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Silicotungstic acid stabilized_pt-ru_nanoparticles_supported_on_carbon_nanofibers_electrodes_for_methanol_oxidation
 

Silicotungstic acid stabilized_pt-ru_nanoparticles_supported_on_carbon_nanofibers_electrodes_for_methanol_oxidation

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    Silicotungstic acid stabilized_pt-ru_nanoparticles_supported_on_carbon_nanofibers_electrodes_for_methanol_oxidation Silicotungstic acid stabilized_pt-ru_nanoparticles_supported_on_carbon_nanofibers_electrodes_for_methanol_oxidation Document Transcript

    • Silicotungstic acid stabilized Pt–Ru nanoparticles supported on carbon nanofibers electrodes for methanol oxidation T. Maiyalagan*,1 Department of Chemistry, School of Science and Humanities, VIT University, Vellore 632014, India a r t i c l e i n f o Article history: Received 14 November 2008 Received in revised form 19 January 2009 Accepted 22 January 2009 Available online 25 February 2009 Keywords: Polyoxometalates Pt–Ru Carbon nanofibers Electrocatalyst Methanol oxidation Fuel cells a b s t r a c t Silicotungstic acid stabilized Pt–Ru nanoparticles supported on Functionalized Carbon Nanofibers have been prepared by a microwave-assisted polyol process. The samples were characterized by XRD, SEM and TEM analysis. The electro-catalytic activities of the prepared composites (20% Pt–Ru/STA–CNF) were examined by using Cyclic Voltammetry (CV) for oxidation of methanol. The electro-catalytic activity of the carbon nanofiber based composite (20% Pt–Ru/STA–CNF) electrode for methanol oxidation showed better perfor- mance than that of commercially available Johnson Mathey 20% Pt–Ru/C and 20% Pt–Ru/ STA–C catalyst. The results imply that carbon nanofiber based STA composite electrodes are excellent potential candidates for application in direct methanol fuel cells. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Fuel cells operated with the electrochemical oxidation of hydrogen or methanol, as fuels at the anode and the reduction of oxygen at the cathode are attractive power sources due to their high conversion efficiencies, low pollution, light weight and high power density. While methanol offers the advantage of easy storage and transportation when compared to hydrogen oxygen fuel cell, its energy density (w2000 Wh/kg) and operating cell voltage (0.4 V) are lower than the theoretical energy density (w6000 Wh/kg) and the thermodynamic potential (w1.2 V) [1,2]. However, the fuel cells could not reach the stage of commercialization due to the high cost which was mainly associated with the noble metal loaded electrodes as well as the membrane. In order to reduce the amount of Pt loading on the electrodes, there have been considerable efforts to increase the dispersion of the metal on the support. The catalyst is very often found to disperse on a conventional carbon support and the support material influences the catalytic activity through metal support interaction [3–6]. The New novel carbon support materials such as graphite nanofibers (GNF) [7,8], carbon nanotubes (CNT) [9–13], carbon nanohorns [14] and carbon nanocoils [15] provide alternate candidates of carbon support for fuel cell applications. GNFs supported Pt–Ru alloy electrocatalysts showed better activity for methanol oxidation. The high electronic conductivity of GNF and the specific crystallographic orientation of the metal particles resulting from well-ordered GNF support are believed to be the factors for the observed enhanced electro- catalytic activity. In heterogeneous catalysis, one of the * Tel.: þ91 0416 2202465; fax: þ91 0416 2243092. E-mail address: maiyalagan@gmail.com 1 Present address: Fuel cells & Electrochemical Research Group, School of Chemical Engineering & Advanced Materials, University of Newcastle upon Tyne, NE1 7RU, United Kingdom. Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.069 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 8 7 4 – 2 8 7 9
    • important tasks is the determination of the number of active sites in the catalyst. For a given catalyst, the number of active sites present is responsible for the observed catalytic activity. A considerable amount of research has been devoted towards understanding the number of active sites as well as the role played by the carrier of the supported catalysts. The most efficient utilization of any supported catalyst depends on the percentage of exposed or the dispersion of the active component on the surface of the carrier material. Among the various factors that influence the dispersion of an active component, the nature of the support and the extent of the active component loading are of considerable importance. Without surface modification, most of the carbon nano- materials lack sufficient binding sites for anchoring precursor metal ions or metal nanoparticles, which usually lead to poor dispersion and the aggregation of metal nanoparticles, espe- cially at high loading conditions. To introduce more binding sites and surface anchoring groups, an acid oxidation process was very frequently adopted to treat carbon-nanomaterials in a mixed acid aqueous solution which introduces surface bound polar hydroxyl and carboxylic acid groups for subse- quent anchoring and reductive conversion of precursor metal ions to metal nanoparticles [16]. Heteropoly acids (HPAs) have attracted attention as promoters in the electrochemical oxidation of methanol [17– 25]. Heteropoly acids have also been involved in a variety of fuel cell applications such as hydrogen evolution, oxygen reduction and membrane fabrication [26–31]. Heteropoly acids are crystalline materials with very high proton conduction. The study of Silicotungstic acid (STA) is of interest due to its special properties which allow their use as catalyst [32–35]. High proton conducting ability makes the STA an attractive species for fuel cell applications [36–38]. Kim et al have reported methanol oxidation on carbon- supported Pt–Sn electrode in STA at various concentrations [39]. HPAs can act as redox mediators for the electrochemical oxidation of CO [40]. Heteropoly acids supported on carbon nanofiber with high surface area are also a useful method for improving catalytic performance. The present study aims at a synthesis, characterization and evaluation of the catalytic activity of Silicotungstic acid, Pt–Ru nanoparticles supported on carbon and carbon nano- fibers towards electro oxidation of methanol and a compar- ison with the activity of conventional 20% Pt–Ru/C electrodes is reported. These materials are characterized and studied, using XRD, SEM, TEM and cyclic voltammetry. The electro- chemical properties of the STA electrode (Pt–Ru–STA/C, Fig. 1 – Schematic diagram illustrating synthesis of electrocatalysts. 10 20 30 40 50 60 70 80 90 Pt(311) (c) (b) (a) (a) 20% Pt-Ru/C (b) 20% Pt-Ru-STA/C (c) 20% Pt-Ru-STA/CNF Pt(220) Pt(200) Pt(111) C(002) 2θ (degrees) Intensity(a.u) Fig. 2 – XRD Spectra of (a) Pt–Ru/C (b) Pt–Ru–STA/C and (c) Pt–Ru–STA/CNF. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 8 7 4 – 2 8 7 9 2875
    • Pt–Ru–STA/CNF) were compared to those of the commercial electrode (Pt–Ru/C), using cyclic voltammetry. The 20% Pt–Ru– STA/CNF exhibited excellent catalytic activity and stability when compared to the 20 wt% Pt–Ru/C. 2. Experimental 2.1. Materials All the chemicals were used in analytical grade. Carbon nanofibers (grade PR 24 LHT) were commercially available from Pyrograph Inc., USA. Silicotungstic acid (H4SiO4$12WO3$xH2O), Hexachloroplatinic acid and ruthe- nium (III) chloride hydrate were procured from Sigma–Aldrich and used as received. Vulcan XC-72 carbon black was purchased from Cabot Inc., Methanol and sulphuric acid were obtained from Fischer chemicals. Nafion 5 wt% solution was obtained from Dupont and was used as received. 2.2. Functionalization of CNF Carbon nanofibers (CNF) were obtained from Pyrograph limited, USA. The as-received CNF was treated with mixed acid aqueous solution of HNO3 and H2SO4 in 1:3 ratios under a magnetic stirrer for 3 h. It was then washed with water and evaporated to dryness. 2.3. Preparation of Pt–Ru-STA supported on carbon (Pt– Ru-STA/C) and Carbon Nanofiber (Pt–Ru–STA/CNF) Pt–Ru–STA/CNF catalysts were prepared by microwave heat- ing of an ethylene glycol (EG) solution of STA, H2PtCl6$6H2O, and RuCl3$xH2O with CNF suspended in the solution. Specific amounts of hexachloroplatinic acid (10 mM), Ruthenium (III) chloride hydrate (10 mM) and STA (10 mM) were loaded on carbon nanofiber to get 20 wt% metal loading by conventional impregnation method. The composition of Pt, Ru and STA was fixed to produce a final atomic ratio of 1:1:1 in the preparation of all Pt–Ru based composite catalysts. 0.4 M KOH (in EG solution) was added drop by drop to adjust the pH of the solution to about 10 to induce the formation of small and uniform metal nanoparticles. The suspensions were then placed in the center of the microwave oven (Sharp NN-S327 WF, 2450 MHz, and 1100 W) and heated for 50 s. The resultant suspension was filtered; and the residue was washed with acetone and dried over night at 25  C in a vacuum oven. A schematic of the detailed procedure for the electrocatalyst preparation has been shown in Fig. 1. 2.4. Characterization The phases and lattice parameters of the catalyst were char- acterized by X-ray diffraction (XRD) patterns employing Shi- madzu XD-D1 diffractometer using Cu Ka radiation (l ¼ 1.5418 A˚ ) operating at 40 kV and 48 mA. XRD samples were obtained by depositing carbon-supported nanoparticles on a glass slide and drying the later in a vacuum overnight. The scanning electron micrographs were obtained using JEOL JSM- 840 model, working at 15 keV. For transmission electron microscopic studies, the composite dispersed in ethanol were placed on the copper grid and the images were obtained using JEOL JEM-3010 model, operating at 300 keV. 2.5. Electrochemical measurements All electrochemical measurements were performed using a BAS Epsilon potentiostat. A three electrode cell was used which consisted of the glassy carbon (0.07 cm2 ) as working electrode, Pt foil and Ag/AgCl (Saturated by KCl solution) electrodes as counter and reference electrodes respectively were used. All the electrochemical experiments were carried out at room temperature in 0.5 M H2SO4 electrolyte. The electrolyte solution was purged with high pure nitrogen for 30 min prior to a series of voltammetric experiments. 2.6. Preparation of the working electrode Glassy Carbon (GC) (Bas Electrode, 0.07 cm2 ) was polished to a mirror finish with 0.05 mm alumina suspensions before each experiment and served as an underlying substrate of the working electrode. In order to prepare the composite elec- trode, the catalysts were dispersed ultrasonically in water at a concentration of 1 mg mlÀ1 and 20 ml aliquot was transferred on to a polished glassy carbon substrate. After the evaporation Fig. 3 – (a,b) SEM images of CNF. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 8 7 4 – 2 8 7 92876
    • of water, the resulting thin catalyst film was covered with 5 wt% Nafion solution. Then the electrode was dried at 353 K and used as the working electrode. A solution of 1 M CH3OH in 0.5 M H2SO4 was used to study the methanol oxidation activity. 3. Results and discussion The crystal structures of all the catalysts were examined by XRD as shown in Fig. 2. All the XRD responses represent the diffraction peaks of (111), (200), (220), and (311) planes, typical character of a Pt in a face-centered cubic (FCC) phase. The diffraction peaks of the catalyst (Pt–Ru/C) were observed to be sharp with a high intensity indicating high crystallinity (Fig. 2a). On the contrary, very broad peaks with weak inten- sity were observed for STA containing composites (Pt–Ru/ STA-C, Pt–Ru/STA–CNF), indicating that they are not fully crystalline in nature (Fig. 2b & c) as observed for the commercial Pt–Ru/C (J. M) (Fig. 2a). No evidence of peaks related to Ru was found in these catalysts [41,42]. When compared with the diffraction peaks of Pt–Ru/C catalyst, it was observed that the peaks of the Pt–Ru/STA–C and Pt–Ru/ STA–CNF shifted slightly to higher angles. This implies that there is some interaction between Pt–Ru particles with STA. The scanning electron micrograph images of commercial carbon nanofibers are shown in Fig. 3. The diameter of the nanofibers was found to be in the range of 100–150 nm and the length was about 100 mm. Fig. 4 shows the images of Pt–Ru– STA/Vulcan and Pt–Ru–STA/CNF respectively. From this it is confirmed that particles are dispersed on the carbon nano- fibers. The TEM image of the prepared Pt–Ru–STA/CNF cata- lysts, shown in Fig. 5 reveals that the Pt–Ru nanoparticles are highly dispersed on the CNF supports, with a mean particle size of ca. 3.9 Æ 0.1 nm. The electro-catalytic activities for methanol oxidation of Pt–Ru–STA/CNF, Pt–Ru–STA/C and commercial Pt–Ru/C elec- trocatalysts were analyzed by cyclic voltammetry in an elec- trolyte of 0.5 M H2SO4 and 1 M CH3OH at 50 mV/s. Fig. 6. shows the cyclic voltammograms (CVs) of the synthesized Pt–Ru– STA/CNF, Pt–Ru–STA/C and commercial Pt–Ru/C catalysts in an electrolyte solution of 0.5 M H2SO4 and 1 M CH3OH. There are two irreversible current peaks during the electrooxidation of methanol that are typically attributed on the forward scan peak at around 0.8 V to methanol electrooxidation and on the backward peak at 0.6 V to the faradic oxidation reaction on the Pt of the residual intermediate species. Both CV curves reveal a similar shape and peak position, which is also a good agreement with previous reports for methanol CVs over sup- ported Pt catalysts. The activity for methanol oxidation follows the order: Pt–Ru/STA–CNF > Pt–Ru/STA–C > Pt–Ru/C (J.M). The experimental results highlight the better perfor- mance for methanol oxidation on Pt–Ru/STA–CNF electro- catalyst prepared by Polyol assisted microwave method over commercially available 20 wt % Pt–Ru/C (J.M) catalyst. The enhanced catalytic activity of Pt–Ru/STA–CNF catalyst is due to higher dispersion of Pt–Ru nanoparticles and better oxida- tion of CO intermediates during methanol oxidation. Similarly Fig. 7. shows the chronoamperometric studies of the synthe- sized sample which reveals the stability of the Pt–Ru/STA–CNF catalyst towards the methanol electrooxidation. The results are consistent with the view that STA is not only stabilizing Fig. 5 – TEM image of 20% Pt–Ru–STA/CNF catalyst. Fig. 4 – SEM images of (a) Pt–Ru–STA/C and (b) Pt–Ru–STA/ CNF. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 8 7 4 – 2 8 7 9 2877
    • the Pt–Ru nanoparticles but is also enhancing the methanol oxidation currents under Chronoamperometric conditions. This catalytic enhancement may be due to the synergistic effect between Pt–Ru and STA. Bearing in mind the activity of tungsten oxides, tungstate units may provide additional –OH groups capable of facilitating oxidation of passivating inter- mediates (COads) on Pt [43,44]. 4. Conclusions In summary, well dispersed Pt–Ru nanoparticles have been synthesized on STA-carbon nanofibers composite. The enhancements in activity and stability over Pt–Ru/STA–CNF catalyst have been attributed to (i) highly dispersed Pt–Ru nanoparticles on the CNF and (ii) enhanced methanol oxidation due to the presence of STA for facilitating the oxidative removal of poisoning species on Pt. These findings suggest that Pt–Ru/STA–CNF should be considered a good electrocatalyst material for direct methanol fuel cells. Acknowledgement The author wishes to thank K.C. Divya for the help in the experimental work reported in this paper. r e f e r e n c e s [1] McNicol BD, Rand DAJ, Williams KR. Fuel cells for road transportation purposes – Yes or no? J Power Sources 2001; 83:47–59. [2] Carrette L, Friedrich KA, Stimming U. Fuel cells: principles, types, fuels, and applications. Chem Phys Chem 2001;1:163–93. [3] Uchida M, Aoyama Y, Tanabe N, Yanagihara N, Eda N, Ohta A. Influences of both carbon supports and heat- treatment of supported catalyst on electrochemical oxidation of methanol. J Electrochem Soc 1995;142:2572–6. [4] Antolini E. Platinum-based ternary catalysts for low temperature fuel cells. Part I. Preparation methods and structural characteristics. Appl Catal B: Environ 2007;724:324–36. [5] Maiyalagan T, Viswnathan B, Varadaraju UV. Electro- oxidation of methanol on TiO2 nanotube supported platinum electrodes. J Nanosci Nanotechnol 2006;6:2067–71. [6] Maiyalagan T, Khan FN. Electrochemical oxidation of methanol on Pt/V2O5–C composite catalysts. Catal Commun 2009;10:433–6. [7] Bessel CA, Laubernds K, Rodriguez NM, Baker RTK. Graphite nanofibers as an electrode for fuel cell applications. J Phys Chem B 2001;105:1115–8. [8] Steigerwalt ES, Deluga GA, Lukehart CM. Pt–Ru/carbon fiber nanocomposites: synthesis, characterization, and performance as anode catalysts of direct methanol fuel cells. A search for exceptional performance. J Phys Chem B 2002;106:760–6. [9] Li W, Liang C, Zhou W, Qiu J, Zhou Z, Sun G, et al. Preparation and characterization of multiwalled carbon nanotube- supported platinum for cathode catalysts of direct methanol fuel cells. J Phys Chem B 2003;107:6292–9. [10] Kim C, Kim YJ, Kim YA, Yanagisawa T, Park KC, Endo M, et al. High performance of cup-stacked-type carbon nanotubes as a Pt–Ru catalyst support for fuel cell applications. J Appl Phys 2004;96:5903–5. [11] Maiyalagan T. Pt–Ru nanoparticles supported PAMAM dendrimer functionalized carbon nanofiber composite catalysts and their application to methanol oxidation. J Solid State Electrochem 2008. doi:10.1007/s10008-008-0730-0. [12] Maiyalagan T. Synthesis and electro-catalytic activity of methanol oxidation on nitrogen containing carbon nanotubes supported Pt electrodes. Appl Catal B: Environ 2008;89:286–95. [13] Maiyalagan T, Viswanathan B. Template synthesis and characterization of nitrogen containing carbon nanotubes. Mater Chem Phys 2005;93:291–5. [14] Maiyalagan T, Viswanathan B, Varadaraju UV. Nitrogen containing carbon nanotubes as supports for Pt – Alternate anodes for fuel cell applications. Electrochem Commun 2005; 7:905–12. [15] Hyeon T, Han S, Sung YE, Park K-W, Kim Y-W. High- performance direct methanol fuel cell electrodes using solid- phase-synthesized carbon nanocoils. Angew Chem Int Ed Engl 2003;42:4352–6. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.005 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 (c) (b) (a)(a) 20% Pt-Ru-STA/CNF (b) 20% Pt-Ru-STA/C (c) 20% Pt-Ru/C Current(A) Potential (V) Fig. 6 – Cyclic Voltammograms of (a) Pt–Ru–STA/CNF (b) Pt– Ru–STA/C and (c) Pt–Ru/C in electrolyte solution of 0.5 M H2SO4 with 1 M CH3OH at a sweep rate of 50 mV/S at room temperature. 0 500 1000 1500 2000 2500 3000 3500 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060 (c) (b) (a) (a) 20% Pt-Ru-STA/CNF (b) 20% Pt-Ru-STA/C (c) 20% Pt-Ru/C Current(A) Time (s) Fig. 7 – Chronoamperometry of (a) Pt–Ru–STA/CNF (b) Pt– Ru–STA/C and (c) Pt–Ru/C polarized at D0.6 V in 0.5 M H2SO4/1 M CH3OH. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 8 7 4 – 2 8 7 92878
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