Ethanol Electrooxidation on Pt-Sn Catalysts Deposited ... - Google Sites

ECS Transactions, 6 (25) 217-223 (2008) 10.1149/1.2943241, © The Electrochemical Society

Ethanol Electrooxidation on Pt-Sn Catalysts Deposited by Pulsed Laser Ablation Pascale Bommersbach, Mohamed Mohamedi, Daniel Guay Institut National de la Recherche Scientifique (INRS) - Centre Énergie, Matériaux et Télécommunications (EMT), 1650 Blvd Lionel-Boulet, CP 1020, Varennes, Québec, J3X 1S2 Canada Pt3Sn catalysts were prepared from pure Pt and Sn targets by crossed beam pulsed laser deposition. The deposition was performed either under vacuum or in the presence of a He background pressure (1, 2, 3, 4 and 5 Torr). The pressure in the deposition chamber influences the surface structure and the morphology of the catalytic nanoparticles, as determined from scanning electron microscopy. Cyclic voltammogram curves reveal that Pt3Sn catalysts deposited under He lower the onset oxidation potential of ethanol compared to Pt3Sn electrodes synthesized under vacuum. This effect is particularly evident for Pt3Sn elaborated at 2 Torr He which provides a high current density value close to 20 mA.cm-2 at 0.7 V vs. RHE. Chronoamperometric measurements conducted at + 0.5 V vs. RHE also demonstrated that Pt3Sn deposited at 2 Torr He is the most active electrocatalyst after 1 hour of electrolysis. Introduction Direct Alcohol Fuel Cells (DAFCs) use methanol or ethanol as fuel directly without external reformer, which makes them simple and compact (1,2). Ethanol is a particularly promising and interesting alternative as a renewable fuel since it can easily be produced in large quantities from biomass. The complete electrooxidation of ethanol to CO2 is difficult to achieve and pure platinum is not the most efficient anodic catalyst for DEFCs (Direct Ethanol Fuel Cells). During the oxidation of ethanol, platinum is poisoned by reaction intermediates and its activity deteriorates with time. To counter this problem, platinum is often modified by adding a second metal (like Ru, Pb, Sb, Rh, Sn, etc), which enhances the electrocatalytic activity of pure platinum. The more pronounced enhancement has been observed with Pt-Sn catalysts. Several authors have investigated mixed Pt-Sn catalysts that show superior performances for ethanol oxidation compared to pure Pt (3-21). These studies have clearly shown that the preparation procedure has a marked effect on the catalyst composition and consequently on its electrocatalytic activity toward ethanol oxidation. The aim of this paper was to characterize Pt3Sn catalysts synthesized by pulsed laser ablation and to evaluate their electrocatalytic performances toward ethanol electrooxidation. The Pulsed Laser Deposition (PLD) method is well adapted for the synthesis of new complex nanomaterials (22). Furthermore, there are several deposition parameters that can influence the structure and properties of the deposited materials (23). In the present work, our attention is focused on the influence of He background pressure in the deposition chamber at fixed substrate-target distance. Pt3Sn catalysts were thus

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ECS Transactions, 6 (25) 217-223 (2008)

synthesized either under vacuum (as reference) or under He (1, 2, 3, 4 and 5 Torr) to determine the optimal He pressure during the synthesis. Scanning electron microscopy was employed to characterize all the catalysts. Their catalytic activities for ethanol oxidation were evaluated by cyclic voltammetry and chronamperometry techniques. Experimental Catalyst Preparation Pt3Sn films were deposited by simultaneously ablating pure platinum and pure tin targets (Kurt J. Lesker Company) by means of a pulsed KrF excimer laser (λ= 248 nm, pulse width = 17 ns and repetition rate 50 Hz). The substrates consist of disks made of graphite (99.997 %, Ø = 5 mm, Goodfellow Corporation) previously polished. Prior to deposition, the chamber was evacuated by means of a turbopump down to a pressure of 4 x 10-5 Pa. The substrate-target distance was fixed to 5 cm and the He background pressure was varied from 1 to 5 Torr by step of 1 Torr. The targets were continuously moved across the laser beam. The [Pt]/[Sn] concentration ratio was adjusted by varying the fluences of each target. Physico-chemical characterization The catalyst samples were examined using a JEOL JSM-6300F Scanning Electron Microscope (SEM) operated at an accelerating voltage of 20 kV. Bulk chemical compositions were determined by Energy Dispersive X-ray (EDX) analysis. Electrochemical Study Electrochemical studies were carried out in a de-aerated solution of H2SO4, 0.5 M + C2H5OH, 1 M. The reference electrode was a RHE and the counter electrode was a platinized Pt plate. The working electrode was the Pt3Sn electrocatalyst deposited on the graphite disk. Current–potential (I–E) and chronoamperometric (I-t) curves were measured by a potentiostat EG&G model 273A. Prior to electrochemical measurements in ethanol containing solution, the surface of the working electrode was cleaned electrochemically by potential cycling in H2SO4, 0.5 M. Before each test, dissolved oxygen was removed from the solution by bubbling argon for 15 min. All the electrochemical measurements were carried out at room temperature. The current densities are referred to the geometric surface area of the electrodes (i.e. 0.2 cm2). Results – Discussion SEM-EDX analysis SEM analysis was carried out to investigate the morphology of electrodes prepared at various He background pressure in the deposition chamber. Figure 1 presents the SEM micrographs of Pt3Sn catalysts prepared by PLD under vacuum and under 1, 2, 3, 4 and 5 Torr He.



VACUUM

3 TORR He

1 TORR He

4 TORR He

2 TORR He

5 TORR He

Figure 1. SEM micrographs of Pt3Sn electrodes elaborated under vacuum and under 1, 2, 3, 4 and 5 Torr He. The surface structure of Pt3Sn catalysts varies with the He background pressure. For catalysts prepared in vacuum, the surface is very smooth and only a few scattered tin particles (as determined by EDX analyses) are observed. Catalysts deposited at higher He pressure shows an increase number of particles. The number of particles at the surface of the catalyst is maximal for Pt3Sn deposited at 2 Torr He. For higher He background pressure, the number of particles decrease. At 5 Torr He, no particles are seen on the surface. With regard to the synthesis of Pt3Sn at 5 Torr He, X-ray photoelectron spectroscopy analysis (results not shown here) reveals that there is neither Pt nor Sn atoms on the substrate (Pt and Sn atoms are observed on all samples prepared at lower He background pressure). As shown elsewhere, the deposition rate first increases and then decreases with the background pressure. The decrease of the deposition rate at the highest He background pressure occurs as a result of a smaller mean free path of the species as the background pressure is increased. Cyclic voltammetry In order to evaluate the electrocatalytic activity of the catalysts for ethanol oxidation, we have tested electrochemically all catalysts, except the one elaborated at 5 Torrs He. Cyclic voltammograms of Pt3Sn catalysts elaborated under vacuum and under He up to 4 Torr are shown in Fig. 2. All measurements were performed in H2SO4, 0.5 M + C2H5OH , 1 M. In each case, 10 consecutive cycles (5 mV.s-1) were realized. We found this procedure yields to stable CV. Pure Pt electrodes synthesized under vacuum and at 2 Torr He are also reported in Fig. 2.



20.0

synthesis atmosphere :

vacuum 1 Torr He 2 Torr He 3 Torr He 4 Torr He

17.5

i/mA.cm

-2

15.0 12.5 10.0 7.5 5.0

Pt (2 Torr He)

2.5 Pt (vacuum)

0.0 0.0 -2.5

Figure 2.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

E/V vs. RHE

Cyclic voltammograms of P3Sn and Pt catalysts elaborated under different atmospheres. The CVs were performed in H2SO4 , 0.5 M + C2H5OH , 1 M, at a scan rate of 5 mV s-1.

Cyclic voltammograms were recorded from 0.05 to 0.7 V vs. RHE to avoid tin leaching (6,7,21). Prior to measurements in presence of ethanol, the surface of the working electrode was cleaned electrochemically by potential cycling in sulphuric acid (0.5 M, scan rate = 50 mV s-1) using the same potential limits. This cleaning step is necessary for obtaining a reproducible surface and to ensure the stability of the deposited particles on the graphite substrate (6). A good electrocatalyst must provide both a low onset potential for ethanol electrooxidation reaction and a high current density at a fixed potential. Figure 3 summarizes the main results extracted from the cyclic voltammograms. For each synthesis conditions, the current densities recorded at 0.5 V vs. RHE and the onset potentials are plotted. It is clear from that Figure that Pt3Sn synthesized at 2 Torr He is the best electrocatalyst. Indeed, this catalyst presents not only the highest current density (almost 10 mA.cm-2 at 0.5 V vs. RHE), but also the lowest onset potential (175 mV vs. RHE). Catalysts synthesized at 1 Torr and 3 Torr He are the second and third most performing catalysts, respectively. Pt3Sn elaborated at 4 Torr He displays a weaker catalytic electroactivity, but it is still higher than that of catalysts elaborated under vacuum. As compared to platinum electrodes, all Pt3Sn samples show higher electrocatalytic activity for ethanol oxidation.


Onset Potential (V vs. RHE)


0.40 0.35 0.30 0.25

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intensity (mA.cm )

0.20 9 8 7 6 5 4 3 2 1 0

Pt

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1

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rr To

e

e

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H

e

e

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H rr To

rr To

um cu va

Figure 3. Evolution of onset potentials and current densities recorded at 0.5 V vs. RHE, for Pt3Sn and Pt elaborated under different atmospheres. Chronoamperometric curves Providing a high current density at low potential is one of the fundamental criteria to judge the quality of a catalyst, but the stability of the catalyst’s activity with time is also another important parameter. By stepping the potential from the open circuit potential to 0.5 V vs. RHE, the current density values were recorded during one hour in 0.5 M H2SO4 + 1 M C2H5OH. The resulting i-t curves are shown in Fig. 4.

10

synthesis atmosphere :

vacuum 1 Torr He 2 Torr He 3 Torr He 4 Torr He

i/mA.cm

-2

8

6

4

2

Pt (2 Torr He)

0 0

10

20

30

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Pt (vacuum)

time / min

Figure 4. Current-density vs. time curves at 0.5 V vs. RHE for Pt3Sn and Pt catalysts elaborated under different He pressures, in H2SO4 , 0.5 M + C2H5OH , 1 M. All i-t curves of Pt3Sn catalysts show the same general trend characterized by three different regions. In the first one, that last a few minutes, the current drops rapidly. This region is followed by a second one where the current decrease is less steep. Finally, in the third region, the current almost reaches a steady state. The variation of current at the



beginning is attributed to the increasing surface coverage with partially oxidized species (6). The continuous slight decrease of current in the second region is due to the poisoning of Pt surface, which prevents ethanol oxidation (24). The rapid poisoning of Pt surface is clearly confirmed by the very low current densities recorded for Pt catalysts (less than 0.3 mA.cm-2 for Pt elaborated at 2 Torr He and even less than 10 µA.cm-2 for Pt catalyst synthesized under vacuum). Tin is able to activate water dissociation at lower potentials than platinum, helping the oxidation of adsorbed intermediates and consequently allowing the reaction of ethanol oxidation to proceed. Currents recorded for catalysts prepared under He are more important than those recorded for catalysts elaborated under vacuum, except for the catalyst prepared under 4 Torr He. The Pt3Sn electrode elaborated under 2 Torr He demonstrates the highest current density, in agreement with the previous voltammetric observations. Conclusion Pt3Sn catalysts were prepared by the pulsed laser deposition method. Only one deposition parameter was varied and the catalysts were elaborated at variable He background pressure. SEM pictures evidenced the presence of tiny particles for catalysts elaborated under 1, 2, 3 and 4 Torr He, with a higher concentration of these particles at 2 Torr He. During ethanol electrooxidation, Pt3Sn electrodes demonstrate a higher electrocatalytic activity (a higher current density and lower onset potential) compared to Pt catalysts. Furthermore, the He background pressure during Pt3Sn synthesis is beneficial to the enhancement of the activity of catalysts, and particularly in this study, 2 Torr He is the pressure for which the Pt3Sn catalyst presents the best catalytic performances toward ethanol electrooxidation. Acknowledgements This work was supported by NSERC (Natural Sciences Engineering Research Council of Canada) - Strategic Grant "Development of novel nanosized electrocatalysts for direct bioethanol fuel cells", the Canadian Foundation for Innovation (CFI), FQRNT, and INRS-EMT. References 1. C. Lamy, J.-M. Léger, and S. Srinivasan, Direct Methanol Fuel Cells – from a 20th century electrochemists’ dream to a 21st century emerging technology, in : J.O’M. Bockris, B.E. Conway (Eds), Modern Aspects of Electrochemistry, vol. 34, Plenum Press, New York, 2000. 2. C. Lamy and J.-M. Léger, in Proc. 2nd international Symposium on New Materials for fuel Cell and Modern Battery Systems, Ed. By O. Savadogo and P .R. Roberge, Ecole Polytechnique, Montréal, pp. 477-488. (1997). 3. S. Song and P. Tsiakaras, Appl. Catalysis, 63, 187 (2006). 4. F. Vigier, C. Coutanceau, F. Hahn, E. M. Belgsir, and C. Lamy, J. Electroanal. Chem., 563, 81 (2004).



5. L. Jiang, G. Sun, S. Sun, J. Liu, S. Tang, H. Li, B. Zhou, and Q. Xin, Electrochim. Acta, 50, 5484 (2005). 6. G. Siné, G. Foti, Ch. Comninellis, J. Electroanal. Chem., 595, 115 (2006). 7. L. Colmenares, H. Wang, Z. Jusys, L. Jiang, S.Yan, C.Q. Sun, R.J. Behm, Electrochim. Acta, 52, 221 (2006). 8. W.J. Zhou, B. Zhou, W.Z. Li, Z.H. Zhou, S.Q. Song, G.Q. Sun, Q. Xin, S. Douvartzides, M. Goula, P. Tsiakaras, J. Power Sources, 126, 16 (2004). 9. W.J. Zhou, S. Q. Song, W. Z. Li, Z. H. Zhou, G. Q. Sun, Q. Xin, S. Douvartzides, and P. Tsiakaras, J. Power Sources, 140, 50 (2005). 10. E. V. Spinacé, M. Linardi, and A. Oliveira Neto, Electrochemistry Communications, 7, 365 (2005). 11. W. J. Zhou, S. Q. Song, W. Z. Li, G. Q. Sun, Q. Xin, S. Kontou, K. Poulianitis, and P. Tsiakaras, Solid State Ionics, 175, 797 (2004). 12. L. Jiang, G. Sun, Z. Zhou, W. Zhou, and Q. Xin, Catalysis Today, 93-95, 665 (2004). 13. S.Q. Song, W.J. Zhou, Z.H. Zhou, L.H. Jiang, G.Q. Sun, Q. Xin, V. Leontidis, S. Kontou, P. Tsiakaras, Int. J. Hydrogen Energ., 30, 995 (2005) 14. J.-M. Léger, S. Rousseau, C. Coutanceau, F. Hahn, and C. Lamy, Electrochim. Acta, 50, 5118 (2005). 15. C. Lamy, S. Rousseau, E.M. Belgsir, C. Coutanceau, and J.-M. Léger, Electrochim. Acta, 49, 3901 (2004). 16. S. Rousseau, C. Coutanceau, C. Lamy, and J.-M. Léger, J. Power Sources, 158, 18 (2006). 17. F. Colmati, E. Antolini, E. Gonzalez, J. Electrochem. Soc., 154, B39 (2007). 18. F. Colmati, E. Antolini, E.R. Gonzalez, Appl. Catal. B-Environ., 73, 106 (2007). 19. P. Bommersbach, M. Mohamedi, D. Guay, J. Electrochem. Soc., accepted. 20. F. Colmati, E. Antolini, and E. R. Gonzalez, J. Power Sources, 157, 98 (2006). 21. H. Wang, Z. Jusys, and R.J. Behm, J. Power Sources, 154, 351 (2006). 22. E. Irissou, PhD Thesis, University of Québec, INRS-EMT (2004). 23. E. Irissou, B. Le Drogoff, M. Chaker, D. Guay, J. Appl. Phys., 94, 4796 (2003). 24. S. Tanaka, M. Umeda, H. Ojima, Y. Usui, O. Kimura, and I. Uchida, J. Power Sources, 152, 34 (2005).
