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Nontrivial role of carbon nanofibers morphology in binderless Pt nanocatalyst supported electrode
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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
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journal homepage: www.elsevier.com/locate/he
Nontrivial role of carbon nanofibers morphology in binderless Pt nanocatalyst supported electrode Ze´hira Hamoudi, My Ali El Khakani, Mohamed Mohamedi* Institut National de la Recherche Scientifique (INRS), E´nergie, Mate´riaux et Te´le´communications (EMT), 1650 Boulevard Lionel Boulet, Varennes, Que´bec, J3X 1S2 Canada
article info
abstract
Article history:
Similar to conventional composite electrodes, developing binderless-based carbon nano-
Received 5 December 2010
structured (CNs) electrodes for fuel cells requires particularly the optimisation of both the
Received in revised form
morphology and the density of the CNs. In this work, carbon nanofibers (CNFs) have been
9 January 2011
optimised and used as catalyst support for Pt nanoparticles (NPs). The nontrivial role of the
Accepted 18 January 2011
CNFs on the catalytic behavior is clearly demonstrated. We have shown that for a similar
Available online 2 March 2011
amount, morphology and dispersion of the Pt NPs fabricated onto CNFs, the density of the latter and to a lesser extent their diameter are the main factors influencing the catalytic
Keywords:
activity. For the particular case of CNFs considered in this work, an optimum activity
Carbon nanofibers
toward methanol fuel cell reaction was obtained when Pt NPs were supported with CNFs
Catalyst support
synthesized with a C2H2/Ar ratio of 0.31.
Platinum nanoparticles
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Methanol electrooxidation
1.
Introduction
As known typically a conventional composite electrode in low temperature fuel cell is made up of a first macroporous layer, which is a carbon cloth or paper. This is the conductive support onto which the microporous gas diffusion layer (gdl) and thereafter the catalytic layer are deposited. In most electrode configurations, the gdl is formed by Polytetrafluoroetylene (PTFE) and carbon black (catalyst support); whereas, the composite catalytic layer consists of carbon supported Pt catalyst and Nafion ionomer [1,2]. The optimal amount of each component of this composite electrode has been found after years of design of experiments and trials. In addition, when in practical electrodes, the carbon supports and the catalysts particles are used in powder form and their processing to promote film formation (i.e. the ink-process) for functional electrodes requires the use of binders to provide mechanical integrity for the catalyst layer.
reserved.
The use of such conventional electrode fabrication is practically non-feasible in case of micro-sized (<1 cm2 active area) fuel cells. It is therefore crucial to develop fabrication techniques which enable to lay down films of catalyst support and catalyst nanoparticles onto electrically conductive substrate (which acts directly as the current collector) without the use of binder materials. One possible and promising approach is the preparation of free-standing (binderless) electrodes fabrication technique. This approach for fuel cell applications is still at its infancy but it certainly has the potential to be considerably more powerful than those currently in use and greatly simplify fabrication of electrode assemblies, the heart of fuel cell devices. It is now agreed that the structure and composition of Pt nanoparticles (NPs) are very important because they have great effect on its catalytic activity. In the form of well-dispersed Pt NPs with dimensions of 2e5 nm seem to represent the best design for the ultimate performance [3e5]. A given carbon
* Corresponding author. E-mail address: [email protected] (M. Mohamedi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.085
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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
catalyst support would mainly provide a mechanical support and an electronic continuity as well as a uniform dispersion of Pt NPs. However, when it comes to developing direct onsubstrate binderless electrodes, the influence of the morphology and density of a particular catalyst support must be investigated for the proper design of high performing electrode structures. Carbon nanofibers (CNFs) possess good electronic conductivity, corrosion resistant properties and are cheap to produce which make them as the best alternative to carbon nanotubes as catalyst support in the short-term particularly for Direct Alcohol Fuel Cells (DAFCs) commercialization for portable applications [6e10]. The present work intends to gain knowledge onto the influence of the morphology and density of CNFs catalyst support on the binderless electrode activity. To avoid the complex interaction among the Pt NPs, the CNFs are synthesized under different conditions but are loaded with
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a similar amount (0.18 mg cm2) and dimensions of Pt NPs. The optimum morphology of CNFs as catalyst support for Pt NPs is investigated with respect to the electrooxidation of methanol, an electrochemical reaction of technological importance to direct methanol fuel cells (DMFCs) [11].
2.
Material and methods
2.1.
Material synthesis
For the CNFs growth, a nickel catalyst layer of 5 nm thickness was deposited by pulsed laser deposition (PLD) technique on one side of an electrically conductive substrate that is made of 3D porous network of carbon microfibers (CMFs) (Toray). This nickel layer was deposited by ablating under vacuum, a pure (99.95%) polycrystalline nickel target by means of a pulsed KrF
Fig. 1 e Electron microscopy analyses: SEM images of (a) CNF0.125, (b) CNF0.31 and (c) CNF0.5 where (a0 , a00 ), (b0 , b00 ) and (c0 , c00 ) are their respective TEM and HR-TEM images.
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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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
Table 1 e Dimensions of carbon nanofibers their structural properties assessed by MicroRaman spectroscopy. Sample Diameter nm1 D band cm1 G band cm1 ID/IG CNF0.125 CNF0.31 CNF0.5
110 256 318
1356.4 1343.6 1337
1580 1592.8 1599
0.90 0.84 0.88
excimer laser (wavelength ¼ 248 nm, pulse duration z 14 ns, repetition rate of 20 Hz) with a fluence of 5 J/cm2. In order to obtain a uniform ablation over the target surface, the target was continuously rotated and translated. The CMFs substrate was placed at 5 cm from the target and the deposition was performed at room temperature. Afterward, the CNFs were grown by chemical vapor deposition (CVD) technique. First, the Ni-coated-CMFs substrate was placed in a furnace and heated to 700 C with a 5 C/min rate under an 80 sccm argon gas flow. When the temperature reached 700 C, the samples were kept under the same heating conditions for 1 h in order to break the Ni film into spherical Ni NPs covering uniformly the CMFs substrate. Subsequent to the Ni NPs formation, the reactive gas (C2H2 in the present study) was introduced simultaneously with argon in various ratios for the CNFs synthesis. After synthesis, the acetylene flow was cut off and the furnace was cooled down to room temperature under flowing argon. In this work, for CNFs optimisation three C2H2/ Ar ratios were considered, i.e., 0.125, 0.31 and 0.5 and will be denoted CNF0.125, CNF0.31 and CNF0.5, respectively. Pt NPs onto CNFs were deposited at room temperature by PLD by ablating a polycrystalline Pt target (99.99% purity, Kurt J. Lesker Co.) by means of a pulsed KrF excimer laser (l ¼ 248 nm, pulse width ¼ 15 ns, and repetition rate of 10 Hz) under 500 mTorr background pressure of He.
2.2.
Fig. 2 e MicroRaman spectra of CNFs: (a) CNF0.125, (b) CNF0.31 and (c) CNF0.5. Inset is the MicroRaman spectrum of the bare CMF substrate.
focused onto the sample to a spot size of 1 mm (MicroRaman spectroscopy, Renishaw Imaging Microscope WireTM).
2.3.
Electrochemical studies
In order to assess the electronic transfer properties of the CNFs as well as their electrical contact to the CMFs substrate, we employed cyclic voltammetry (CV) in a benchmark
Material characterization
The surface morphology of the as-prepared samples was examined by means of a scanning electron microscope (SEM, JEOL, JSM 6300F apparatus) operated at an accelerating voltage of 15 kV and a transmission electron microscopy (TEM, JEOL JEM-2100F) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was conducted for elemental composition of the material’s surface and its chemical environment. The XPS study was performed with a VG Instruments Escalab 220i-XL surface microanalysis system equipped with hemispherical analyzer and Al Ka X-ray source (1486.6 eV). Survey scans in the range 0e1000 eV were recorded at 100 eV pass energy with a step size of 1 eV. Core level spectra were obtained for C1s and recorded at 20 eV pass energy with a step size of 0.1 eV. Curve fitting of the XPS data was carried out with casaXPS version 2.2.107. All spectra have been recalibrated with respect to the C1s core level peak of adventitious carbon contamination. The metallic components of the Pt4f region were fitted using a Gaussian/Lorentzian asymmetrically modified line shape. The graphitic structure of the CNFs was assessed with MicroRaman by using the 514.5 nm (2.41 eV) laser radiation of an Arþ laser with a circular polarization. The laser beam was
Fig. 3 e Electron transfer properties with cyclic voltammetry in a 1.0 mM K4Fe(CN)6 D 1.0 M KCl solution at (dotted) CNF0.125, (full) CNF0.31 and (dashed) CNF0.5. The scan rate was 5 mV/s.
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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
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Fig. 4 e Electron microscopy analyses: TEM images of (a) Pt/CNF0.125, (b) Pt/CNF0.31 and (c) Pt/CNF0.5. (d) Typical SAED patterns of Pt NPs/CNF. (e) Typical Pt NPs size distribution onto CNFs.
solution consisting of 1.0 mM K4Fe(CN)6 and 1.0 M KCl solution, which usually serves as a benchmark for investigating electrochemistry of various carbon structures [12e14]. Fuel cell reactions were studied in 0.5 M H2SO4 and in a mixture of 1 M CH3OH þ 0.5 M H2SO4 deaerated solutions. All electrochemical measurements were conducted at room temperature using a two compartments electrochemical cell with the reference electrode and counter electrode being an Ag/AgCl and a platinum coil, respectively. The reference electrode was separated from the analyte solution by a Luggin capillary that is very close to the working electrode to minimze the IR drop. Data acquisition was conducted with a potentiostat/galvanostat Autolab from EcoChemie.
3.
>20 mm, and within 10e15 mm, respectively. In addition, the C2H2/Ar ratio seems also to influence both the inner structure and the diameter of the CNFs as shown by TEM (Fig. 1a0 ec0 ) and HR-TEM (Fig. 1a00 ec00 ). The obtained inner structure is plain for
Results and discussion
Fig. 1aec shows SEM images of the CMFs substrate covered with CNF0.125, CNF0.31 and CNF0.5, respectively. In general, the CMFs substrate is uniformly coated with CNFs being mostoften wavy and somewhat entangled. In terms of density, it can be seen however that C2H2/Ar ratio had a marked effect, i.e., low C2H2/Ar ratio (0.125) led to low density of CNFs (Fig. 1a), whereas high C2H2/Ar ratio (0.5) yielded to protruding CNFs (small bright points in Fig. 1c and inset). It is only with C2H2/Ar ratio (0.31) that high density of CNFs was obtained (Fig. 1b). Average length of CNF0.125, CNF0.31 and CNF0.5, were >10 mm,
Fig. 5 e X-ray photoelectron spectra of Pt4f core level peaks of Pt NPs deposited onto CNFs. (a) Pt/CNF0.125, (b) Pt/CNF0.31, and (c) Pt/CNF0.5.
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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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
Fig. 6 e Cyclic voltammetry in a deaerated 0.5 M H2SO4 solution at Pt/CNF electrodes with 50 mV/s scan rate. (a) Pt/CNF0.125, (b) Pt/CNF0.31, and (c) Pt/CNF0.5. (d) Measured ECSA from the Had/Hdes regions in the CVs shown in aec.
CNF0.125, hollow of the herringbone and fishbone type for CNF0.31 and CNF0.5, respectively. It was also observed that CNFs diameter increased as the C2H2/Ar ratio increased (Table 1). It is beyond the scope of this paper to study by which mechanism the C2H2/Ar ratio affects the morphology and density of CNFs since our main interest is to investigate such obtained CNFs in electrochemical applications. It should be however noted that neither XPS nor the thermogravimetric analysis has revealed the presence of nickel on the surface of the carbon nanofibers or residue after total combustion, respectively. In addition, characterizing such structures with the underlying carbon microfibers substrate by TEM was very challenging. A great deal of efforts and time were taken to obtain fairly analyzable images of Fig. 1. From these images, no nickel is seen remaining at the interior of the carbon nanofibers or on their tips which were mostly open. It is likely then that the growth of carbon nanofibers follows a tip-growth mechanism during which the nickel is extruded out of the carbon nanofibers and driven by the gases toward the outlet of the CVD set-up. The MicroRaman spectra of the CNFs samples are shown in Fig. 2 altogether with the spectrum related to the bare CMFs substrate (inset). All the samples exhibit mainly two Raman bands, within ranges of 1330e1359 cm1 (D band) and 1580e1599 cm1 (G band). The D and G bands reflect the
defective (non-graphitic) structure and the graphitic structure (sp2), respectively. Generally, the ratio of the intensities of D to G band (ID/IG) can be used as an indicator of the quality of the carbon structure [15]. If both bands possess similar intensity, this indicates a higher quantity of structural defects. Table 1 resumes the characteristics of the D and G bands for the bare CMFs substrate and for CNFs synthesized under different conditions of C2H2/Ar ratios. Fairly similar ID/IG values were observed for all synthetised, indicating the presence of high quantity of structural defects. It has to be noted the bare CMFs substrate showed an ID/IG of w0.17. Second, shifts in the D and G bands are observed as function of the C2H2/Ar ratio. These shifts are mainly due to the differences between CNFs internal structure and the diameter difference as observed by SEM and TEM analyses in Fig. 1. This kind of nanofibers exposes numerous graphene edges. The presence of these edges has been reported to be beneficial for catalysis applications either catalysts or as supports [16]. Fig. 3 displays cyclic voltammograms (CVs) measurements of the CNFs in 1.0 mM K4Fe(CN)6 and 1.0 M KCl solution at potential scan rate of 5 mV/s. The [Fe(CN)4-]6/[Fe(CN)3-]6 redox pairs is highly resolved at the CNFs synthesized under different conditions of C2H2/Ar ratios with higher redox currents obtained at CNF0.31. Furthermore, the anodic peak to cathodic peak
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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
Fig. 7 e Cyclic voltammetry in a deaerated 1 M CH3OHD0.5 M H2SO4 solution at Pt/CNF electrodes with 50 mV/s scan rate. (b) Anodic peak current mass activity.
separation, DEp, measured in Fig. 3 is of 114, 80 and 102 mV for the CNF0.125, CNF0.31 and CNF0.5, respectively, which suggests a faster electron transfer at the CNF0.31 electrode [12e14], which can be attributed to high density of state in the latter [17]. Owing to the fact that CNFs exhibited enhanced electron transfer rates, they could offer higher ability to stabilize the high dispersion of catalyst particles for methanol oxidation reaction (MOR). To explore this avenue, Pt NPs onto CNFs have been deposited by PLD technique following the conditions reported in the experimental section. TEM images of Pt NPs deposited onto CNFs are shown in Fig. 4aec. As can be seen a fairly similar dispersion of Pt particles was achieved. The crystallographic orientation was further studied by recording the Selected Area Electron Diffraction (SAED) pattern (Fig. 4d). Besides the (002) planes of carbon, the crystal lattice fringes of Pt are revealed as the lattice planes of (111), (200) and (220) with particle size having a mean diameter of 2.25 nm 0.5 nm (Fig. 4e). High-resolution XPS Pt4f core level spectrum of Pt NPs deposited onto CNFs is reported in Fig. 5. The Pt4f core level spectrum displays two peaks whose maximum intensities are located between 70 and 78 eV binding energy range. The binding energy difference (DEbind) varies between 3.24 and 3.32 eV between these two maxima is that expected from Pt4f7/2 and Pt4f5/2 core level peaks. The position of these two peaks is consistent with the fact that Pt is in a metallic state [18].
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The characteristic of Pt NPs surface electrochemistry was studied by cyclic voltammetry. Fig. 6 compares resulting CVs at Pt NPs supported by CNFs in 0.5 M H2SO4 deaerated solution. All electrodes exhibited fairly defined hydrogen adsorption and desorption regions in the potential region of ca.-0.3e0.2 V. It is though observed that peaks characteristics of Had/Hdes are more resolved and their currents are highest at the Pt/CNF0.31 electrode, which illustrates the better utilisation of Pt when supported on CNF0.31. To measure the effect of the CNFs on the electrochemical activity of catalysts (Pt), the electroactive surface area (ECSA) of Pt was determined by calculating the charge corresponding to the Had/Hdes region from the cyclic voltammograms [19]. A plot of the ECSA for Pt NPs vs. CNFs obtained at the three C2H2/Ar ratio synthesis condition interestingly revealed an optimum ECSA of 181 cm2/mg corresponding to Pt/CNF0.31 electrode (Fig. 6d). These results clearly demonstrate that the morphology of the carbon nanofibers support alters the electrochemical properties of Pt NPs. The typical CVs for MOR at the Pt NPs supported on the CNFs electrodes in a 0.5 M H2SO4 and 1 M CH3OH solution are shown in Fig. 7a. The overall shapes of the CVs of all samples are in good agreement with the literature [20]. The current mass activity of the electrode estimated from the forward anodic peak current density clearly demonstrates the influence of CNF morphology and density where the Pt/CNF0.31 exhibited the highest values for methanol oxidation (Fig. 7b). Additionally, in most of the literature, the typical methanol oxidation current peak on the Pt catalyst is at about 0.69 V versus Ag/AgCl in the anodic scan [21]. The peaks of methanol oxidation are at 0.63 V at our Pt/CNFs electrodes regardless of the CNFs morphology and density.
4.
Conclusion
Developing binderless-based carbon nanostructured electrodes requires particularly the optimisation of both the morphology and the density of the carbon support which plays a nontrivial role on the catalytic behavior as revealed by the present study. We have shown that for a similar amount, morphology and dispersion of the catalyst (Pt NPs) fabricated on carbon nanofibers, the density of the latter and to a lesser extent their diameter are the main factors influencing the catalytic activity. For the particular case of carbon nanofibers considered in this work, an optimum performance toward methanol electrooxidation was obtained when Pt NPs were supported with CNFs synthesized with a C2H2/Ar ratio of 0.31.
Acknowledgement This work was supported by the Natural Sciences Engineering Research Council of Canada (NSERC), the Fonds Que´be´cois pour la Recherche en Nature et Technologie (FQRNT), and the Centre Que´be´cois sur les Mate´riaux Fonctionnels (CQMF).
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references [11] [1] Di Blasi A, Baglio V, Denaro T, Antonucci T, Arico` AS. Optimization of electrode properties for low temperature DMFC applications. J New Mat Electrochem Syst 2008;11:165e74. [2] Tarasenko AA, Lysenko AA, Lysenko VA. Porous carboncarbon composites for fuel cells. Fibre Chem 2007;39:159e62. [3] Kim YI, Soundararajan D, Park CW, Kim SH, Park JH, Ko JM. Electrocatalytic properties of carbon nanofiber websupported nanocrystalline Pt catalyst as applied to direct methanol fuel cell. Int J Electrochem Sci 2009;4:1548e59. [4] Jiang Z, Yu X, Jiang ZJ, Menga Y, Shi Y. Synthesis of monodispersed Pt nanoparticles on plasma processed carbon nanotubes for methanol electro-oxidation reaction. J Mater Chem 2009;19:6720e6. [5] Wu G, Chen YS, Xu BQ. Remarkable support effect of SWNTs in Pt catalyst for methanol electrooxidation. Electrochem Commun 2005;7:1237e43. [6] Maiyalagan T. Silicotungstic acid stabilized PteRu nanoparticles supported on carbon nanofibers electrodes for methanol oxidation. Int J Hydrogen Energy 2009;34:2874e9. [7] Maiyalagan T. PteRu nanoparticles supported PAMAM dendrimer functionalized carbon nanofiber composite catalysts and their application to methanol oxidation. J Solid State Electrochem 2009;13:1561e6. [8] Balan BK, Sreekuttan Unni M, Kurungot S. Carbon nanofiber with selectively decorated pt both on inner and outer walls as an efficient electrocatalyst for fuel cell applications. J Phys Chem C 2009;113:17572e8. [9] Sebastia´n D, Caldero´n JC, Gonza´lez-Expo´sito JA, Pastor E, Martı´nez-Huerta MV, Suelves I, et al. Influence of carbon nanofiber properties as electrocatalyst support on the electrochemical performance for PEM fuel cells. Int J Hydrogen Energy 2010;35:9934e42. [10] Qin Yuan-Hang, Yang Hou-Hua, Zhang Xin-Sheng, Li Ping, Ma Chun-An. Effect of carbon nanofibers microstructure on electrocatalytic activities of Pd electrocatalysts for ethanol
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[17]
[18]
[19]
[20]
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oxidation in alkaline medium. Int J Hydrogen Energy 2010;35: 7667e74. Liu H, Song C, Zhang L, Zhang J, Wang H, Wilkinson DP. A review of anode catalysis in the direct methanol fuel cell. J Power Sources 2006;155:95e110. Justin Gooding J. Nanostructuring electrodes with carbon nanotubes: a review on electrochemistry and applications for sensing. Electrochim Acta 2005;50:3049e60. Banks CE, Compton RG. New electrodes for old: from carbon nanotubes to edge plane pyrolytic graphite. Analyst 2006;131: 15e21. Antiochia R, Lavagnini I, Magno F, Valentini F, Palleschi G. Single-wall carbon nanotube paste electrodes: a comparison with carbon paste, platinum and glassy carbon electrodes via cyclic voltammetric data. Electroanalysis 2004;16:1451e8. Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53:1126e30. Zhao TJ, Sun WZ, Gu XY, Ronning M, Chen D, Dai YC. Rational design of the carbon nanofiber catalysts for oxidative dehydrogenation of ethylbenzene. Appl Catal A: Gen 2007;323:135e46. Nugent JM, Santhanam KSV, Rubio A, Ajayan PM. Fast electron transfer kinetics on multiwalled carbon nanotube microbundle electrodes. Nano Lett 2001;2:87e91. Wagner CD, Riggs WM, Davis LE, Moulder JF. In: Muilenberg GE, editor. Handbook of X-ray photoelectronic spectroscopy. Eden Prairie, MN, USA: Perkin-Elmer Corp.; 1979. Lee SJ, Mukerjee S, Mcbreen J, Rho YW, Kho YT, Lee TH. Effects of Nafion impregnation on performances of PEMFC electrodes. Electrochim Acta 1998;43:3693e701. Liang HP, Zhang HM, Hu JS, Guo YG, Wan LJ, Bai CL. Pt hollow nanospheres: facile synthesis and enhanced electrocatalysts. Angew Chem Int Ed 2004;43:1540e3. Zheng SF, Hu JS, Zhong LS, Wan LJ, Song WG. In Situ one-step method for preparing carbon nanotubes and pt composite catalysts and their performance for methanol oxidation. J Phys Chem C 2007;111:11174e9.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Nontrivial role of carbon nanofibers morphology in binderless Pt nanocatalyst supported electrode Ze´hira Hamoudi, My Ali El Khakani, Mohamed Mohamedi* Institut National de la Recherche Scientifique (INRS), E´nergie, Mate´riaux et Te´le´communications (EMT), 1650 Boulevard Lionel Boulet, Varennes, Que´bec, J3X 1S2 Canada
article info
abstract
Article history:
Similar to conventional composite electrodes, developing binderless-based carbon nano-
Received 5 December 2010
structured (CNs) electrodes for fuel cells requires particularly the optimisation of both the
Received in revised form
morphology and the density of the CNs. In this work, carbon nanofibers (CNFs) have been
9 January 2011
optimised and used as catalyst support for Pt nanoparticles (NPs). The nontrivial role of the
Accepted 18 January 2011
CNFs on the catalytic behavior is clearly demonstrated. We have shown that for a similar
Available online 2 March 2011
amount, morphology and dispersion of the Pt NPs fabricated onto CNFs, the density of the latter and to a lesser extent their diameter are the main factors influencing the catalytic
Keywords:
activity. For the particular case of CNFs considered in this work, an optimum activity
Carbon nanofibers
toward methanol fuel cell reaction was obtained when Pt NPs were supported with CNFs
Catalyst support
synthesized with a C2H2/Ar ratio of 0.31.
Platinum nanoparticles
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Methanol electrooxidation
1.
Introduction
As known typically a conventional composite electrode in low temperature fuel cell is made up of a first macroporous layer, which is a carbon cloth or paper. This is the conductive support onto which the microporous gas diffusion layer (gdl) and thereafter the catalytic layer are deposited. In most electrode configurations, the gdl is formed by Polytetrafluoroetylene (PTFE) and carbon black (catalyst support); whereas, the composite catalytic layer consists of carbon supported Pt catalyst and Nafion ionomer [1,2]. The optimal amount of each component of this composite electrode has been found after years of design of experiments and trials. In addition, when in practical electrodes, the carbon supports and the catalysts particles are used in powder form and their processing to promote film formation (i.e. the ink-process) for functional electrodes requires the use of binders to provide mechanical integrity for the catalyst layer.
reserved.
The use of such conventional electrode fabrication is practically non-feasible in case of micro-sized (<1 cm2 active area) fuel cells. It is therefore crucial to develop fabrication techniques which enable to lay down films of catalyst support and catalyst nanoparticles onto electrically conductive substrate (which acts directly as the current collector) without the use of binder materials. One possible and promising approach is the preparation of free-standing (binderless) electrodes fabrication technique. This approach for fuel cell applications is still at its infancy but it certainly has the potential to be considerably more powerful than those currently in use and greatly simplify fabrication of electrode assemblies, the heart of fuel cell devices. It is now agreed that the structure and composition of Pt nanoparticles (NPs) are very important because they have great effect on its catalytic activity. In the form of well-dispersed Pt NPs with dimensions of 2e5 nm seem to represent the best design for the ultimate performance [3e5]. A given carbon
* Corresponding author. E-mail address: [email protected] (M. Mohamedi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.085
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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
catalyst support would mainly provide a mechanical support and an electronic continuity as well as a uniform dispersion of Pt NPs. However, when it comes to developing direct onsubstrate binderless electrodes, the influence of the morphology and density of a particular catalyst support must be investigated for the proper design of high performing electrode structures. Carbon nanofibers (CNFs) possess good electronic conductivity, corrosion resistant properties and are cheap to produce which make them as the best alternative to carbon nanotubes as catalyst support in the short-term particularly for Direct Alcohol Fuel Cells (DAFCs) commercialization for portable applications [6e10]. The present work intends to gain knowledge onto the influence of the morphology and density of CNFs catalyst support on the binderless electrode activity. To avoid the complex interaction among the Pt NPs, the CNFs are synthesized under different conditions but are loaded with
4683
a similar amount (0.18 mg cm2) and dimensions of Pt NPs. The optimum morphology of CNFs as catalyst support for Pt NPs is investigated with respect to the electrooxidation of methanol, an electrochemical reaction of technological importance to direct methanol fuel cells (DMFCs) [11].
2.
Material and methods
2.1.
Material synthesis
For the CNFs growth, a nickel catalyst layer of 5 nm thickness was deposited by pulsed laser deposition (PLD) technique on one side of an electrically conductive substrate that is made of 3D porous network of carbon microfibers (CMFs) (Toray). This nickel layer was deposited by ablating under vacuum, a pure (99.95%) polycrystalline nickel target by means of a pulsed KrF
Fig. 1 e Electron microscopy analyses: SEM images of (a) CNF0.125, (b) CNF0.31 and (c) CNF0.5 where (a0 , a00 ), (b0 , b00 ) and (c0 , c00 ) are their respective TEM and HR-TEM images.
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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 6 ( 2 0 1 1 ) 4 6 8 2 e4 6 8 8
Table 1 e Dimensions of carbon nanofibers their structural properties assessed by MicroRaman spectroscopy. Sample Diameter nm1 D band cm1 G band cm1 ID/IG CNF0.125 CNF0.31 CNF0.5
110 256 318
1356.4 1343.6 1337
1580 1592.8 1599
0.90 0.84 0.88
excimer laser (wavelength ¼ 248 nm, pulse duration z 14 ns, repetition rate of 20 Hz) with a fluence of 5 J/cm2. In order to obtain a uniform ablation over the target surface, the target was continuously rotated and translated. The CMFs substrate was placed at 5 cm from the target and the deposition was performed at room temperature. Afterward, the CNFs were grown by chemical vapor deposition (CVD) technique. First, the Ni-coated-CMFs substrate was placed in a furnace and heated to 700 C with a 5 C/min rate under an 80 sccm argon gas flow. When the temperature reached 700 C, the samples were kept under the same heating conditions for 1 h in order to break the Ni film into spherical Ni NPs covering uniformly the CMFs substrate. Subsequent to the Ni NPs formation, the reactive gas (C2H2 in the present study) was introduced simultaneously with argon in various ratios for the CNFs synthesis. After synthesis, the acetylene flow was cut off and the furnace was cooled down to room temperature under flowing argon. In this work, for CNFs optimisation three C2H2/ Ar ratios were considered, i.e., 0.125, 0.31 and 0.5 and will be denoted CNF0.125, CNF0.31 and CNF0.5, respectively. Pt NPs onto CNFs were deposited at room temperature by PLD by ablating a polycrystalline Pt target (99.99% purity, Kurt J. Lesker Co.) by means of a pulsed KrF excimer laser (l ¼ 248 nm, pulse width ¼ 15 ns, and repetition rate of 10 Hz) under 500 mTorr background pressure of He.
2.2.
Fig. 2 e MicroRaman spectra of CNFs: (a) CNF0.125, (b) CNF0.31 and (c) CNF0.5. Inset is the MicroRaman spectrum of the bare CMF substrate.
focused onto the sample to a spot size of 1 mm (MicroRaman spectroscopy, Renishaw Imaging Microscope WireTM).
2.3.
Electrochemical studies
In order to assess the electronic transfer properties of the CNFs as well as their electrical contact to the CMFs substrate, we employed cyclic voltammetry (CV) in a benchmark
Material characterization
The surface morphology of the as-prepared samples was examined by means of a scanning electron microscope (SEM, JEOL, JSM 6300F apparatus) operated at an accelerating voltage of 15 kV and a transmission electron microscopy (TEM, JEOL JEM-2100F) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was conducted for elemental composition of the material’s surface and its chemical environment. The XPS study was performed with a VG Instruments Escalab 220i-XL surface microanalysis system equipped with hemispherical analyzer and Al Ka X-ray source (1486.6 eV). Survey scans in the range 0e1000 eV were recorded at 100 eV pass energy with a step size of 1 eV. Core level spectra were obtained for C1s and recorded at 20 eV pass energy with a step size of 0.1 eV. Curve fitting of the XPS data was carried out with casaXPS version 2.2.107. All spectra have been recalibrated with respect to the C1s core level peak of adventitious carbon contamination. The metallic components of the Pt4f region were fitted using a Gaussian/Lorentzian asymmetrically modified line shape. The graphitic structure of the CNFs was assessed with MicroRaman by using the 514.5 nm (2.41 eV) laser radiation of an Arþ laser with a circular polarization. The laser beam was
Fig. 3 e Electron transfer properties with cyclic voltammetry in a 1.0 mM K4Fe(CN)6 D 1.0 M KCl solution at (dotted) CNF0.125, (full) CNF0.31 and (dashed) CNF0.5. The scan rate was 5 mV/s.
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Fig. 4 e Electron microscopy analyses: TEM images of (a) Pt/CNF0.125, (b) Pt/CNF0.31 and (c) Pt/CNF0.5. (d) Typical SAED patterns of Pt NPs/CNF. (e) Typical Pt NPs size distribution onto CNFs.
solution consisting of 1.0 mM K4Fe(CN)6 and 1.0 M KCl solution, which usually serves as a benchmark for investigating electrochemistry of various carbon structures [12e14]. Fuel cell reactions were studied in 0.5 M H2SO4 and in a mixture of 1 M CH3OH þ 0.5 M H2SO4 deaerated solutions. All electrochemical measurements were conducted at room temperature using a two compartments electrochemical cell with the reference electrode and counter electrode being an Ag/AgCl and a platinum coil, respectively. The reference electrode was separated from the analyte solution by a Luggin capillary that is very close to the working electrode to minimze the IR drop. Data acquisition was conducted with a potentiostat/galvanostat Autolab from EcoChemie.
3.
>20 mm, and within 10e15 mm, respectively. In addition, the C2H2/Ar ratio seems also to influence both the inner structure and the diameter of the CNFs as shown by TEM (Fig. 1a0 ec0 ) and HR-TEM (Fig. 1a00 ec00 ). The obtained inner structure is plain for
Results and discussion
Fig. 1aec shows SEM images of the CMFs substrate covered with CNF0.125, CNF0.31 and CNF0.5, respectively. In general, the CMFs substrate is uniformly coated with CNFs being mostoften wavy and somewhat entangled. In terms of density, it can be seen however that C2H2/Ar ratio had a marked effect, i.e., low C2H2/Ar ratio (0.125) led to low density of CNFs (Fig. 1a), whereas high C2H2/Ar ratio (0.5) yielded to protruding CNFs (small bright points in Fig. 1c and inset). It is only with C2H2/Ar ratio (0.31) that high density of CNFs was obtained (Fig. 1b). Average length of CNF0.125, CNF0.31 and CNF0.5, were >10 mm,
Fig. 5 e X-ray photoelectron spectra of Pt4f core level peaks of Pt NPs deposited onto CNFs. (a) Pt/CNF0.125, (b) Pt/CNF0.31, and (c) Pt/CNF0.5.
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Fig. 6 e Cyclic voltammetry in a deaerated 0.5 M H2SO4 solution at Pt/CNF electrodes with 50 mV/s scan rate. (a) Pt/CNF0.125, (b) Pt/CNF0.31, and (c) Pt/CNF0.5. (d) Measured ECSA from the Had/Hdes regions in the CVs shown in aec.
CNF0.125, hollow of the herringbone and fishbone type for CNF0.31 and CNF0.5, respectively. It was also observed that CNFs diameter increased as the C2H2/Ar ratio increased (Table 1). It is beyond the scope of this paper to study by which mechanism the C2H2/Ar ratio affects the morphology and density of CNFs since our main interest is to investigate such obtained CNFs in electrochemical applications. It should be however noted that neither XPS nor the thermogravimetric analysis has revealed the presence of nickel on the surface of the carbon nanofibers or residue after total combustion, respectively. In addition, characterizing such structures with the underlying carbon microfibers substrate by TEM was very challenging. A great deal of efforts and time were taken to obtain fairly analyzable images of Fig. 1. From these images, no nickel is seen remaining at the interior of the carbon nanofibers or on their tips which were mostly open. It is likely then that the growth of carbon nanofibers follows a tip-growth mechanism during which the nickel is extruded out of the carbon nanofibers and driven by the gases toward the outlet of the CVD set-up. The MicroRaman spectra of the CNFs samples are shown in Fig. 2 altogether with the spectrum related to the bare CMFs substrate (inset). All the samples exhibit mainly two Raman bands, within ranges of 1330e1359 cm1 (D band) and 1580e1599 cm1 (G band). The D and G bands reflect the
defective (non-graphitic) structure and the graphitic structure (sp2), respectively. Generally, the ratio of the intensities of D to G band (ID/IG) can be used as an indicator of the quality of the carbon structure [15]. If both bands possess similar intensity, this indicates a higher quantity of structural defects. Table 1 resumes the characteristics of the D and G bands for the bare CMFs substrate and for CNFs synthesized under different conditions of C2H2/Ar ratios. Fairly similar ID/IG values were observed for all synthetised, indicating the presence of high quantity of structural defects. It has to be noted the bare CMFs substrate showed an ID/IG of w0.17. Second, shifts in the D and G bands are observed as function of the C2H2/Ar ratio. These shifts are mainly due to the differences between CNFs internal structure and the diameter difference as observed by SEM and TEM analyses in Fig. 1. This kind of nanofibers exposes numerous graphene edges. The presence of these edges has been reported to be beneficial for catalysis applications either catalysts or as supports [16]. Fig. 3 displays cyclic voltammograms (CVs) measurements of the CNFs in 1.0 mM K4Fe(CN)6 and 1.0 M KCl solution at potential scan rate of 5 mV/s. The [Fe(CN)4-]6/[Fe(CN)3-]6 redox pairs is highly resolved at the CNFs synthesized under different conditions of C2H2/Ar ratios with higher redox currents obtained at CNF0.31. Furthermore, the anodic peak to cathodic peak
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Fig. 7 e Cyclic voltammetry in a deaerated 1 M CH3OHD0.5 M H2SO4 solution at Pt/CNF electrodes with 50 mV/s scan rate. (b) Anodic peak current mass activity.
separation, DEp, measured in Fig. 3 is of 114, 80 and 102 mV for the CNF0.125, CNF0.31 and CNF0.5, respectively, which suggests a faster electron transfer at the CNF0.31 electrode [12e14], which can be attributed to high density of state in the latter [17]. Owing to the fact that CNFs exhibited enhanced electron transfer rates, they could offer higher ability to stabilize the high dispersion of catalyst particles for methanol oxidation reaction (MOR). To explore this avenue, Pt NPs onto CNFs have been deposited by PLD technique following the conditions reported in the experimental section. TEM images of Pt NPs deposited onto CNFs are shown in Fig. 4aec. As can be seen a fairly similar dispersion of Pt particles was achieved. The crystallographic orientation was further studied by recording the Selected Area Electron Diffraction (SAED) pattern (Fig. 4d). Besides the (002) planes of carbon, the crystal lattice fringes of Pt are revealed as the lattice planes of (111), (200) and (220) with particle size having a mean diameter of 2.25 nm 0.5 nm (Fig. 4e). High-resolution XPS Pt4f core level spectrum of Pt NPs deposited onto CNFs is reported in Fig. 5. The Pt4f core level spectrum displays two peaks whose maximum intensities are located between 70 and 78 eV binding energy range. The binding energy difference (DEbind) varies between 3.24 and 3.32 eV between these two maxima is that expected from Pt4f7/2 and Pt4f5/2 core level peaks. The position of these two peaks is consistent with the fact that Pt is in a metallic state [18].
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The characteristic of Pt NPs surface electrochemistry was studied by cyclic voltammetry. Fig. 6 compares resulting CVs at Pt NPs supported by CNFs in 0.5 M H2SO4 deaerated solution. All electrodes exhibited fairly defined hydrogen adsorption and desorption regions in the potential region of ca.-0.3e0.2 V. It is though observed that peaks characteristics of Had/Hdes are more resolved and their currents are highest at the Pt/CNF0.31 electrode, which illustrates the better utilisation of Pt when supported on CNF0.31. To measure the effect of the CNFs on the electrochemical activity of catalysts (Pt), the electroactive surface area (ECSA) of Pt was determined by calculating the charge corresponding to the Had/Hdes region from the cyclic voltammograms [19]. A plot of the ECSA for Pt NPs vs. CNFs obtained at the three C2H2/Ar ratio synthesis condition interestingly revealed an optimum ECSA of 181 cm2/mg corresponding to Pt/CNF0.31 electrode (Fig. 6d). These results clearly demonstrate that the morphology of the carbon nanofibers support alters the electrochemical properties of Pt NPs. The typical CVs for MOR at the Pt NPs supported on the CNFs electrodes in a 0.5 M H2SO4 and 1 M CH3OH solution are shown in Fig. 7a. The overall shapes of the CVs of all samples are in good agreement with the literature [20]. The current mass activity of the electrode estimated from the forward anodic peak current density clearly demonstrates the influence of CNF morphology and density where the Pt/CNF0.31 exhibited the highest values for methanol oxidation (Fig. 7b). Additionally, in most of the literature, the typical methanol oxidation current peak on the Pt catalyst is at about 0.69 V versus Ag/AgCl in the anodic scan [21]. The peaks of methanol oxidation are at 0.63 V at our Pt/CNFs electrodes regardless of the CNFs morphology and density.
4.
Conclusion
Developing binderless-based carbon nanostructured electrodes requires particularly the optimisation of both the morphology and the density of the carbon support which plays a nontrivial role on the catalytic behavior as revealed by the present study. We have shown that for a similar amount, morphology and dispersion of the catalyst (Pt NPs) fabricated on carbon nanofibers, the density of the latter and to a lesser extent their diameter are the main factors influencing the catalytic activity. For the particular case of carbon nanofibers considered in this work, an optimum performance toward methanol electrooxidation was obtained when Pt NPs were supported with CNFs synthesized with a C2H2/Ar ratio of 0.31.
Acknowledgement This work was supported by the Natural Sciences Engineering Research Council of Canada (NSERC), the Fonds Que´be´cois pour la Recherche en Nature et Technologie (FQRNT), and the Centre Que´be´cois sur les Mate´riaux Fonctionnels (CQMF).
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