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Electrochimica Acta 66 (2012) 1– 6

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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he effect of Sn content in Pt–SnO2/CNTs for methanol electro-oxidation

e Lin, Shichao Zhang ∗, Shaohui Yan, Guanrao Liuchool of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100191, China

r t i c l e i n f o

rticle history:eceived 9 September 2011eceived in revised form3 November 2011ccepted 22 December 2011vailable online 31 December 2011

eywords:

a b s t r a c t

The electrocatalyst is one of several impediments for the application of direct methanol fuel cells, whichincludes the mediocre performance on anode and its endurance against intermediates of methanoloxidation such as carbon monoxide.

In this research, a hydrolyze–hydrothermal method has been applied to synthesize the SnO2/CNTsnano support, followed by the Pt loading process to prepare the Pt–SnO2/CNTs in sequence. CTAB wasadded into system during hydrothermal process to promote uniform dispersion of the tin oxide on sur-face of CNTs. The total noble metal loading is 20% in weight. The catalysts were characterized by cyclic

tarbon nano-tubesydrothermalin oxideslectro oxidation

voltammetry, and chronoamperometry. For comparison, a series of catalysts with different Sn contentwere also synthesized.

The onset potential of the Pt–SnO2/CNTs is much lower than Pt/C, the current density of Pt–SnO2/CNTswere nearly double of which on the commercial Pt/C, indicating that SnO2 plays a very important rolein methanol oxidation and CO oxidation at low potentials. CV curves and chronoamperometry indicatedthat there was a positive correlation between Sn content and the electrochemical activity.

. Introduction

Fuel cell converts chemical energy directly into electrical powerith high efficiency, in most likelihood, via the course which results

n low emission of contaminations. Before fuel–cell technology gain significant share of electrical power market, intensely substantialssues are required to be addressed. In spite of the attractive systemfficiencies and environmental benefits associated with fuel–cellechnology, it has proved difficult to develop the early scientificxperiments into commercially viable industrial products [1].

For electro-oxidation of methanol, the oxidation of adsorbed COn Pt is slow, and is facilitated by adjacent absorbed OH species [1].ransition metal oxides have been widely used for various elec-rochemical and catalytic applications. Besides the conventionaltRu binary catalysts, transition metal and their oxidations haveeen highly regarded as the efficient components in Pt base cat-lysts by many researching groups [2]. In recent years, tin-basedxides/CNTs composites have attracted much interest as electrodeaterials [3]. Much attention has been focused on carbon based

latinum nano-particles modified by transition metal oxides whichre used in the process of methanol and ethanol electro-oxidation

4–6]. Kowal [7] synthesized a ternary PtRhSnO2/C catalyst byepositing platinum and rhodium atoms on carbon-supported tinioxide nanoparticles that is capable of oxidizing ethanol with high

∗ Corresponding author. Tel.: +86 10 82338148; fax: +86 10 82339319.E-mail address: csc@buaa.edu.cn (S. Zhang).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.12.109

© 2012 Elsevier Ltd. All rights reserved.

efficiency. Lim [8] reported that the Sn can improve the activity ofPt nanoparticles on methanol oxidation by removing the interme-diates through its oxygen-containing species.

Carbon nanotubes (CNTs), due to its unique properties, havebeen considered as powerful nano materials to functionalizeother materials aiming at improving their electrical conductivity,mechanical and thermal properties in the past ten years [3]. Multi-walled carbon nano-tubes (MWCNTs) are considered one of themost important catalyst supports in such a scientific trend.

In this research, we used the hydrolyze–hydrothermal routereported by Fujihara [9] to synthesize SnO2/CNTs nano-composites.Here, carbon nano support and tin precursor were mixed at the verybeginning in the purpose to get a homogeneous product. After Ptloading, the Pt–SnO2/CNTs was finally prepared. We synthesized aseries of catalysts which contain different Pt/Sn atom ratio to studythe content effect of Sn in Pt–SnO2/CNTs catalysts. The obtainedcatalyst showed high electrochemical activity and good perfor-mances against intermediates during methanol electro-oxidation.This was attributed to the uniform distribution of tin oxide and itsmodification effects to Pt nano-particles on the surfaces of carbonnano-tubes.

2. Experimental

2.1. Synthesis of SnO2/CNTs

MWCNTs with about 50 nm of outer diameter and 2–5 �m oflength were purchased from Chengdu Organic Chemicals Co. Ltd.,

2 imica Acta 66 (2012) 1– 6

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Fig. 1. XRD patterns of the Pt–SnO2/CNTs with atom ratio at (a) Pt/Sn = 1.5; (b)

Y. Lin et al. / Electroch

hinese Academy of Sciences. For the purification and functionalnd functionalization, CNTs were treated by refluxing in nitric acidnd sulfuric acid (1:3) mixture under 95 ◦C water bath for 5 h,ollowed by washing with deionized water until pH value was.

Here, it was necessary that tin oxides should be deposited onarbon support before Pt loading, otherwise the low conductiv-ty of SnO2 may decrease Pt active surface, this happened to havehe same view with Kowal et al. [7]. As reported by Fujihara [9], aertain amount of SnCl4·5H2O was dissolved in deionized waterith a concentration of 0.05 M. 80 mg of MWCNTs were added

nto system and ultrasonicated for 0.5 h to form a homogeneouslack suspension, SnCl4 solution was then heated for 20 min in5 ◦C water bath under refluxing conditions to promote a completeydrolysis of SnCl4 and formation of SnO2 nano sol. After elec-romagnetic stirring for 12 h to ensure a complete adsorption, theroduct was centrifugal washed by deionized water over 10 timeso remove chloride ions from the system. 2% of Cetyltrimethylam-

onium bromide (CTAB) was added as structure-directing agents3]. The hydrothermal treatment was carried under 150 ◦C for 24 h,ollowed by excessive washing and drying, and The SnO2/CNTsomposites were prepared.

.2. Pt loading

For Pt loading, we used a simple solvent thermal route.nO2/CNTs nano support was sonochemical dispersed into 30 mLf ethylene glycol until forming a uniform suspension, 0.0386 Mf chloroplatinic acid aqueous solution was carefully added drop-ise into the system. The pH value was adjusted to 13 using NaOH,

hen followed by a heat treatment under 160 ◦C for 5 h using Teflonutoclave. The Pt–SnO2/CNTs was finally collected after filtering

nd drying at 60 ◦C for 12 h. The total noble metal loading was con-rolled as 20% in weight. A series of catalysts with Pt/Sn atom ratiot 1.5, 2 and 3 were prepared and defined as Pt3Sn2, Pt2Sn Pt3Snnd Pt4Sn, respectively.

Fig. 2. EDX spectrum of (a) Pt3Sn2; (

Pt/Sn = 2; (c) Pt/Sn = 3; (d) Pt/Sn = 4.

2.3. Characterization

The structural characteristics of the catalysts were studied bypowder X-ray diffraction and transmission electron microscope(TEM). The X-ray diffraction patterns were recorded on Rint D/max2000 wide angle goniometer (Rigaku Co. Ltd.) using Cu target,K� radiation with a scanning range (2 theta) from 20◦ to 90◦ at40 kV/40 mA, divergence slit = 1◦. A JEOL JEM-2010F field emissiontransmission electron microscope operating at 200 kV was used forTEM studies. The metal content of the catalysts was determined byOxford INCA energy dispersive X-ray spectroscopy (EDX) equipped

on S-530 Scanning Electron Microscope (HITACHI Co. Ltd). Thesamples for SEM characterization were prepared by dispersing the

b) Pt2Sn; (c) Pt3Sn; (4) Pt4Sn.

Y. Lin et al. / Electrochimica Acta 66 (2012) 1– 6 3

Fig. 3. TEM image (a, c, e and g) and particle size distribution (b, d, f and h) of Pt nano-particles in Pt–SnO2/CNTs with Pt/Sn atom ratio at 1.5, 2, 3, and 4.

4 Y. Lin et al. / Electrochimica Acta 66 (2012) 1– 6

Table 1Metal content of Pt–SnO2/CNTs.

Sample Pt load (wt.%) Pt content (at.%) Sn load (wt.%) Sn content (at.%) Pt/Sn (Atom.)

Pt3Sn2 21.17 ± 2.04 1.82 9.46 ± 0.51 1.34 1.36

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Pt2Sn 19.08 ± 0.85 1.52

Pt3Sn 23.37 ± 0.82 1.94

Pt4Sn 17.54 ± 0.87 1.33

btained catalysts on the surface of copper column ( ̊ = 2 mm) setn polytetrafluoroethylene (PTFE) substrate.

.4. Electrochemical characterization

The electrochemical properties were examined by CHI 660Dotential static instrument (CHI instruments, Inc.). Glassy carbonlectrodes (˚4) were used as the working electrode to supporthe catalysts. The electrodes were polished with Al2O3 paste until

mirror-like state, then treated in a supersonic bath for 0.5 min,esidual polishing material were removed from surfaces by soni-ation after cleaning out by second distilled water and anhydrousthanol in sequence. A specified amount of catalysts was dispersedn isopropyl alcohol, sonicated for 1 h, carefully dropped onto theurface of GC electrode and dried at room temperature. 5 �L ofafion ethanol solution (1 wt.% Dupont) was then added on theatalytic layer. A conventional three-electrode cell was used for

ests. Platinum sheet and saturated calomel electrode (SCE) weresed as counter and reference electrodes respectively. After remov-

ng all the dissolved air by nitrogen flow for 20 min. The surface ofhe Pt/C catalyst on the glassy carbon electrode was then cleaned

Fig. 4. HRTEM images of Pt–SnO2/CNTs with atom ratio at (

5.61 ± 0.51 0.74 2.054.53 ± 0.42 0.62 3.132.69 ± 0.44 0.34 3.91

electrochemically by cycling the potential between 0.05 and 1.4 Vat a sweep rate of 0.05 V/s−1 until the CV profiles no longer change.Cyclic voltammograms of the catalysts were then executed in0.5 M sulfuric acid in order to measure the electrochemical surfacearea (ECSA) by calculating the H2 adsorption peaks. For methanolelectro-oxidation, the obtained catalysts were measured in 0.5 Msulfuric acid + 1 M methanol aqueous solution shortly after scannedin sulfuric acid. All the solutions in the electrochemical measure-ments were prepared from secondary distilled water. Moreover, thecommercial Pt/C (by Johnson Matthey Co.) was also characterizedunder the same conditions as obtained catalyst for comparison.

3. Result and discussion

3.1. Physical analysis of Pt–SnO2/CNTs

The XRD patterns of the Pt–SnO2/CNTs synthesized by in-situhydrolyze–hydrothermal method are shown in Fig. 1. The diffrac-tion peaks in the XRD pattern at 2� of 39.6◦, 46.1◦, 67.4◦, and 81.3◦

can be assigned to be the reflections from the (1 1 1), (2 0 0), (2 2 0),

a) Pt/Sn = 1.5; (b) Pt/Sn = 2; (c) Pt/Sn = 3; (d) Pt/Sn = 4.

imica Acta 66 (2012) 1– 6 5

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nd (3 1 1) crystalline planes of the face-centered-cubic (fcc) Pt,espectively. The high values of the peak intensities indicate thexistence of good crystallinity in the obtained Pt–SnO2/CNTs. Theiffraction peaks of SnO2 (1 0 1) and SnO2 (2 1 1) planes can be alsobserved at 33.2◦ and 52.8◦ respectively. With the reducing contentf Sn in Pt–SnO2/CNTs, the SnO2 diffraction peaks present lower sig-al intensities, all the SnO2 diffraction peak were extended whilehe intensity of each plane was rather weak, this may attribute tohe low content and broadening effect of SnO2 nano particles.

EDX was shown from Fig. 2(a–d), corresponding to Pt3Sn2,t2Sn, Pt3Sn and Pt4Sn respectively. Normalized results of metalontent were listed in Table 1. The atomic ratio of Pt/Sn was 1.36,.05, 3.13 and 3.91 for Pt3Sn2, Pt2Sn, Pt3Sn and Pt4Sn. The calcu-

ated atomic ratio was agreed with theoretical values, show that theydrolyze–hydrothermal method reported by Fujihara [9] can belso well applied in case of Sn loading onto carbon nano substrate.

Typical TEM images of the Pt–SnO2/CNTs with Pt/Sn atom ratio.5, 2, 3, and 4, were shown in Fig. 2. It can be acquired from the fig-re that the Pt nano-particles are homogeneously deposited ontoarbon nano-tubes, with a diameter about 3–6 nm in general. It cane observed from the images that the nano particles of Pt2Sn pre-ented a dense distribution mostly onto the outer walls of MWCNTs.

Fig. 3(b, d, f and h) is statistical result from more than 150 Ptano-particles showing the distribution of the particle size. Fort3Sn, the frequency of 2–4 nm Pt nano particles is higher thanther two samples. Although the Pt nano particles smaller than

nm exert higher activity due to its smaller diameter, agglomera-ion may also happen in electrochemical or other process. This cane also observed from Fig. 3(e). It is clear that the size distributioneak of Pt nano-particles in the Pt2Sn is much more narrow then thethers, the Pt particles in Pt2Sn catalyst mainly distributed from 3 to

nm, average particle size is 4.4 ± 1.0 nm by calculation, which waslso less than Pt3Sn2 (6.0 ± 1.0 nm) Pt3Sn (4.6 ± 1.0 nm) and Pt4Sn4.7 ± 1.0 nm). This indicating that Pt formed a extremely uniformispersion on SnO2/CNTs with Pt/Sn = 2:1 which is significant toheir electrochemical activities for methanol oxidation. Especially,ince the most of Pt particles in Pt3Sn2 was bigger than 5 nm, it wasifficult to control the diameter of Pt nano particles be under 5 nmue to the high content of SnO2.

High-resolution transmission electron microscopy (HRTEM)mages were shown in Fig. 4(a–d) identified by the white arrows,he lattice fringes of all the samples are at 0.2265 nm approxi-

ately, which can be attributed to Ptfcc(1 1 1) interplanar distanceorresponding to JCPDS 65-2868. Moreover, there is no direct evi-ence of tin oxides in HRTEM images, this may caused by the lowontent of Sn, another explanation is that the tin oxides may exists amorphous phase in nano scale, which means its Crystallinitys poor. This is agreed with X-ray diffraction patterns in Fig. 1,specially for Pt4Sn.

.2. Electrochemical analysis

The cyclic voltammograms of the obtained Pt–SnO2/CNTs andommercial Pt/C catalyst in 0.5 M H2SO4 + 1 M CH3OH aqueousolution are shown in Fig. 5(a and b). To compare the obtained car-on nano-tubes support Pt–SnO2 catalyst with commercial Pt/C,he current densities of each sample are normalized by their ECSAhich are estimated by the integrated charge in the hydrogen

dsorption from the CV curve in sulfuric acid. According to Fig. 5(a),ith the increase of Sn content in Pt–SnO2/CNTs, the peak currentensity (j/mA cm−2Pt) was significantly increased. The anodic peakurrent density of Pt2Sn is almost double of which on the com-

ercial Pt/C, this indicate the SnO2 plays a very important role

n methanol oxidation and CO oxidation at low potentials. For thet2Sn, it is noteworthy that the forward peak current (If) is higherhan the backward current (Ib). Gutierrez [10] reported that the

Fig. 5. (a) Cyclic voltammograms (by Johnson Matthey Co.) and (b) zoomed regionof the Pt–SnO2/CNTs and commercial Pt/C catalysts in 0.5 M H2SO4 + 1 M CH3OHaqueous solution, scanning rate is 0.05 V/s.

removal of the incompletely oxidized carbonaceous species formedin the forward scan promotes the appearance of an asymmetricanodic peak. In the reverse scan, it was one of the possible routesthat the SnO2 was able to promote regeneration of Pt active surfacewith absorptive OHads due to bifunctional mechanism [11] as Eqs.(1)–(2) below.

SnO2 + OH− → SnO2 OHads + e− (1)

Pt (CO)ads + OH− → Pt + CO2 + H2O + e− (2)

Since the If/Ib ratio of the Pt2Sn and Pt3Sn were higher than 1,while which of commercial Pt/C is not, the endurance against inter-mediates of catalysts during methanol oxidation can be increaseddue to the addition of SnO2. However, the If/Ib ratio of the Pt3Sn2was lower than Pt2Sn and Pt3Sn, this may be caused by the blockingof Pt active surface with high content of SnO2 in catalyst. In Fig. 5(b),it was inferred from the zoomed area in forward scan that all theonset potentials (Eonset) of Pt–SnO2/CNTs were lower than which ofJM. The onset potentials of Pt2Sn and Pt3Sn were the same about0.10 V, which is lower than the others, verified the phenomenon ofthe obtained catalysts in the CV curves.

Chronoamperometry was processed at half wave potential of

each single sample. The resultative I/t curve was showed in Fig. 6.To exclude the impulse current, the data of first 200 s was removedin order to get a more uniform result. With the increment of Sn con-tent, the current density decreased. Although the Pt3Sn show the

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ig. 6. Chronoamperometry of (a) Pt3Sn2; (b) Pt2Sn; (c) Pt3Sn; (4) Pt4Sn in 0.5 M2SO4 + 1 M CH3OH aqueous solution.

ighest electro catalytic performance of all the samples, its currentecreased distinctly from the beginning. This may be caused by theact that some of the SnO2 dissoluted into acid electrolyte under aonstant positive potential. According to the curves, the Pt2Sn kept

balance between electrochemical activity and stability againstntermediates during methanol oxidation.

. Conclusion

In summary, we synthesized a series of carbon nano-tube sup-ort SnO2 nano-composite with different Sn content by in-situydrothermal method, after Pt loading, the Pt–SnO2/CNTs wasbtained. EDX and X-ray diffraction patterns showed the tin oxidean be deposited on carbon support uniformly and the Pt nano par-icles are well dispersed onto carbon nano support. Even though

he addition of tin oxide may lead to a conductivity decreasing ofhe electrode, the electrochemical activity was improved by theddition of Sn. After electrochemical analysis in sulfuric acid, Itas proved that the electrochemical performance for methanol

[

Acta 66 (2012) 1– 6

oxidation and endurance against adsorptive intermediates wereimpressive while Pt/Sn = 2.0 in atom ratio. Moreover, the catalystwith Pt/Sn = 1.5 represented a high activity for methanol oxidationbut poor endurance against poisoning because of its high con-tent of tin oxides. This indicated that electrochemical activity ofPt–SnO2/CNTs show positive correlation with Sn content. This con-clusion was also verified by the TEM images and size distributionbar graphs.

Acknowledgments

This work is supported by Nature and Science Foundation of Bei-jing (2051001) and the National Basic Research Program of China(2007CB936502).

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