Electrocatalytic Properties of Platinum Catalysts Prepared by Pulse

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J. Phys. Chem. C 2008, 112, 12894–12898

Electrocatalytic Properties of Platinum Catalysts Prepared by Pulse Electrodeposition Method Using SnO2 as an Assisting Reagent Feng Ye, Jingjing Li, Tongtao Wang, Yun Liu, Haojie Wei, Jianling Li, and Xindong Wang* Department of Physical Chemistry, UniVersity of Science and Technology Beijing, 30 College Road, Beijing 100083, China ReceiVed: April 13, 2008; ReVised Manuscript ReceiVed: June 10, 2008

PtSnO2/C catalysts with uniformly Pt spherical nanoclusters were synthesized by a novel pulse electrodeposition method using SnO2 as an assisting reagent. The morphology and crystallinity of the catalysts were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively. Spherical nanoclusters obtained by introducing SnO2 reagent into supports were more and finer than Pt/C catalyst nanoclusters prepared by the same method without SnO2. The electrocatalytic activities of catalysts toward methanol oxidation reaction and oxygen reduction reaction were investigated by cyclic voltammetry measurements. In comparison with Pt/C catalysts, the PtSnO2/C catalysts had larger active surfaces and exhibited higher electrocatalytic activity for methanol oxidation and oxygen reduction. The possible reason for the increased activity was analyzed and ascribed to both the promotion of CO electro-oxidation reaction kinetics and oxygen sources provided by SnO2. It is implied in the study that the method of mixing cocatalyzing materials with Pt-based catalysts is promising and will be potential in design and fabrication of electrocatalysis. 1. Introduction More attention has been paid to direct methanol fuel cells (DMFCs) because a direct-fed liquid fuel cell is perfect for portable power and mobile applications due to its high energy density and compact system and low operating temperatures.1-7 The Pt or Pt-based alloys have excellent catalytic activity for DMFCs involving methanol oxidation and oxygen reduction in acidic solution. However, it is well-known that pure platinum is easily poisoned by the carbonyl species generated by oxidation of methanol.8 In order to overcome CO poisoning and improve the catalyst performance, various Pt-based binary catalysts or Pt/metal oxide such as PtRu,9,10 PtSn,11 PtMo,12,13 PtMyOx,14 and PtWO315 have been intensively investigated and developed for DMFCs. However, the costs of the noble metal materials are often very high. Of course, there are still other related problems for DMFCs which directly restrict its performance. For example, the type of supporting materials and the dispersion of catalysts affect the efficiency of the catalytic activity16 and the kinetics of the anodic methanol oxidation.17,18 A suitable supporting material may have a significant effect on the catalytic activity due to its interactions and surface reactivity with supported electrocatalysts.19 Recently certain metal oxides appeared to be preferred components for dispersing metallic particles in fuel cells owing to their unique electrical and catalytic properties. Some metal oxides have been recently used as substrate materials for dispersing of the Pt particles in DMFCs,20,21 which remarkably enhanced catalytic activities and reduced the catalyst cost. The increase in the catalytic activity was attributed to the decrease of poisoning effect, which resulted from synergic effects of the metal oxides-platinum nanoparticles composites. SnO2, one of the most widely used rare metal oxide catalysts, is efficient for electrical and catalytic activity.22 Recently, Jiang et al.23 * Corresponding author. E-mail: [email protected]. Phone: +86-1062332651. Fax: +86-1062333477.

compared the catalytic activity of a partially alloyed PtSn catalyst with that of a quasi nonalloyed PtSnOx catalyst. They showed that the slight change in lattice parameter of Pt in the PtSnOx catalyst was in favor of ethanol adsorption, and the formed oxides in the vicinity of Pt nanoparticles provided the oxygen species which could remove the CO-like species in the form of ethanolic residues on Pt active sites. In this paper, the appropriate SnO2 nanoparticles were directly mixed with the XC-72 by ultrasonic dispersing and then platinum nanoclusters were prepared using an efficient electrodeposition method, which was easy to control without protective reagents and thermal treatments. The prepared catalysts were characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM).The performance of the prepared PtSnO2/C electrocatalysts for methanol oxidation was monitored and compared with that of Pt/C catalyst prepared by the same method. Both methanol electro-oxidation current and oxygen reduction current were remarkably increased. The possible reasons of the effects of SnO2 on the higher catalytic activity of prepared PtSnO2/C catalysts were also discussed. 2. Experimental Section 2.1. Preparation of Carbon Black/Nafion Substrates. The Pt catalysts were electrochemically deposited on carbon black (Vulcan XC-72)/Nafion and carbon black + SnO2/Nafion substrates supported on graphite electrodes (diameter 8 mm), respectively. First, the polished graphite electrodes were ultrasonically cleaned in ethanol for 40 min, followed by thorough rinsing with deionized water and drying in a vacuum oven. And then 5.0 g dm-3 Vulcan XC-72 was ultrasonically dispersed in a mixed solution of ethanol and 2% (volume ratio of [Nafion]/ [ethanol] )0.02) Nafion solution (5 wt. %). For carbon black+SnO2/Nafion substrate, 5.0 g dm-3 Vulcan XC-72 and 2.5 g dm-3 SnO2 were ultrasonically dispersed in a mixed solution of ethanol and Nafion solution (5 wt. %) abovementioned. Finally, carbon black ink with or without SnO2 was

10.1021/jp803188s CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

Electrocatalytic Properties of Platinum Catalysts

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12895

Figure 1. Scanning electron micrographs of Pt/C (SDC) by direct current electrodepostion.

Figure 3. Current-time and voltage-time curves for the pulse electrodeposition process in 0.5 M H2SO4 + 1.0 M H2PtCl6 solution. (a) Current-time curves for 100 s in the total 1200 s; (b) corresponding voltage-time curves.

Figure 2. Scanning electron micrographs of S1 (Pt/C) and S2 (PtSnO2/ C) by pulse electrodeposition.

pipetted on the graphite support to form an uncatalyzed carbon black + SnO2/Nafion substrate and uncatalyzed carbon black/ Nafion substrate, respectively. The electrodes were dried at 75 °C in a vacuum oven to vaporize the ethanol. In this paper, the geometric area of the carbon black substrate is 0.5 cm2. 2.2. Electrochemical Measurement. All of the electrochemical experiments were performed using Potentiostat of Princeton Applied Research model VMP2 at 30 °C. A conventional three-electrode cell was used in these experiments. The carbon substrate served as the basal working electrode, and a Pt sheet and a saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. All potentials in this paper are quoted with reference to SCE.

The samples of Pt catalysts were pulse electrodeposited in 0.5 M H2SO4 +1.0 mM H2PtCl6 solutions. The ton/toff is 1s /5 s for pulse electrodepositon. The total charge amount and deposition current density are 1.2 C cm-2 and -6.0 mA cm-2, respectively. In comparison with pulse electrodeposition, Pt catalyst sample (named SDC) was also fabricated by direct current (DC) electrodeposition using the same deposition solution, total charge amount, and deposition current density as pulse electrodepositon, for the purpose of showing effects of duty cycle on the particle size and morphology of the Pt deposit.24 The electrodeposition parameters and the concentration of the deposition solutions were optimized results from series of experiments. After electrodeposition, all samples were thoroughly rinsed with deionized water and dried at 60 ° in a vacuum oven. Cyclic voltammetry (CV) in 0.5 M H2SO4 between -0.25 and +1.1 V with scan rate of 100 mV s-1 was used to determine the electrochemically active surface area of the Pt catalysts. CV in the 0.5 M H2SO4 +1.0 M CH3OH solution scanning between 0 and 0.85 V at 10 mV s-1 was used to study the activity of the catalysts for methanol oxidation reaction. Prior to the CVs pure argon was passed through the electrolyte solution to remove oxygen dissolved in it. The CVs were performed till a reproductive curve was acquired. Linear sweep (negative sweep) voltammetry from 1.0 to 0.0 V at 10 mV s-1 in O2-saturated 0.5 M H2SO4 was used to test the activity of the samples for oxygen reduction reaction. 2.3. Physical Characterization. Scanning electron microscopy (SEM) images were performed using ZEISS and LEO1530 FESEM instruments. X-ray diffraction (XRD) measure-

12896 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Ye et al.

ments were carried out in a MXP21 VAHF system (Cu KR/35 kV/200mA). 3. Results and Discussions The SEM image of DC electrodeposited Pt sample (SDC) is shown in Figure 1. Spherical Pt particles (white) with particle sizes ranging between 400 and 500 nm are observed on the top of the Vulcan XC-72/Nafion substrate (dark). Figure 2 shows that S1 and S2 which are fabricated by pulse electrodeposition in the same deposition solutions with different substrates, are similar in morphologies. Comparing SDC with S1 and S2, the morphologies evolve from spherical particles to spherical nanoclusters (about 200∼300 nm) with the changing of the deposition duty cycle, which indicates that the deposition duty cycle is one of the crucial factors that influence the particle size and morphology of the Pt deposit. In accordance with that of previous studies,24,25 it demonstrates that the pulse duty cycle and the peak current density determine the nucleation rate and the growth of Pt deposit during electrodeposition. Moreover, from Figure 2, it can be seen that Pt particles of S2 are finer than those of S1. That is to say, the particle sizes of catalysts prepared with SnO2 are smaller than those of catalysts prepared without SnO2 by the same method, indicating that the addition of SnO2 facilitates the dispersion of Pt particles on the carbon support. The shape of the crystal becomes nanoclusters during the pulse electrodeposition, as shown in Figure 2. Pulse electrodeposition can produce uniform metal nanoclusters compared to what is achieved using direct current deposition.26 This increases the surface area of the catalyst in appearance. Figure 3 presents a general concept of pulse-current technique and current-time response plotted voltage vs time. The insets in Figure 3b show two magnifications for voltage vs deposition time from time of 200-235 and 795-830 s, respectively. Figure 3a shows the ton/toff in electrochemical deposition is 1 s/5 s. Figure 3b illustrates voltage-time relation in the whole deposition process corresponding to total current-time. Furthermore, the mechanism of the pulse electrodeposition of platinum is studied by the response of current-time. In Figure 3b, a sharp decrease of voltage appears during the first few seconds of the electrodeposition, which corresponds to the double-layer charging process. Then the voltage increases until stabilization state due to the free growth of independent nuclei for deposition or the formation of the new nucleation sites. In general, lower overpotential is thought to be in favor of underpotential deposition on electrodeposited platinum. Taking magnification from time of 200-235 s as an example, the voltage-time sawtooth curves show up. After that, with the reaction time increasing, the voltage decreases and gradually trend to steady state due to the growth of independent nuclei and crystals. From the insets from 795-830 s in Figure 3b, it can be seen that the response curves of current-time are consistent with applied current-time curves. During pulse electrodeposition the oxygen containing surface functional groups of Vulcan XC-72 and SnO2 form anchoring sites for the metal ions. The SnO2 interacting with the support surface groups might prevent the surface migration of platinum atoms during electrodeposition and could minimize the growing propensity of platinum particles. This is probably because that the presence of oxygen containing surface functional groups can enhance hydrophilism and thereby facilitate the access of the electrolyte to the internal pore structure and increase the effective accessible surface area of the support materials. Meanwhile, SnO2 as oxygen storage material can also provide

Figure 4. X-ray diffraction patterns of the substrate, SDC, S1, and S2.

Figure 5. Cyclic voltammograms (100 mV s-1) of SDC, S1, and S2 in 0.5 M H2SO4.

oxygen sources. The excellent oxygen storage behavior of SnO2 is the result of a unique and delicate balance between structural, kinetic, textural factors. Hence, more anchoring sites are provided for the deposition of platinum particles due to the synergistic effect between SnO2 and Vulcan XC-72. Platinum particles prefer being inserted into voids formed by SnO2 on the Vulcan XC-72 other than deposited on SnO2. It is clear that Vulcan XC-72 has a much better electronic conductivity than SnO2. As a result, highly dispersed platinum particles are uniformly formed on the supports. Although the exact interaction between SnO2 and Vulcan XC-72 is not yet fully understood, we believe that supports are of great importance to nucleation and growing propensity of platinum particles. It is also shown in Figure 2 that the dark areas of the Vulcan XC-72/Nafion substrate on S2 is smaller than those on SDC and S1, which means S2 has higher platinum (white) coverage on their substrates than S1 and SDC. The XRD patterns for the electrodeposited Pt catalysts are presented in Figure 4. The diffraction peaks of the substrate between 30° and 90° are indexed to a graphite phase. The diffraction peaks at about 39°, 46°, 68°, 81°, and 86° of S1, S2, and S3 are assigned to Pt (111), (200), (220), (311), and (222) planes, respectively, which represent the typical character of a platinum fcc structure. All three Pt catalysts show stronger (111) peaks than other peaks. In the (220) peak region, there are no reflection signals associated with the carbon substrate. The calculated results indicate that the (111)/(220) intensity ratios of SDC, S1, and S2 are 4.65, 4.32, and 6.05, respectively. At the same time, the (200)/(220) intensity ratios of SDC, S1, and S2 are 1.65, 1.63, and 2.02, respectively. So it can be seen

Electrocatalytic Properties of Platinum Catalysts

Figure 6. Cyclic voltammograms of SDC, S1, and S2 in 0.5 M H2SO4 + 1.0 M CH3OH solution.

Figure 7. Linear sweep voltammograms (10 mV s-1, negative sweep) of SDC, S1, and S2 in O2-saturated 0.5 M H2SO4.

that the intensity ratios of S1 are almost the same as the ratios of SDC, whereas the ratios of S2 are slightly larger than those of S1, indicating that deposition condition has no effects on the growth direction of Pt crystal, whereas SnO2 has influence on it to some extent. Figure 5 shows cyclic voltammograms in 0.5 M H2SO4 for SDC, S1, and S2. The voltammograms of one direct current and two pulse electrodeposited Pt catalysts exhibit the characteristic features of clean polycrystalline Pt.27 Two reversible hydrogen adsorption/desorption peaks are clearly identified in the potential region from +100 to -250 mV. The catalytic activity of the Pt catalyst is usually evaluated by its active surface area. The electrochemical surface area of a Pt catalyst is usually determined by the hydrogen underpotential deposition (H-UPD) method. The H+ adsorption charge density was evaluated by integrating the cathode current for hydrogen adsorption reaction with correcting for the double-

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12897 layer charging current (see the shaded region of the I-E curve in Figure 5).28 For a polycrystalline Pt electrode, the charge associated with the above reaction is 210 µC cm-2.29 The electrochemically active specific surface areas of SDC, S1, and S2, which are normalized to the geometric area of the carbon black/Nafion substrates, are 25.78, 34.82, and 80.69 cm2, respectively. The electrochemically active specific surface areas of S1 and S2 are 1.35 and 3.13 times as large as that of SDC, as expected from their different morphologies and the coverage of platinum shown in Figure 2. Figure 6 presents the cyclic voltammograms for methanol oxidation reaction obtained on SDC, S1, and S2. In Figure 6, the onset potentials of the methanol oxidation reaction on the three catalysts are the same but the order of the current density at the same potential is S2 > S1 > SDC. The current density of the positive sweep at 0.7 V (I0.7V) of S1DC, S1, and S2 is 68.47, 98.92, and 183.89 mA cm-2, respectively. The I0.7V of S1 and S2 is 1.45 and 2.69 times as much as that of SDC, respectively, and the I0.7V of S2 is 1.86 times larger than that of S1. The electrocatalytic properties of SDC, S1, and S2 for oxygen reduction reaction have been investigated by linear sweep voltammetry in O2-saturated 0.5 M H2SO4. The corresponding results are presented in Figure 7. The peak current density of SDC, S1, and S2 is 9.37, 14.06, and 20.72 mA cm-2, respectively. The peak current density of S1 and S2 is almost 1.5 and 2.2 times as much as that of SDC, respectively. The above results imply that sample S2 possesses the highest electrocatalytic activities for methanol oxidation reaction and oxygen reduction reaction among three samples. The increase in the electrocatalytic activities may mainly be attributed to higher active specific surface area of S2. Since CO species are the main poisoning intermediates, a good catalyst for methanol electro-oxidation should possess excellent CO electro-oxidizing ability. It is necessary to analyze the mechanism for the enhanced electro-catalyzing activity of Pt for methanol electro-oxidation (as shown in Figure 6). Such a result indicates that SnO2 can make CO oxidation easier under lower potential, which may be one reason for the increased methanol electro-oxidizing current in Figure 6. As the CO electro-oxidation can be promoted by SnO2, the mechanism of CO electro-oxidation can be tentatively deduced as

SnO2 + H2O f SnO2-OHads + H+ + e

(1)

Pt-COads + SnO2-OHads f Pt + SnO2 + CO2H+ + e (2) It can be thought that only when Pt nanoparticles have contacts with SnO2 can the CO electro-oxidation be promoted. Figure 8 illustrates schematic diagram of structural designing of PtSnO2/C and the total process. As shown in Figure 8, there are some Pt particles that have no contact with SnO2. Even so, the methanol electro-oxidation current is still greatly promoted with SnO2. It is well-known that Pt0 has high electrocatalytic

Figure 8. Schematic diagram of structural designing of PtSnO2/C and the process from electrodepostion to electro-oxide.

12898 J. Phys. Chem. C, Vol. 112, No. 33, 2008 activity for the methanol oxidation, while metal oxide may serve as the source of oxygen that is required for surface CO removal, thus enhancing methanol oxidation kinetics.30-33 It might be a reason for the excellent catalytic activity toward methanol oxidation of the PtSnO2/C catalyst. 4. Conclusions Highly active PtSnO2/C catalysts were synthesized by pulse electrodeposition method using SnO2 as an assisting agent. SEM and XRD results showed that particles of the PtSnO2/C catalysts were uniformly dispersed on the support and had similar morphologies with more and finer clusters forming. It might be dispersion of SnO2 and underpotential deposition at electrodeposited platinum that lead to the change in morphology and crystal size of the Pt catalysts. It may open a way for the utilization of low-electron-conductivity materials in electrocatalysis. Meanwhile, it may provide access to using transition metal oxides with various morphology and structures in catalyst layer. Using SnO2 reagent can notably increase the active surface areas, and then enhance electrocatalytic activity of Pt catalysts for both methanol oxidation and oxygen reduction. Such an electrode with the combined properties has potential applications for fuel cells. The approach also provides a route to fabricate electrode based on depositing noble metal nanoparticles onto the metal oxides and carbon black grown on the fuel cell backings. Acknowledgment. This work was supported by National Nature Science Foundation of China (Grant No. 50528404, 90510001) and the Grant 863 Project of China (2006AA03Z224, 2007AA05Z150). References and Notes (1) Jung, D. H.; Lee, C. H.; Kim, C. S.; Shin, D. R. J. Power Sources 1998, 71, 169. (2) Ye, Q.; Zhao, T. S.; Yang, H.; Prabhuram, J. Electrochem. SolidState Lett. 2005, 8, 52. (3) Mcnicol, B.; Rand, D.; Williams, K. J. Power Sources 1999, 83, 15. (4) Dillon, R.; Srinivasan, S.; Arico, A. S.; Antonucci, V. J. Power Sources 2004, 127, 112. (5) Ren, X. M.; Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1996, 143, L12. (6) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169.

Ye et al. (7) Vigier, F.; Coutanceau, C.; Perrard, A.; Belgsir, E. M.; Lamy, C. J. Appl. Electrochem. 2004, 34, 439. (8) Jusys, Z.; Kaiser, J.; Behm, R. J. Electrochim. Acta 2002, 47, 3693. (9) Cao, D. X.; Bergens, S. H. J. Power Sources 2004, 134, 170. (10) Roth, C.; Benker, N.; Buhrmester, T.; Mazurek, M.; Loster, M.; Fuess, H.; Koningsberger, D. C.; Ramaker, D. E. J. Am. Chem. Soc. 2005, 127, 14607. (11) Colmati, F.; Antolini, E.; Gonalez, E. R. J. Electrochem. Soc. 2007, 154, B39. (12) Mukerjee, S.; Urian, R. C.; Lee, S. J.; Ticianelli, J.; McBreen, E. A. J. Electrochem. Soc. 2004, 151, A1094. (13) Lebedeva, N. P.; Janssen, G. J. M. Electrochim. Acta 2005, 51, 29. (14) Oliveira, M. B.; Profeti, L. R.; Olivi, P. Electrochem. Commun. 2005, 7, 703. (15) Jayaraman, S.; Jaramillo, T.; Baeck, S. H.; McFarland, E. J. Phys. Chem. B 2005, 109, 22958. (16) Wang, H. J.; Yu, H.; Peng, F.; Lv, P. Electrochem. Commun. 2006, 8, 499. (17) Iwasita, T.; Nart, F. C. J. Electroanal. Chem. Interfacial Electrochem. 1991, 308, 277. (18) Jarvi, T. D.; Sriramulu, S.; Stuve, E. M. J. Phys. Chem. B 1997, 101, 3649. (19) Wu, G.; Chen, Y. S.; Xu, B. Q. Electrochem. Commun. 2005, 7, 1237. (20) Jiang, L. H.; Sun, G. Q.; Zhou, Z. H.; Sun, S. G.; Wang, Q.; Yan, S. Y.; Li, H. Q.; Tian, J.; Guo, J. S.; Zhou, B.; Xin, Q. J. Phys. Chem. B 2005, 109, 8774. (21) Wang, J. S.; Yu, S. S.; Xi, J. Y.; Chen, L. Q.; Zhu, W. T.; Qiu, X. P. Electrochem. Solid-State Lett. 2007, 10, B114. (22) Cheng, B.; Russell, J. M.; Shi, W.; Zhang, L.; Samulski, E. T. J. Am. Chem. Soc. 2004, 126, 5972. (23) Jiang, L.; Sun, G.; Sun, S.; Liu, J.; Tang, S.; Li, H.; Zhou, B.; Xin, Q. Electrochim. Acta 2005, 50, 5384. (24) Choi, K. H.; Kim, H. S.; Lee, T. H. J. Power Sources 1998, 75, 230. (25) Kim, H. S.; Subramanian, N. P.; Popov, B. N. J. Power Sources 2004, 138, 14. (26) Enea, O.; Riedo, B; Dietler, G. Nano Lett. 2002, 2, 241. (27) Gollas, B.; Elliott, J. M.; Bartlett, P. N. Electrochim. Acta 2000, 45, 3711. (28) Qiao, H.; Kunimatsu, M.; Okada, T. J. Power Sources 2005, 139, 30. (29) Wang, J.; Swain, G. M. J. Electrochem. Soc. 2003, 150, E24. (30) Zhang, X.; Chan, K. Y. Chem. Mater. 2003, 15, 451. (31) Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E. S.; Sung, Y. E.; Kim, H.; Thomas, S.; Wieckowski, A. J. Phys. Chem. B 2000, 104, 3518. (32) Anderson, A. B.; Grantscharova, E.; Seong, S. J. Electrochem. Soc. 1996, 143, 2075. (33) Iwasita, T.; Hoster, H.; Anacker, A. J.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522.

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