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    Nitrogen containing carbon nanotubes as supports for pt – alternate anodes for fuel cell applications Nitrogen containing carbon nanotubes as supports for pt – alternate anodes for fuel cell applications Document Transcript

    • Electrochemistry Communications 7 (2005) 905–912 www.elsevier.com/locate/elecom Nitrogen containing carbon nanotubes as supports for Pt – Alternate anodes for fuel cell applications T. Maiyalagan, B. Viswanathan *, U.V. Varadaraju Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India Received 7 June 2005; accepted 7 July 2005 Available online 8 August 2005Abstract Aligned nitrogen containing carbon nanotubes have been synthesized using Anodisc alumina membrane as template. Highly dis-persed platinum nanoparticles have been supported on the nitrogen containing carbon nanotubes. Nitrogen containing carbonnanotubes as platinum catalyst supports were characterized by electron microscopic technique and electrochemical analysis. TheEDX patterns show the presence of Pt and the micrograph of TEM shows that the Pt particles are uniformly distributed on thesurface of the nitrogen containing carbon nanotube with an average particle size of 3 nm. Cyclic voltammetry studies revealed ahigher catalytic activity of the nitrogen containing carbon nanotube supported Pt catalysts.Ó 2005 Elsevier B.V. All rights reserved.Keywords: Nitrogen containing carbon nanotubes; Template synthesis; Alumina template; Catalyst support; Methanol oxidation1. Introduction Pt based catalysts. High noble metal loadings on the electrode [4,5] and the use of perfluorosulfonic acid Since the last decade, fuel cells have been receiving an membranes significantly contribute to the cost of the de-increased attention due to the depletion of fossil fuels vices. An efficient way to decrease the loadings of pre-and rising environmental pollution. Fuel cells have been cious platinum metal catalysts and higher utilization ofdemonstrated as interesting and very promising alterna- Pt particles is by better dispersion of the desired metaltives to solve the problem of clean electric power gener- on the suitable support [6]. In general, small particle sizeation with high efficiency. Among the different types of and high dispersion of platinum on the support will re-fuel cells, direct methanol fuel cells (DMFCs) are excel- sult in high electrocatalytic activity. Carbon materialslent power sources for portable applications owing to its possess suitable properties for the design of electrodeshigh energy density, ease of handling liquid fuel, low in electrochemical devices. Carbon is an ideal materialoperating temperatures (60–100 °C) and quick start up for supporting nano-sized metallic particles in the elec-[1,2]. Furthermore, methanol fuel cell seems to be highly trode for fuel cell applications. No other material exceptpromising for large-scale commercialization in contrast carbon material has the essential properties of electronicto hydrogen-fed cells, especially in transportation [3]. conductivity, corrosion resistance, surface properties,The limitation of methanol fuel cell system is due to and the low cost required for the commercialization oflow catalytic activity of the electrodes, especially the an- fuel cells. In general, the conventional supports namelyodes and at present, there is no practical alternative to carbon black is used for the dispersion of Pt particles [7]. The appearance of novel carbon support materials, * Corresponding author. Tel.: +91 044 22574200; fax: +91 44 such as graphite nanofibers (GNFs) [8,9], carbon nano-22574202. tubes (CNTs) [10–17], carbon nanohorns [18], and car- E-mail address: bvnathan@iitm.ac.in (B. Viswanathan). bon nanocoils [19–22], provides new opportunities of1388-2481/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.elecom.2005.07.007
    • 906 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912carbon supports for fuel cell applications. Bessel et al. [8] alumina template by wetting method [28]. Afterand Steigerwalt et al. [9] used GNFs as supports for Pt complete solvent evaporation, the membrane was placedand Pt–Ru alloy electrocatalysts and observed better in a quartz tube (30 cm length, 3.0 cm diameter), kept inactivity for methanol oxidation. The high electronic a tubular furnace and carbonized at 1173 K under Arconductivity of GNFs and the specific crystallographic gas flow. After 3 h of carbonization, the quartz tubeorientation of the metal particles resulting from well- was cooled to room temperature. The resulting templateordered GNF support were believed to be the important with carbon–nitrogen composite was immersed in 48%factors for the observed enhanced electrocatalytic activ- HF at room temperature for 24 h to remove the aluminaity. The morphology and the nature of the functional template and the nitrogen containing CNTs weregroups of the support influence the activity of fuel cell obtained as an insoluble fraction. The nanotubes wereelectrocatalyts [23–26]. Carbon with sulphur or nitrogen then washed with distilled water to remove the residualbased functionality [25], can influence the activity of the HF and dried at 393 K.catalyst. The present report focuses on the efforts undertaken 2.3. Loading of Pt catalyst inside nanotubeto develop unconventional supports based platinumcatalysts for methanol oxidation. Nitrogen containing Platinum nanoclusters were loaded inside the N-CNTcarbon nanotubes were used to disperse the platinum as follows; the C/alumina composite obtained (beforeparticles effectively without sintering and to increase the dissolution of template membrane) was immersedthe catalytic activity for methanol oxidation. The tubu- in 73 mM H2PtCl6 (aq) for 12 h. After immersion, thelar morphology and the nitrogen functionality of the membrane was dried in air and the ions were reducedsupport have influence on the dispersion as well as to the corresponding metal(s) by a 3 h exposure to flow-the stability of the electrode. In this communication ing H2 gas at 823 K. The underlying alumina was thenthe preparation of highly dispersed platinum supported dissolved by immersing the composite in 48% HF foron nitrogen containing carbon nanotubes, the evalua- 24 h. This procedure resulted in the formation of Pttion of the activity for the methanol oxidation of these nanocluster loaded N-CNT and the complete removalelectrodes and comparison with the activity of conven- of fluorine and aluminum was confirmed by EDXtional electrodes are reported. analysis. 2.4. Preparation of working electrode2. Experimental Glassy carbon (GC) (Bas electrode, 0.07 cm2) was2.1. Materials polished to a mirror finish with 0.05 m alumina suspen- sions before each experiment and served as an underly- All the chemicals used were of analytical grade. Poly- ing substrate of the working electrode. In order tovinyl pyrrolidone (Sisco Research Laboratories, India), prepare the composite electrode, the nanotubes weredichloromethane and concentrated HF (both from dispersed ultrasonically in water at a concentration ofMerck) were used. Hexachloroplatinic acid was ob- 1 mg mlÀ1 and 20 ll aliquot was transferred on to atained from Aldrich. 20 wt% Pt/Vulcan carbons were polished glassy carbon substrate. After the evaporationprocured from E-TEK. Methanol and sulphuric acid of water, the resulting thin catalyst film was coveredwere obtained from Fischer chemicals. The alumina with 5 wt% Nafion solution. Then the electrode wastemplate membranes (Anodisc 47) with 200 nm diameter dried at 353 K and used as the working electrode.pores were obtained from Whatman Corp. Nafion5 wt% solution was obtained from Dupont and was used 2.5. Characterization methodsas received. The chemical composition of the nanotubes was2.2. Synthesis of nitrogen containing carbon nanotubes determined by elemental analysis using Hereaus CHN analyzer after the removal of alumina template. The Pyrolysis of nitrogen containing polymers is a facile scanning electron micrographs were obtained usingmethod for the preparation of carbon nanotube materi- JEOL JSM-840 model, working at 15 keV. The nano-als containing nitrogen substitution in the carbon frame- tubes were sonicated in acetone for 20 min and thenwork. Nitrogen containing carbon nanotubes were were dropped on the cleaned Si substrates. The AFMsynthesized by impregnating polyvinylpyrrolidone imaging was performed in air using the Nanoscope IIIA(PVP) inside the alumina membrane template and subse- atomic force microscope (Digital Instruments, St. Bar-quent carbonization of the polymer [27]. Polyvinylpyr- bara, CA) operated in contact mode. For transmissionrolidone (PVP – 5 g) was dissolved in dichloromethane electron microscopic studies, the nanotubes dispersed(20 ml) and impregnated directly in the pores of the in ethanol were placed on the copper grid and the
    • T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 907images were obtained using Phillips 420 model, operat- nanotubes is shown in Fig. 1(a). Fig. 1(b) shows lateraling at 120 keV. view of the nitrogen containing carbon nanotubes with low magnification and Fig. 1(c) shows lateral view of2.6. Electrochemical measurements the nitrogen containing carbon nanotubes with high magnification. The hollow structure and well alignment All electrochemical studies were carried out using a of the nitrogen containing carbon nanotubes have beenBAS 100 electrochemical analyzer. A conventional identified by SEM.three-electrode cell consisting of the GC (0.07 cm2) AFM images of the nitrogen containing carbon nano-working electrode, Pt plate (5 cm2) as counter electrode tubes deposited on a silicon substrate are shown inand Ag/AgCl reference electrode were used for the cyclic Fig. 2. The AFM tip was carefully scanned across thevoltammetry (CV) studies. The CV experiments were tube surface in a direction perpendicular to the tubeperformed using 1 M H2SO4 solution in the absence axis. From the AFM images, it is inferred that a partand presence of 1 M CH3OH at a scan rate of of the long nanotube is appearing to be cylindrical in50 mV sÀ1. All the solutions were prepared by using ul- shape and is found to be terminated by a symmetrictra pure water (Millipore, 18 MX). The electrolytes were hemispherical cap. Because of the finite size of thedegassed with nitrogen gas before the electrochemical AFM tip, convolution between the blunt AFM tip andmeasurements. the tube body will be giving rise to an apparently greater lateral dimension than the actual diameter of the tube [29].3. Results and discussion The TEM images are shown in Fig. 3(a)–(c). The open end of the tubes observed by TEM shows that Elemental analysis was conducted to examine the nanotubes are hollow and the outer diameter ofwhether nitrogen has really entered the carbon nanotube the nanotube closely match with the pore diameter offramework. It has been found that the samples prepared template used, with a diameter of 200 nm and a lengthcontained about 87.2% carbon and 6.6% nitrogen (w/w). of approx. 40–50 lm. It is evident from the micrographsThe SEM images of the nitrogen containing carbon that there is no amorphous material present in the nano-nanotubes support are shown in Fig. 1(a)–(c). Top view tube. Fig. 3(c) shows the TEM image of Pt nanoparticlesof the vertically aligned nitrogen containing carbon filled carbon nanotubes. TEM pictures reveal that the PtFig. 1. SEM images of the nitrogen containing carbon nanotubes: (a) the top view of the nanotubes; (b) side view of the vertically aligned nanotubesand (c) high magnification lateral view of the nanotubes.
    • 908 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 Fig. 2. AFM image of the nitrogen containing carbon nanotubes.Fig. 3. TEM images of the nitrogen containing carbon nanotubes: (a) at lower magnification; (b) at higher magnification image of the individualnanotube (an arrow indicating the open end of the tube) and (c) Pt filled nitrogen containing carbon nanotubes.
    • T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 909particles have been homogeneously dispersed on the the current increases quickly due to dehydrogenationnanotubes and particle sizes were found to be around of methanol followed by the oxidation of absorbed3 nm. The optimal Pt particle size for reactions in the methanol residues and reaches a maximum in the poten-H2/O2 fuel cell is 3 nm [30]. The importance of the Pt tial range between 0.8 and 1.0 V vs. Ag/AgCl. In theparticle size on the activity for methanol oxidation is cathodic scan, the re-oxidation of methanol is clearlydue to the structure sensitive nature of the reaction observed due to the reduction of oxide of platinum.and the fact that particles with different sizes will Electrocatalytic activity of methanol oxidation has beenhave different dominant crystal planes and hence the dif- found to be strongly influenced by the metal dispersion.ferent intercrystallite distances, which might influence Pure Pt electrode shows an activity of 0.167 mA cmÀ2.methanol adsorption. The commercial Pt/C has a very The Pt/N-CNT shows a higher activity of 13.3 mA cmÀ2high specific surface area but contributed mostly by where as conventional 20% Pt/Vulcan (E-TEK)micropores less than 1 nm and are therefore more diffi- electrode shows less activity of 1.3 mA cmÀ2 comparedcult to be fully accessible. It has been reported that the to nitrogen containing carbon nanotube supportedmean value of particle size for 20% Pt/Vulcan electrode. The nitrogen containing carbon nanotube(E-TEK) catalyst was 2.6 nm [31]. The EDX pattern of supported electrodes shows a ten fold increase in thethe as synthesized catalyst shows the presence of Pt catalytic activity compared to the E-TEK electrode.particles in the carbon nanotubes and also the complete The Pt/N-CNT electrode showed higher electrocatalyticremoval of fluorine and aluminum has also been activity for methanol oxidation than commercialconfirmed in Fig. 4. It has been reported that the Pt/Vulcan (E-TEK) electrode. The anodic currentelectronic and physical structures of a Pt particle density of Pt/N-CNT electrode is found to be higherdeposited on carbon differ from those of the bulk Pt. than that of Pt/Vulcan (E-TEK) electrode, which indi-The electronic change in Pt/C is considered as a result cates that the catalyst prepared with nitrogen containingof functional groups of the carbon support that might carbon nanotubes as the support has excellent catalyticinfluence the electronic structure of Pt particulate activity on methanol electrooxidation.[32–36]. The nitrogen functional group on the carbon The onset potential and the forward-scan peak currentnanotubes surface intensifies the electron withdrawing density for the different electrodes are given in Table 1.effect against Pt and the decreased electron density of The onset potential of methanol oxidation at nitrogenplatinum facilitate oxidation of methanol. containing carbon nanotube supported catalysts occurs Fig. 5(a)–(c) shows the cyclic voltammogram of at 0.22 V, which is relatively more negative to that ofmethanol oxidation. Fig. 5(c) shows the cyclic voltam- the other catalysts. This may be attributed to the high dis-mogram of Pt/N-CNT electrode in 1 M H2SO4/1 M persion of platinum catalysts and the nitrogen functionalCH3OH run at a scan rate of 50 mV sÀ1. The electrocat- groups on its surface. The higher electrocatalytic activityalytic activity of methanol oxidation at the Pt/N-CNT of the nitrogen containing carbon nanotube supportedelectrodes was evaluated and compared with that of electrode is due to higher dispersion and a good interac-the conventional electrodes. During the anodic scan, tion between the support and the Pt particles The Vulcan Fig. 4. EDX pattern of Pt/N-CNT electrode.
    • 910 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 Fig. 5. Cyclic voltammograms of (a) pure Pt; (b) Pt/Vulcan (E-TEK) and (c) Pt/N-CNT in 1 M H2SO4/1 M CH3OH run at 50 mV sÀ1.Table 1 also been reported [38]. All these results indicate thatElectrochemical parameters for methanol oxidation on the various the nitrogen functionality on CNT influences theelectrodes catalytic activity of the catalyst. The enhanced electrocat-Electrocatalyst Methanol oxidation Forward peak current alytic effect of the nitrogen containing carbon nanotube onset potential (V) density (mA cmÀ2) vs. Ag/AgCl supported electrodes could also be partly due to the following factors which require further investigation:Bulk Pt 0.4 0.16720% Pt/C (E-TEK) 0.45 1.3 (1) higher dispersion on the nitrogen containing carbonPt/N-CNT 0.22 13.3 nanotube support increases the availability of an enhanced electrochemically active surface area, (2) appearance of the specific active sites at the metal– support boundary and, (3) strong metal–supportcarbon support has randomly distributed pores of vary- interaction.ing sizes which may make fuel and product diffusion Long-term stability is important for practical applica-difficult whereas the tubular three-dimensional morphol- tions. Fig. 6 shows the current density–time plots of var-ogy of the nitrogen containing carbon nanotubes makes ious electrodes in 1 M H2SO4 and 1 M CH3OH at 0.6 V.the fuel diffusion easier. The Vulcan carbon contains high The performance of Pt electrodes was found to be poorlevels of sulfur (ca. 5000 ppm or greater), which could compared to the E-TEK and Pt/N-CNT electrode. Thepotentially poison the fuel-cell electrocatalysts [37]. nitrogen containing carbon nanotube electrodes are theNitrogen containing carbon nanotubes used in this study most stable for direct methanol oxidation. The increas-contains heterocyclic nitrogen so that it preferentially at- ing order of stability of various electrodes is; Pt < Pt/taches the Pt particles. The selective attachment of Au Vulcan (E-TEK) < Pt/N-CNT. We are currently investi-nanoparticles on nitrogen doped carbon nanotubes has gating whether nitrogen has a catalytic role that contrib-
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