Electrochemical Behaviors of PtRu/CNTs Catalysts Prepared by Pulse Potential Plating Methods

Transcription

1 Solid State Phenomena Vols (2007) pp Online: (2007) Trans Tech Publications, Switzerland doi: / Electrochemical Behaviors of PtRu/CNTs Catalysts Prepared by Pulse Potential Plating Methods Seok Kim 1, Yunhee Kwon 1, Yongju Jung 2 and Soo-Jin Park 1,3,a 1 Advanced Materials Division, Korea Research Institute of Chemical Technology, P. O. Box 107, Yuseong, Daejeon , Korea (South) 2 Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, P. O. Box 105, Yuseong, Daejeon , Korea (South) 3 Department of Chemistry, Inha Univ., 253, Nam-gu, Incheon , Korea (South) a Corresponding author: sjpark@inha.ac.kr Keywords: pulse potential plating method, Carbon nanotubes, pulse interval, plating time Abstract. In this work, PtRu/CNTs catalysts were prepared by pulse potential plating methods. The particle size and loading level of catalysts were measured by changing the plating time and pulse interval of pulse potential plating method. Electrochemical activities of PtRu/CNTs catalysts were measured by cyclic voltammetry (CV). PtRu/CNTs catalysts showed an increased the electrochemical activity of methanol oxidation up to 24min and a slightly decreased activity over 24min. The electrochemical activity was increased with increasing of the pulse interval. Consequently, it was found that optimal plating time was 24min and optimal pulse interval was 0.5s for electrochemical activity of PtRu/CNTs catalysts. Introduction Direct methanol fuel cells (DMFCs), which use methanol as the fuel, have many advantages over other fuel cell systems, because methanol has a high energy density and is a liquid at room temperature. The performance of DMFCs has markedly in the past 5 years. Despite of many efforts devoted to the DMFCs development, there still remain problems to be overcome in terms of efficiency and power density [1,2]. One of the reasons is the relatively slow kinetics of the methanol oxidation reaction at the anode, which lead to high over-potentials. Platinum has high activity for methanol oxidation and as used as anode electrocatalysts for many. However, Pt electrocatalyst will be poisoned by the immediate of methanol oxidation, such as CO. Since the 1970s, to promote methanol electrooxidation at platinum, modification of the catalyst surface has been made by the addition of a second metal to platinum [3]. The PtRu binary metallic catalyst is commonly accepted as the best eletrocatalyst for methanol oxidation. The fundamental mechanism studies for PtRu catalysts indicate that methanol is oxidized according to a bifuntional mechanism : surface-sited Pt atoms oxidatively dehydrogenate the chemisorbed methyl moiety in consecutive steps to CO 2 at DMFCs potentials, Pt adsorbed CO is removed via a oxygen-transfer step from eletrogenerated Ru-OH. Ru transfers oxygen more effectively than Pt due to its ability to oxidatively absorb water at less positive potentials. Generally, electrocatalysts with small particle size and high dispersion will result in high electrocatalytic activity [4,5]. The importance of the structure of the supporting materials for dispersion of the catalyst has been concerned. The supporting materials with high surface are essential to disperse catalyst particles and reduce the catalyst loading under the condition of keeping the high All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/09/15,12:19:42)

2 1040 Advances in Nanomaterials and Processing catalytic activity. Carbon nanotubes (CNTs), because of their interesting properties, such as nanometer size and high surface area, have been received an increasing attention in recent years for their application if catalyst support. PtRu catalysts were usually prepared by chemical reduction method. However, the impurities are easy to be involved from the bath solution, which may decrease the catalytic activity metal catalysts. Recently, the electrochemical deposition of metal catalysts has been received more and more attentions due to its advantages, such as high purity of deposits, simple produce for deposition, easy control of the loading mass [6,7]. In this work, PtRu/CNTs catalysts prepared by pulse potential plating method have been investigated. The particle size and loading level of catalysts were measured by changing the plating time and pulse interval of pulse potential plating method. Electrochemical activities of PtRu/CNTs catalysts were measured by cyclic voltammetry (CV). The crystallinity of the PtRu/CNTs catalysts was evaluated using X-ray diffraction (XRD). Catalyst morphology was investigated by transmission electron microscopy (TEM). Experimental Procedure Deposition and electrocatalytical properties of PtRu nanoparticles on CNTs have been investigated by an Autolab with PGSTAT 30 (Eco Chemie B.V.; Netherlands). A standard three-electrode cell was employed. The CNTs electrode with a definite area of 1.70cm 2 was used as the working electrode. A platinum wire as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. PtRu nanoparticles were electrodeposited on the CNTs electrode by pulse potential plating method from distilled water with ruthenium chloride and chloroplatinic acid. In the deposition solution, the concentration of Pt and Ru was kept to be constant (20mM). The CNTs electrode was cycled in the range of -0.3 V to -0.8 V at a sweep rate of 50mV s -1 in deposition solution for the plating time. PtRu/CNTs electrode was cycled by changing the plating time. After PtRu particle deposition, the electrochemical properties of the PtRu/CNTs electrode were investigated in 1.0 M CH 3 OH M H 2 SO 4 aqueous solutions by cyclic voltammertry. All experiments were carried out at room temperature (25 ). The crystallinity of the PtRu/CNTs catalysts was evaluated using X-ray diffraction (XRD) performed on a Rigaku D/MAX-ΙΙΙB X-Ray diffractometer using a CuKα source. The X-ray diffractograms were obtained for 2θ values varying between 20 and 85. The mean sizes of the particles were determined from the X-ray dffractograms, using a Scherrer equation. L = 0.9λ B2θ cosθ max (1) Where λ is the X-ray wavelength ( A for the CuKα radiation), B 2θ is the width of the diffraction peak at half-height and θ max is the angle at the peak maximum position. TEM analysis was performed on a FEI Tecnai G2 20 equipment. Catalyst sample were suspended in ethanol. Catalyst morphology and chemical composition were investigated by TEM. The loading mass of PtRu was determined by ICP-AES. ICP-AES analysis was performed on a Jobin Yvon Ultima-C Inductively Coupled Plasma-Atomic Emission Spectrometer.

3 Solid State Phenomena Vols Results and Discussion Size and loading level of PtRu/CNTs catalysts. The element analysis of the PtRu/CNTs catalysts have been investigated by XRD. Fig. 1. and Fig. 2. show the XRD patterns for the PtRu/CNTs catalysts prepared by changing the plating time and the pulse interval. The peaks at 2θ=40, 47, 68, 82 are associated with the (111), (200), (220), and (311), respectively. Another peak at 2θ=54 can also be seen and may be associated to the presence of metallic Ru. All the PtRu/CNT catalysts prepared the diffraction patterns similar to those of the Pt, except that 2θ values were shifted to slightly higher values. These peaks indicated that the Pt was present in the face-centered cubic (fcc) structure. The XRD result confirms obviously that Pt is co-electrodeposited on the surface of CNTs. The average size of the Pt particles was calculated from broadening of the (220) diffraction peak using a Scherrer equation. Intensity (a.u.) Pt (111) Pt (200) Pt (220) Pt (311) (d) (c) (b) (a) θ Fig. 1. XRD patterns of catalysts prepared by plating time of (a)6, (b)12, (c)24, and (d)36 min. Intensity (a.u.) Pt (111) Pt (200) Pt (220) Pt (311) (d) (c) (b) (a) θ Fig. 2. XRD patterns of catalysts prepared by pulse interval of (a)0.03 s, (b)0.06 s, (c)0.2 s, and (d)0.5 s The average particle sizes calculated from the XRD peak width were found to be consistent with those from the TEM results are given in Table 1. and Table 2. The average particle sizes were between 3 and 12 nm. Table. 1 Average size of PtRu/CNTs catalysts prepared by a different plating time. Plating Time (min) XRD (nm) TEM (nm) Table. 2 Average size of PtRu/CNTs catalysts prepared by a different pulse interval. Pulse interval (s) XRD(nm) TEM (nm) Deposition and Electrochemical properties of PtRu/CNTs catalysts. Electrochemical properties of PtRu/CNTs catalysts have been investigated by cyclic voltammetry in 1 M CH 3 OH M H 2 SO 4 aqueous solution. Fig. 3. and Fig. 4. show representative voltammograms of the prepared PtRu/CNTs catalysts, which present a similar behavior to that of polycrystalline Pt with well-defined hydrogen adsorption/desorption peaks and Pt oxidation/reduction regions. The voltammetric behavior depends on the Pt content. The electrochemical activity increase with the increase of the plating time, the maximum is found

4 1042 Advances in Nanomaterials and Processing at 24 min, and then the electrochemical activity decreases with the increase plating time. Also, electrochemical activity was increased with increasing of the pulse interval. Because of PtRu/CNTs catalysts showed an increased the Pt content of plating time up to 36 min. Also, Pt content was increased with increasing of the pulse interval. And small particles and high dispersion of catalysts may result in large valuable catalyst surface area and good electrocatalytic properties for methanol oxidation. Optimal plating time was 24min, but Pt content is better plating time of 36 min. because of 24 min catalysts average size is 2-3 nm, but 36 min catalysts average size is 3-4 nm. Additionally, 24 min catalysts were homogenously distributed in the supported catalysts. -1.5x x10-2 Plating Time 6min 12min 24min 36min -3.0x x10-2 Pulse Interval 0.03s 0.06s 0.2s 0.5s I (A) -5.0x10-3 I (A) -1.0x mv (Ag/AgCl) Fig. 3. Cyclic voltammograms of catalysts prepared a different plating time. 1.0x mv (Ag/AgCl) Fig. 4. Cyclic voltammograms of catalysts prepared For a different pules interval. Conclusions In this work, preparation and characterization of PtRu/CNT catalysts had been investigated. PtRu particles were electrodeposited on carbon nanotubes by pulse potential method from distilled water solution with ruthenium chloride and chloroplatinic acid. The particle size of PtRu catalyst was about 3-12 nm. The electrochemical activity increased with the increasing of the plating time, the maximum was found at 24 min, and then the electrochemical activity decreased with the increase plating time. Electrochemical activity was increased with increasing of the pulse interval. Consequently, it was found that optimal plating time was 24 min and optimal pulse interval was 0.5 s for a high electrochemical activity of PtRu/CNTs catalysts. References [1] X. Ren, P. Zenlenay, S. Thomas, J. Davey and S. Gottesfeld: J. Power sources Vol. 86 (2000), p. 111 [2] K. Miura: J. Int. Eng. Chem., Vol. 11 (6) (2005), P.797 [3] A. Lima, C. Coutanceau, J. M. Leger and C. Lamy: J. Appl. Electrochem. Vol. 31 (4)(2001), p. 379 [4] M. Gotz and H. Wendt: Electrochim. Acta Vol. 43 (24) (1998), p [5] S. Kim and S. J. Park: J. Power Source (2006) in press [6] H. Laborde, J. M. Leger and C. Lamy: J. Appl. Electrochem. Vol. 24 (1994), p [7] C. H. Lee, C. W. Lee, E. I. Kim and S. E. Bae: Int. J. Hydrog. Energy Vol. 27 (4) (2002), p. 445

5 Advances in Nanomaterials and Processing / Electrochemical Behaviors of PtRu/CNTs Catalysts Prepared by Pulse Potential Plating Methods / DOI References [4] M. Gotz and H. Wendt: Electrochim. Acta Vol. 43 (24) (1998), p doi: /s (98)