dropped by a microsyringe onto the top surface of the GC electrode. The asobtained catalyst modified GC electrode was employed as the working electrode in our experiments. 2.3. Instrument and measurement X-ray diffraction (XRD) analyses were carried out on a Bruker D8-ADVANCE diffractometer with Cu K␣ radiation of wavelength = 0.15418 nm. The morphology of the samples was observed by transmission electron microscopy (TEM, JEOL JEM-2100) and the Pd/SnO2 -GNS-2 catalyst was analyzed by energy-dispersive spectroscopy (EDS) attached to the field emission scanning electron microscope (FESEM, LEO-1550). All electrochemical measurements were carried out with a three-electrode test cell using a CHI 660c electrochemical work-
Fig. 1. Schematic representation of the synthesis of Pd/SnO2 -GNS catalysts.
station (Shanghai, China). The conventional three-electrode system was used with a modified GC electrode as working electrode, a Pt wire as counter electrode and a saturated Ag/AgCl electrode as reference electrode. To determine the real electrochemical surface area (ECSA) of the as-prepared catalysts, CO stripping voltammetry was conducted. CO was pre-adsorbed at a potential of 0.05 V for 30 min by bubbling CO into 0.5 mol L−1 H2 SO4 solution, and CO dissolved in solution was subsequently removed by purging highpurity N2 for 45 min. The ECSA was derived by integrating the COad oxidation area, after subtracting the background current in the CO stripping voltammograms, and assuming 420 C cm−2 as the oxidation charge of monolayer CO on a smooth Pd surface [30]. All electrolytes were deaerated by bubbling N2 for 20 min and protected with a nitrogen atmosphere during the entire experimental procedure. All experiments were carried out at a temperature of 25 ± 1 ◦ C. 3. Results and discussion 3.1. Physical characterization of catalysts Structural features of the obtained composite samples were confirmed by the XRD patterns (shown in Fig. 2). As shown in SnO2 -GNS-2, the major diffraction line can be indexed to the tetragonal SnO2 phase. The typical diffraction peak of GO (2 = 9.7◦ ) disappeared after deposition of SnO2 nanoparticles, which could be attributed to fact that GO nanosheets are partially reduced to graphene [2,28]. Further, the broad diffraction peaks of SnO2 indicate the formation of small crystalline particles. For the pattern of Pd/GNS, the catalyst has crystalline structure with peaks emerging at 39.8◦ , 46◦ , 67.8◦ and 81.6◦ , corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face-centered cubic structure of Pd, respectively. In contrast, the characteristic diffraction peaks of Pd slightly shift to a lower angle in the case of Pd/SnO2 -GNS-2, which can be attributed to the effect from crystallined SnO2 . It is worthwhile to note that a characteristic peak at 51.8◦ corresponding to the SnO2 (2 1 1) plane is detectable, which further indicating the coexistence of Pd and SnO2 in the Pd/SnO2 -GNS-2 catalyst. On the
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Fig. 2. XRD patterns of SnO2 -GNS-2, Pd/GNS and Pd/SnO2 -GNS-2. The inset is the XRD pattern of GO.
Fig. 4. EDS spectrum of Pd/SnO2 -GNS-2 catalyst.
basis of Sherrer’s equation through line broadening of the Pd (2 2 0) peak, the average size of Pd nanoparticles for Pd/SnO2 -GNS-2 and Pd/GNS was calculated as 5.3 and 6.4 nm, respectively. Fig. 3 shows the typical TEM images of as-synthesized samples. Fig. 3a shows a clean GO sheet with a smooth finish and plenty of wrinkles owing to the thin structure of the sheet. Fig. 3b presents a TEM image of the SnO2 -GNS-2 composite. Clearly, SnO2 nanoparticles are uniformly coated on 2D graphene nanosheets with high particle densities. The average particle size is determined from the high-resolution TEM image (inset in Fig. 3b) to be about 1.5 nm. In Fig. 3c, it can be seen that Pd nanoparticles are deposited on GNS,
however, their dispersion is not uniform and some Pd nanoparticles agglomerated together. By comparison, it is clearly seen in Fig. 3d that SnO2 -GNS composite is uniformly decorated by the nanosized Pd particles with much few aggregations. It could be assumed that SnO2 nanoparticles on the GNS surface act as anchoring sites for Pd nanoparticles and be beneficial to restricting the Pd migration. This unique structure appears to provide a suitable support for many uniform Pd nanoparticles, thereby favoring the high performance of ethanol oxidation. Furthermore, an EDS spectrum of the Pd/SnO2 -GNS-2 catalyst is shown in Fig. 4. The EDS analysis confirms the presence of Pd, Sn, O and C. Combing the facts of the
Fig. 3. TEM images of (a) GO, (b) SnO2 -GNS-2, (c) Pd/GNS and (d) Pd/SnO2 -GNS-2. The inset image of (b) is the high-resolution TEM image of SnO2 -GNS-2.
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Fig. 5. CO stripping voltammograms of the as-prepared catalysts in 0.5 mol L−1 H2 SO4 solution after 30 min CO adsorption.
crystal plane formation and a large amount of Pd and Sn detected, it can be inferred that the nanoparticles mainly consist of Pd and SnO2 . 3.2. Electrochemical properties of catalysts Fig. 5 shows the CO stripping voltammograms of Pd/SnO2 -GNS and Pd/GNS catalysts recorded in 0.5 mol L−1 H2 SO4 at a scan rate of 25 mV s−1 . The differences in the profile of these curves for four catalysts can clearly be seen. The Pd/GNS catalyst shows a single CO stripping peak centered at around 0.77 V, which is derived from the electrooxidation of adsorbed CO monolayer on pure Pd surface [30,31]. For the Pd/SnO2 -GNS catalysts, the CO stripping peaks become broadened and the peak potentials for CO stripping shift to lower potentials, which suggests the presence of SnO2 that favor CO oxidation at a lower potential. Moreover, a lower onset potential of CO oxidation on all Pd/SnO2 -GNS catalysts than on Pd/GNS is due to the addition of SnO2 . These results show that the Pd/SnO2 GNS catalysts possess much higher activity toward CO oxidation than Pd/GNS. By integrating the COad oxidation area, the ECSA for each sample can be obtained and is listed in Table 1. It is interesting to note that Pd/SnO2 -GNS catalysts exhibit larger ECSA to a different degree than Pd/GNS. It is due to the fact that Pd nanoparticles are smaller and more uniformly dispersed on the surfaces of SnO2 -GNS, when compared with GNS. It is well known that more uniformly dispersed of catalyst particles show higher mass activity. Furthermore, SnO2 nanoparticles coated onto the GNS though van der Waals interaction prevented the aggregation and restacking of the reduced GO during the reduction process [32]. Thereby, the SnO2 -GNS composites may possess larger surface areas, which pro-
Table 1 Comparison of different parameters between Pd/GNS and Pd/SnO2 -GNS catalysts. Sample
ECSA (m2 g−1 )
Onset potentiala of ethanol oxidation (V)
Peak current density of ethanol oxidation (mA cm−2 )
Pd/GNS Pd/SnO2 -GNS-1 Pd/SnO2 -GNS-2 Pd/SnO2 -GNS-3
58.3 209.4 215.8 112.9
−0.5 −0.59 −0.6 −0.58
29.1 43.2 46.1 34.6
a
vs. Ag/AgCl.
Fig. 6. Cyclic voltammograms for ethanol oxidation on Pd/SnO2 -GNS and Pd/GNS catalysts in 0.25 mol L−1 NaOH and 0.25 mol L−1 C2 H5 OH solution at a scan rate of 50 mV s−1 . Current values are reported for per unit ECSA.
duce much more accessible Pd sites for efficient catalytic activity in comparison with GNS used as catalyst support. Now, it can be concluded that the SnO2 in the Pd/SnO2 -GNS catalysts can increase the ECSA of Pd and enhance its CO oxidation ability. In order to evaluate the effect of SnO2 on the Pd activity, the electrocatalytic activities of all prepared catalysts for ethanol oxidation were estimated by cyclic voltammetry. Fig. 6 shows the cyclic voltammograms of the Pd/SnO2 -GNS and Pd/GNS catalysts in 0.25 mol L−1 NaOH solution containing 0.25 mol L−1 C2 H5 OH. All of the currents are reported for per unit ECSA. The obtained CV curves exhibit very prominent characteristic peaks for the ethanol oxidation at about 30 mV. It can be seen from the data listed in Table 1 that the peak current densities of ethanol oxidation on Pd/SnO2 -GNS catalysts increase with different degrees towards Pd/GNS. They are 43.2, 46.1 and 34.6 mA cm−2 for Pd/SnO2 -GNS-1, Pd/SnO2 -GNS-2 and Pd/SnO2 -GNS-3, respectively. All those values are higher than the 29.1 mA cm−2 obtained for the Pd/GNS catalyst. When bare SnO2 -GNS without Pd nanoparticles was used as electrode, there was no current response for ethanol oxidation, indicating that SnO2 nanoparticles have no catalytic activity for ethanol oxidation (not shown). Further, it can be observed from the inset in Fig. 6 that the onset potentials of ethanol oxidation on Pd/SnO2 -GNS catalysts are more negative than that on Pd/GNS. The change in the onset potential demonstrates the improved reaction kinetics of ethanol oxidation reaction on Pd/SnO2 -GNS catalysts due to synergistic effect between Pd and SnO2 [33]. The results reveal that Pd supported on SnO2 -GNS composite significantly improves the catalytic activity for ethanol oxidation in comparison with GNS used as catalyst support. As shown in Fig. 7, the content of SnO2 in the Pd/SnO2 -GNS catalysts affects the catalytic activity for ethanol oxidation. With the SnO2 content increasing, the peak current density increases at first and then decreases with the excess amount of SnO2 in the catalysts. It can be seen that the best performance of the Pd/SnO2 -GNS catalysts is obtained when the mass ratio of SnCl2 ·2H2 O precursor salt to GO is 1:2; too high and too low SnO2 contents both cause a decrease in the activity of catalyst. All the results indicate that SnO2 -GNS used as catalyst support improves the catalytic activity for ethanol oxidation in comparison with GNS and the best activity comes from the appropriate amount of SnO2 in the catalysts. The chronoamperometric technique is an effective method to evaluate the catalytic activity and stability of catalyst materials. Fig. 8 shows the typical current density–time responses of Pd/SnO2 GNS and Pd/GNS catalysts for ethanol oxidation measured at a
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dissociation–adsorption of ethanol proceeds quickly [Eqs. (1)–(3)].
Fig. 7. Variation of current density for ethanol oxidation with a Pd/SnO2 -GNS catalyst having a SnCl2 ·2H2 O to GO mass ratio of without SnCl2 ·2H2 O, 1:4, 1:2 and 1:1.
fixed potential of −0.3 V. In principle, all of them present a gradual current decay before a steady current status is attained, which is attributed to the formation of some Pd oxides and/or adsorbed intermediates in ethanol oxidation reaction. Gradually, the current decayed and a pseudo-steady state is achieved. The current density measured for the Pd/SnO2 -GNS catalysts are higher than that of Pd/GNS during the whole testing time. For different Pd/SnO2 -GNS catalysts, the current density of Pd/SnO2 -GNS-2 is higher than other samples. These results are consistent with the CV measurements mentioned above. For Pt based catalysts, the incorporation of metal oxides into the anode has been shown to minimize the CO poisoning effects in a methanol fuel cell [34,35], but very few mechanistic studied on the oxidation of ethanol on Pd-metal oxides catalysts have been reported. The beneficial effect of the added metal oxide was attributed to the fact that OHads could easily form on the introduced metal oxides, which is similar with the well-known bi-functional mechanism between Pt and Ru [36]. Referring to the depiction of Pt-metal oxides and the generally accepted oxidation sequence for ethanol oxidation on Pd in alkaline media [37], we propose a tentative reaction mechanism for the ethanol oxidation on Pd/SnO2 -GNS catalysts as follows. The surface adsorbed hydroxyl of SnO2 may remove the adsorbed acyl on the surface of Pd, and then the
SnO2 + OH− → SnO2 -OHads + e−
(1)
Pd-(CH3 CH2 OH)ads + 3OH− → Pd-(COCH3 )ads + 3H2 O + 3e−
(2)
Pd-(COCH3 )ads + SnO2 -OHads → Pd-CH3 COOH + SnO2
(3)
SnO2 would increase the concentration of OHads species on the catalyst surface, favoring the oxidation of the acetaldehyde into acetic acid via acylads -OHads coupling, thereby releasing the active sites on Pd for further electrochemical reaction [38]. It is worth noting that the Pd/SnO2 -GNS catalysts are more effective for ethanol oxidation than Pd/GNS catalyst by cyclic voltammetric studies and chronoamperometric determination. The amount of SnO2 in the Pd/SnO2 -GNS catalysts also affects the catalytic activity. Thus, a high content of SnO2 has the following consequences: (a) higher content of active Pd/SnO2 interface; (b) more OH species for removing reaction intermediates; (c) partial blocking of Pd active sites; (d) higher resistance of the catalysts due to the semiconducting nature of SnO2 . Among these, the first two factors lead to higher catalytic activity, while the latter two lead to lower catalytic activity [39]. Hence, it is needed to balance all the factors so as to obtain the best catalytic activity. 4. Conclusions In summary, Pd/SnO2 -GNS catalysts were successfully prepared and their electrochemical properties have been investigated by cyclic voltammetry and chronoamperometry. Because of the high ECSA and good scavenging capacity towards the intermediate species formed during the ethanol oxidation, the Pd/SnO2 -GNS catalysts show much more enhanced catalytic activity when compared with Pd/GNS catalyst and the optimal mass ratio of SnCl2 ·2H2 O precursor salt to GO is about 1:2. These show that the SnO2 -GNS composite may have the splendid future as the catalyst support for fuel cell. Acknowledgements The authors gratefully acknowledge the support by National Basic Research Program of China (973 Program) (no. 2007CB209700), National Natural Science Foundation of China (no. 50701023), Graduated Student Innovation Foundation of Jiangsu Province (CX09B 075Z and CX09B 076Z) and NUAA Research Funding (no. NS2010165). References
Fig. 8. Chronoamperometric curves for ethanol oxidation on Pd/SnO2 -GNS and Pd/GNS catalysts in 0.25 mol L−1 NaOH and 0.25 mol L−1 C2 H5 OH solution at −0.3 V.
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