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PtRu Nanoparticles Supported on Ozone-Treated Mesoporous Carbon Thin Film As Highly Active Anode Materials for Direct Methanol Fuel Cells Meng-Liang Lin,† Man-Yin Lo,‡ and Chung-Yuan Mou*,† Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan ReceiVed: May 18, 2009; ReVised Manuscript ReceiVed: July 19, 2009
Ordered mesoporous thin-film carbon (TFC) material, of short channels vertical to the film, was synthesized by hard templating and deposited with PtRu nanocatalyst as an anodic material in a direct methanol fuel cell (DMFC). A series of PtRu bimetallic nanoparticles supported on ozone-pretreated carbon were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) analysis. For 20 wt % PtRu catalyst supported on a 30 min ozone-pretreated TFC gave an unprecedented current density of 410 mA/mgPtRu at 0.5 V relative to reference hydrogen electrode (RHE) at 60 °C. The ozone treatment method introduces an easily controllable way for surface modification of mesoporous carbon which was found to modulate the surface composition and structure of the deposited PtRu nanoparticles. The greatly enhanced methanol electrochemical oxidation activity, at an optimum ozone treatment, was ascribed to facile transport in the TFC structure, high dispersion of bimetallic nanoparticles, and an optimized surface Pt and Ru ensembles of 70:30 ratios for a bifunctional efficiency. 1. Introduction Designing three-dimensional nanostructured electrode materials for electrocatalysis is important for high-performance electrochemical devices such as fuel cells. Careful construction of the nanoparticle/support assembly for high surface reactivity, facile transport of reactants and products, good electronic conductivity, and desirable ionic conductivity would enhance the molecular conversion.1 All these factors matter in a welldesigned electrode materials in fuel cell. The synergy between catalyst and the surface state and the three-dimensional nanostructure of the support is an important subject which is, however, less explored. For the development of direct methanol fuel cell (DMFC), one particularly needs novel electrocatalyst materials assembly to decrease the loadings of expensive metal catalyst while maintaining high mass current density. Mesoporous carbon materials synthesized by carbon-casting ordered porous silica templates2,3 are of great interest in many applications such as catalyst supports4 and adsorbents.5 Ordered mesoporous carbon (OMC) has the advantages of high surface area, tunable pore size, interconnected pore network, and tailorable surface properties.6-9 Recently, OMCs as support for Pt-based nanocatalysts have received a lot of attention as electrode materials in fuel cell applications.10,11 An especially important application is in the anode materials of DMFCs.12-16 Various designing features for electrode materials have been emphasized in recent reports. Liu et al. varied Pt/Ru ratios in depositing the alloy nanoparticles in mesoporous carbon to find the optimum electrocatalytic activities in DMFC.12a Lei et al. employed a mesoporous carbon material with higher degree of graphitization to give good electronic conductivity in DMFC application.12b Yu’s group has been developing PtRu catalyst * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. Tel.: +886-2-33665251. Fax: +886-2-23660954. † National Taiwan University. ‡ Industrial Technology Research Institute.
supported on highly integrated interconnected porous carbon systems with periodic order, which allow efficient gas diffusion.14 Ren et al. used a mesocellular carbon dual-pore structure to provide efficient mass transfer of ions and molecules in DMFC.15 Zhao et al. used anodic aluminum oxide membrane together with the soft template of block copolymer surfactant F127 to create a dual structure of fibers of mesoporous carbon as a support of Pt for a good performance in electrooxidation of methanol.16 However, the synergy between the catalyst and the surface state and of the carbon support is less explored.17-20 One needs to study in greater details the interactions between the metal and the carbon material in order to bring out the best performance of all the design features. Recently, we presented a novel OMC framework with very short nanochannels to increase dispersion of catalysts and to facilitate the transport of reactants and products.21 The new carbon electrode material is based on the mesoporous carbon CMK-3 of film morphology with short channels (less than 100 nm) perpendicular to the film surface. The thin-film morphology simultaneously improved its electric contact with electrode surface and the mass transport of the reactants in and out of the short channels where PtRu nanoparticles are confined. Eliminating the transport hindrance effect caused by long-distance migration or structure blockage of fuel molecules is thus a criterion in designing electrode materials. The morphology characteristics of larger pore size and shorter channel length could be of great benefit for application as fuel cell catalyst supports. The better metal dispersion would increase the utilization efficiency of the nanocatalysts. In addition to designing textural properties, the exploit of surface modification chemistry of carbon with the purpose of improving the performance of these fuel cells is another interesting new approach. The modification processes could be grafting or modifying functional species on the surface of target carbon supports. The use of surface-oxidized carbon materials, such as carbon nanotubes (CNT), has been found to be critical
10.1021/jp904611p CCC: $40.75 2009 American Chemical Society Published on Web 08/19/2009
PtRu Nanoparticles as DMFC Anode Material SCHEME 1: Syntheses of Mesoporous Carbon (TFC) and Ozone-Pretreated Carbon Material Supported with PtRu Nanocatalysts (PtRu/TFC_X)
for the attachment of Pt and its particle size control.22,23 Various oxidizing agents such as HNO3,23 H2SO4-HNO3, and H2O217 have been used in surface modification processes to create some functional groups such as carboxyl, hydroxyl, and carbonyl groups to be defect sites on carbon.24 However, few reports can be found in the literature concerning the effect of the functionalization on the dispersion and anchoring of platinum particles on the support.17-19,25 The origin of the functionalization effect on the particle size and dispersion of the metal particles is not well-understood.25,26 Ozone has been used as an oxidizing agent in industry for the oxidation of carbon black. It is also a good substitute for liquid-phase oxidant such as nitric acid. Ozone provides a clean and mild method for causing structural damage to the carbon surface or creating the hydrophilicity. Ozone has also been used in the functionalization and purification of carbon nanotube to expand its chemical processability and reactivity.27 Ozone treatment was done at relatively low temperature to generate carboxylic acid/ester, ketone/aldehyde, and alcohol groups on the carbon surface.28 For methanol electrooxidation reaction, there has been only one report on ozone pretreatment on the commercially available XC-72 carbon black to tune the deposition of Pt catalyst.29 We believe that a combination of wellconstructed mesoporous carbon architecture and nanocatalysts (particle size, dispersity, and alloying degree, etc.) would lead to a substantial improvement in catalytic activity. Controlling the deposition sites and dispersion of the Pt-Ru nanoparticles by use of ozone treatments on the mesoporous carbon surface is an approach previously unexplored. In Scheme 1, we summarize our approach in making the electrocatalyst with the ordered mesoporous thin-film carbon (TFC) material as pretreated by ozone for deposition of PtRu bimetallic nanocatalysts. In this work, mesoporous carbon samples with different ozone exposure times were examined as supports for PtRu nanocatalyst in the electrochemical oxidation of methanol. The structure and composition of the carbon-supported bimetallics, e.g., particle size, oxidation state, and alloying degree, were extensively characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) to reveal the intrinsic effects in enhancing catalytic activity. 2. Experimental Section 2.1. Syntheses. The ordered mesoporous TFC was synthesized according to our previously reported method.21,30 First, an SBA-15 thin film with perpendicular channels was made.21 Repeated loading of glucose and dehydration by concentrated sulfuric acid were performed. The silica content was then dissolved with 10 wt % hydrogen fluoride solution, thus recovered the denoted TFC carbon product. The ozone treatment of the carbon support was performed on a Supreme Aqua ozonolysis apparatus. In this instrument, the flow rate of pure oxygen (O2) gas to the arc discharge was
J. Phys. Chem. C, Vol. 113, No. 36, 2009 16159 kept at 30 mL/min, and ozone was formed in a 7 W discharge at a pressure under 1 atm. Typically, 0.1 g of TFC carbon was first dispersed in 10 g of H2O by extensive ultrasonication, then ozone was purged into the mixture under vigorous stirring at room temperature. After reaction, the mixture was flushed with O2 for 5 min to remove residual O3. The product was recovered by centrifugation, washed with H2O, and dried in air at 60 °C. The modified carbon materials were denoted as TFC_X (X ) 15, 30, 60, and 120 min), with X being the ozone exposure time. PtRu nanocatalysts were synthesized and deposited on carbon supports by using a wet chemical reduction method.31 An amount of 0.1 g of TFC carbon material (with or without ozone treatment) was added in a solution containing a desired amount of 0.01 M H2PtCl6 (Acroˆs) and 0.01 M RuCl3 (Aldrich). The mixture was stirred for 30 min, and an excess of 0.1 M NaBH4 (Aldrich) solution was added into the mixture drop by drop. After stirring for 1 h, the solid was recovered by centrifugation, and extensively washed with H2O twice. The product was dried in air at 60 °C and denoted as PtRu/TFC or PtRu/TFC_X (X ) 15, 30, 60, and 120 min). 2.2. Electrochemical Activity Tests. The electrochemical activity measurements were carried out by cyclic voltammetry (CV) using an Autolab PGSTAT 30 potentiostat equipped with a rotating disk electrode (RDE).32 A conventional threecompartment electrochemical cell consisting of glassy carbon (GC) electrode with an area of 0.196 cm2 as the working electrode, Pt as the counter electrode, and reference hydrogen electrode (RHE) as the reference electrode was used. The GC electrode was polished to a mirror finish with a 0.05 µm alumina suspension before each experiment. The catalyst ink was prepared by adding 5 mg of 20 wt % PtRu/C sample in 2.5 mL of H2O and ultrasonically dispersed for 30 min. Then, 20 µL of suspension was pipetted onto the top surface of a GC electrode, followed by drying at 60 °C for 1 h in air. Thus, we obtained a PtRu metal loading as 0.04 mg/cm2 on the working electrode. After the electrode was cooled down, 20 µL of 1 wt % Nafion solution was pipetted onto the ink surface. CV study of methanol oxidation was measured in 0.5 M H2SO4 and 1.0 M CH3OH solution and characterized by the steady-state current density of the 10th CV sweep at 0.5 V with a scan rate of 10 mV/s. The measurements were carried out at 60 °C with working electrode rotated in 1600 rpm. CO stripping voltammogram curves were collected by linear sweep voltammetry (LSV) in a three-electrode cell.33 First, N2 gas was purged into 0.5 M H2SO4 solution for 30 min. Then, 99.9% carbon monoxide (CO) gas was purged into 0.5 M H2SO4 solution for 15 min to allow complete adsorption of CO onto the catalyst surface while maintaining working electrode potential at 0.2 V. The excess CO in the solution was removed by bubbling N2 gas for another 30 min by holding the potential at 0.2 V. Finally, the stripping voltammograms were done between 0.2 and 1.0 V in 0.5 M H2SO4 solution with scan rate 10 mV/s at 60 °C. For chronoamperometry, the electrode potential was fixed at 0.5 V. 2.3. Characterizations. The powder XRD patterns were collected on a PANalytical X’Pert PRO instrument. Nitrogen adsorption-desorption isotherms were obtained at -196 °C on a Micromeritics ASAP 2010 apparatus. The specific surface area ofthesampleswascalculatedaccordingtotheBrunauer-Emmett-Teller (BET) method, and the pore size distribution curves were obtained from the analysis of nitrogen adsorption isotherms using the Barrett-Joyner-Halenda (BJH) method. TEM images were obtained using a Hitachi H-7100 instrument with an operating voltage of 75 kV. Elemental analysis was determined
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by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Jarrel-Ash ICAP 9000 instrument with simultaneous wavelength detection range through 170-800 nm. All the samples were predissolved in HNO3/HCl/HF mixed solution. XPS measurements were performed with a Thermo VG Scientific ESCALAB 250 equipped with an Al KR radiation source (1486.6 eV) under a residual pressure of ∼1 × 10-9 Torr. The measured spectra were deconvoluted by a leastsquares procedure to a product of Gaussian-Lorentzian functions after background subtraction by using the software XPSPEAK. X-ray absorption spectra (XAS) were recorded at beamline 01C1 of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The electron storage ring is operated at 1.5 GeV and 300 mA. A double Si(111) crystal monochromator was employed for energy selection with a resolution ∆E/E better than 1 × 10-4 at the Pt LIII-edge and Ru K-edge. All the experiments were conducted in a stainless steel cell with Krypton film cap in two sides for beam path to avoid exposure of air.34 Before each measurement, the sample was reduced with 10% H2 for 30 min to remove the surface oxygen. All spectra were recorded at room temperature in a transmission mode and with the double-crystal monochromator detuned to eliminate the effect of higher harmonics in the X-ray beam. A standard compound, Pt foil or Ru powder, was measured simultaneously so that energy calibration could be performed scan by scan. Raw XAS data were analyzed following standard procedures. The EXAFS function was obtained by subtracting the postedge background from the overall absorption and normalized with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space with χ(k) multiplied by k3 to compensate for EXAFS oscillations in the high-k region. Subsequently, k3-weighted χ(k) data in k-space ranging from 3.0 to 14.3 Å-1 for the Pt LIII-edge and from 3.5 to 12.9 Å-1 for the Ru K-edge, respectively, were Fourier transformed to r-space. A nonlinear least-squares curve fitting was carried out with regard to the data in r-space ranging from 1.60 to 3.10 Å for Pt and from 1.69 to 3.10 Å for Ru, respectively. Reference phase and amplitude for the Pt-Pt and Ru-Ru coordination shells were each obtained from a Pt foil and a Ru powder. All the computations were implemented in the UWXAFS software package with the backscattering amplitude and phase shift for the atom pairs being calculated by using FEFF 7 code.35,36 3. Results 3.1. Synthesis and Characterization of the Carbon-Supported Nanocatalysts. By impregnating carbon supports with H2PtCl6 and RuCl3 and reducing chemically with NaBH4, PtRu bimetallic alloy nanoparticles were homogeneously dispersed onto the external and internal surfaces of mesoporous carbon thin film (Scheme 1). Figure 1A shows the low-angle XRD patterns of 20 wt % metal-loaded PtRu/TFC (TFC ) mesoporous thin-film carbon) and the ozone-treated sample, PtRu/ TFC_X. All of the samples reveal a main (100) and small (110) (200) diffraction peaks at low angles, indicating the mesostructure of two-dimensional hexagonal symmetry of SBA-15. The intensity of the (100) diffraction peak decreased significantly in the PtRu/TFC_120 sample, suggesting the mesostructure of TFC was extensively damaged as the result of long exposure to the ozone. Figure 1B shows the wide-angle XRD patterns of the samples corresponding to those of Figure 1A. All of the prepared samples displayed similar diffraction profiles of facecentered cubic (fcc) structure of Pt crystallites, except that the 2θ values were shifted slightly to higher values, as (220)
Lin et al.
Figure 1. (A) Low-angle and (B) wide-angle XRD patterns of (a) 20 wt % PtRu/TFC and 20 wt % PtRu/TFC_X, X ) (b) 15, (c) 30, (d) 60, and (e) 120 min.
diffraction peaks from 67.5° to 68.0°, corresponding to a decrease in the lattice constant due to the incorporation of Ru atoms.37 The sample of PtRu/TFC_120 shows the least shift for the (220) peak (∼67.7°) and could be attributed to poor alloying. No recognizable Ru hexagonal close-packed (hcp) structure or RuO2 tetragonal phase was observed in all the samples, suggesting that Ru was mostly incorporated into the Pt fcc lattice and formed Pt-Ru bimetallic alloy.38 However, the crystal domain size of RuO2 might be too small (
