Energy Fuels 2010, 24, 3727–3732 Published on Web 03/10/2010
: DOI:10.1021/ef901275q
Electrochemical Behavior of Pt Nanoparticles Supported on Meso- and Microporous Carbons for Fuel Cells† )
Fabing Su,*,‡,§ Chee Kok Poh,§ Zhiqun Tian,§ Guangwen Xu,‡ Guangyong Koh,§ Zhan Wang,§ Zhaolin Liu, and Jianyi Lin§
)
‡ State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, §Institute of Chemical Engineering and Sciences, 1 Pesek Road, Jurong Island, Singapore 6278332, Singapore, and Institute of Materials Research and Engineering of Singapore, 3 Research Link, Singapore 117602, Singapore
Received November 1, 2009. Revised Manuscript Received February 21, 2010
Porous structure of the electrocatalyst support is of importance for mass transfer of reactants and products in electrochemical reactions of fuel cells. This study reports the comparative investigation of the electrochemical performance of Pt nanoparticles supported on the meso- and microporous carbons as well as commercial Pt catalyst E-TEK (40 wt % Pt loading) for the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in fuel cells. Ordered mesoporous carbon (OMC) synthesized using the template method was employed as the representative of mesoporous carbon, and carbon black BP2000 was used as the microporous carbon because of its microporous structure with a high surface area comparable to that of OMC. The samples were characterized by nitrogen adsorption, X-ray diffraction, small-angle X-ray scattering, thermogravimetric analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results showed that, for MOR, the Pt/OMC catalyst possessed a significantly higher catalytic activity measured by cyclic voltammetry than that of Pt/BP2000 and its performance even exceeded that of commercial catalyst E-TEK. The electrochemical impedance measurement indicated that Pt/OMC has a smaller charge-transfer resistance and faster overall MOR rate than both Pt/BP2000 and E-TEK catalysts. In contrast, for ORR, the mass activity of Pt/BP2000 is higher than that of Pt/OMC on a rotating disk electrode but comparable to that of E-TEK. The study may suggest that the mesoporous structure of the carbon support is important for liquid-phase electrochemical reactions, while micropores are more suitable for gas reactions at the electrodes of fuel cells. This work would be helpful in understanding the molecular transport of reactants and products in the pore nanostructure of carbon-supported Pt electrocatalysts for fuel cell application.
liquid-phase electrocatalysis of small molecules requiring more accessible surface for high dispersion of active metal nanoparticles, facile mass transport, and good mobility of electrolyte ions. High-performance fuel cell electrode materials require nanoarchitecture with an established nanoscopic reaction zone and efficient molecular transport of gas- or liquid-phase reactants and products to and from the carbon-supported nanoscale electrocatalyst.7,8 The desire to build a catalyst nanoarchitecture to improve the accessibility of fuel and oxidant to the active sites of the catalyst surface has led to efforts to incorporate the deposition of Pt nanoparticles as part of the synthesis process of nanostructured carbon supports. Recently, ordered mesoporous carbons (OMCs) synthesized using a template strategy have been found to be promising Pt electrocatalyst supports for DMFCs and PEMFCs because of their prominent advantages, such as high surface area, relatively uniform pore size, ordered pore structure, interconnected pore network, tailorable surface properties, and good thermal and mechanical stabilities.9,10 These characteristics
1. Introduction Pt/carbon catalysts have been widely used as electrode materials for chemical energy conversion of a fuel to electricity via electrocatalytic reactions,1-4 for example, methanol oxidation reaction (MOR) at the anode of direct methanol fuel cells (DMFCs) and oxygen reduction reaction (ORR) at the cathode of proton-exchange membrane fuel cells (PEMFCs). Considering the high use of noble Pt metal and electrochemical performance of electrodes, research on the carbon support materials with a developed pore structure, high surface area, nanoscaled morphology, tunable surface chemistry, and good electric conductivity is of significance for enhanced properties of fuel cells.5,6 Carbon supports having a large specific area and abundant nanoporous architecture are preferred for gas- or † This paper has been designated for the Asia Pacific Conference on Sustainable Energy and Environmental Technologies (APCSEET) special section. *To whom correspondence should be addressed. Telephone: þ86-1062526956. E-mail:
[email protected]. (1) Winter, M.; Brodd, R. J. Chem. Rev. 2004, 104 (10), 4245–4270. (2) Wei, S.; Wu, D.; Shang, X.; Fu, R. Energy Fuels 2009, 23 (2), 908– 911. (3) Cai, K.-D.; Yin, G.-P.; Wang, J.-J.; Lu, L.-L. Energy Fuels 2009, 23 (2), 903–907. (4) Park, H. I.; Mushtaq, U.; Perello, D.; Lee, I.; Cho, S. K.; Star, A.; Yun, M. Energy Fuels 2007, 21 (5), 2984–2990. (5) Dicks, A. L. J. Power Sources 2006, 156 (2), 128–141. (6) McCreery, R. L. Chem. Rev. 2008, 108 (7), 2646–2687.
r 2010 American Chemical Society
(7) Zhu, M.; Sun, G.; Yan, S.; Li, H.; Xin, Q. Energy Fuels 2008, 23 (1), 403–407. (8) Yen, C. H.; Shimizu, K.; Lin, Y.-Y.; Bailey, F.; Cheng, I. F.; Wai, C. M. Energy Fuels 2007, 21 (4), 2268–2271. (9) Su, F.; Zhou, Z.; Guo, W.; Liu, J.; Tian, X. N.; Zhao, X. S. Template approaches to preparing porous carbon. In Chemistry and Physics of Carbon; Radovic, L. R., Ed.; Marcel Dekker: New York, 2008; Vol. 30, pp 63-128.
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enabled electrocatalysts to possess high dispersion and use of Pt nanoparticles and fast diffusion of the mass products in MOR and/or ORR.11-15 However, although many studies have demonstrated the importance of the pore structure within carbon supports for fuel cell electrodes16 and even with the fabrication art developed over several decades,17 the porous nanostructure of the electrocatalyst is not yet sufficiently optimized, so that all of the surface atoms and pore system of the electrocatalyst are accessible to fuel or oxidant molecules.18 Herein, we comparatively investigated the electrochemical performance of Pt nanoparticles supported on the carbon materials with different pore structures, namely, meso- and microporous carbons, in MOR for DMFCs and ORR for PEMFCs. OMC as the representative of mesoporous carbon was prepared using the template method.19 The uniform mesoporous structure of OMC offers wide opportunities for a fundamental catalysis study.20,21 We used commercial BP2000 carbon black as the model of microporous carbon for a comparison because it has a surface area comparable to OMC, leading to a relatively fair comparison to the Pt support. Normally, carbon black XC-72 and BP2000 as commercial electrocatalyst supports are microporous, with a particle diameter of less than 100 nm. The results showed that, for MOR, the Pt/OMC catalyst exhibited a higher catalytic activity than that of Pt/BP2000 because of its smaller chargetransfer resistance and faster overall MOR rate. On the contrary, for ORR, Pt/BP2000 showed better mass activity. The study would be important in understanding the effect of the carbon support pore structure on the performance of Pt catalysts for fuel cells.
temperature for 24 h, followed by washing with copious amounts of deionized water and drying in air at 150 °C for 5 h to yield a carbon sample OMC. Carbon black BP2000 as a representative of microporous carbon for a comparison was purchased from the Cabot Corporation. The wet impregnation method was used here to prepare Pt electrocatalysts. A total of 0.30 g of carbon support (OMC or BP2000) was first added to a solution containing 0.45 g of chloroplatinic acid hydrate (H2PtCl6 3 xH2O) (∼36 wt % Pt, Sigma-Aldrich), 1.5 g of deionized water, and 0.5 g of ethanol. The resultant mixture was then mixed homogenously, evaporated at 100 °C for 2 h, and then dried at 150 °C overnight under vacuum. Subsequently, the solid product was treated in an atmosphere of argon (10 mL/min) and hydrogen (5 mL/min) at 400 °C (ramping at 2 °C/min from 10 °C) for 2 h to reduce Pt compounds into metallic Pt. The catalysts were washed with copious water for removal of residual chloride ion and finally dried at 80 °C overnight under vacuum. The catalysts obtained using OMC and BP2000 as supports were assigned as Pt/OMC and Pt/BP2000, respectively. The impregnation method used here may provide a relatively high dispersion of Pt nanoparticles within the pore channels of OMC compared to liquid reduction approaches, such as the E-G method22 or NaBH4 reduction,12 in which, most Pt nanoparticles would be dispersed on the external surface of carbon supports. 2.2. Characterization. The porous structure of the samples was investigated using N2 adsorption at -196 °C using an Autosorb-6B volumetric adsorption analyzer (Quantachrome). Prior to the measurement, the samples were degassed at 200 °C for 10 h under vacuum. The specific surface area was determined according to the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05-0.2. Pore size distribution (PSD) curves were derived from the Barrett-Joyner-Halenda (BJH) method using the adsorption branches. The pore size was estimated from the maximum position of the BJH PSD curve. The X-ray diffraction (XRD) patterns in Bragg’s angle (2θ) ranging from 10 to 90° at room temperature were collected on a Bruker D8 diffractometer with Cu KR radiation of wavelength λ = 0.1541 nm. The mesostructure of the carbon OMC was characterized using the small-angle X-ray scattering (SAXS) technique on a Bruker NanoStar with Cu KR radiation of wavelength λ = 0.154 18 nm. Thermogravimetric analysis (TGA) was conducted on a thermogravimetric analyzer TGA Q500 (Thermal Analysis Instruments, Burlington, MA) in air, with a flow rate of 100 mL/min and a temperature ramp of 10 °C/min. The microscopic features of the samples were observed with a fieldemission transmission electron microscope (TEM) (Tecnai G2 TF20 S-twin, FEI Company) operated at 200 kV. The surface chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 250 spectrometer (Thermo Electron, Altrincham, U.K.), using a non-monochromatized Al KR X-ray source (1486 eV). The operating pressure in the analysis chamber was maintained below 1 10-9 Torr. Wide-scan spectra in the binding energy range of 1100-0 eV were recorded in 1 eV step size with a pass energy of 50 eV. High-resolution spectra of the elemental signals were recorded in 0.05 eV steps with a pass energy of 20 eV. The calibration of binding energy (BE) of the spectra was referenced to the C1s electron bond energy at 284.5 eV. After the linear baseline for non-metal element signals (using the Shirley baseline for a metal element signal) was subtracted, curve fitting was performed using the nonlinear least-squares algorithm assuming a Gaussian peak shape. The electrochemical performance of Pt catalysts was measured by cyclic voltammetry (CV) at room temperature in a three-electrode cell using an Autolab PGSTAT302 electrochemical test system (Eco Chemie, Utrecht, The Netherland).
2. Experimental Section 2.1. Synthesis. The OMC sample was prepared using sucrose as the carbon precursor and ordered mesoporous silica SBA-15 as the template according to the method reported elsewhere.19 Briefly, 1.0 g of SBA-15, 1.25 g of sucrose, and 0.14 g of H2SO4 were added to 5.0 g of deionized water, and the resulting mixture was dried overnight at 80 °C. Then, the solid product was ground and heated to 160 °C for 4 h. Following this, 0.8 g of sucrose, 2.0 g of deionized water, 0.09 g of H2SO4, and 1.0 g of ethanol were then added to the solid and dried at 80 °C for 4 h and 160 °C overnight. Finally, the solid product was carbonized at 900 °C under an atmosphere of nitrogen for 5 h. The silica template was removed using a 10% HF solution at room (10) Stein, A.; Wang, Z.; Fierke, M. A. Adv. Mater. 2009, 21 (3), 265– 293. (11) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412 (6843), 169–172. (12) Su, F.; Zeng, J.; Bao, X.; Yu, Y.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2005, 17 (15), 3960–3967. (13) Liu, S.-H.; Yu, W.-Y.; Chen, C.-H.; Lo, A.-Y.; Hwang, B.-J.; Chien, S.-H.; Liu, S.-B. Chem. Mater. 2008, 20 (4), 1622–1628. (14) Fang, B.; Kim, J. H.; Kim, M.; Yu, J.-S. Chem. Mater. 2009, 21 (5), 789–796. (15) Fang, B.; Kim, M.; Kim, J. H.; Yu, J.-S. Langmuir 2008, 24 (20), 12068–12072. (16) Rao, V.; Simonov, P. A.; Savinova, E. R.; Plaksin, G. V.; Cherepanova, S. V.; Kryukova, G. N.; Stimming, U. J. Power Sources 2005, 145 (2), 178–187. (17) Shao, Y.; Liu, J.; Wang, Y.; Lin, Y. J. Mater. Chem. 2009, 19 (1), 46–59. (18) Rolison, D. R. Science 2003, 299 (5613), 1698–1701. (19) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122 (43), 10712–10713. (20) Su, F.; Lee, F. Y.; Lv, L.; Liu, J.; Tian, X. N.; Zhao, X. S. Adv. Funct. Mater. 2007, 17 (12), 1926–1931. (21) Su, F.; Lv, L.; Lee, F. Y.; Liu, T.; Cooper, A. I.; Zhao, X. S. J. Am. Chem. Soc. 2007, 129 (46), 14213–14223.
(22) Tian, Z. Q.; Jiang, S. P.; Liang, Y. M.; Shen, P. K. J. Phys. Chem. B 2006, 110 (11), 5343–5350.
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Figure 2. TG curves of catalysts Pt/OMC, Pt/BP2000, and E-TEK and carbon supports OMC and BP2000.
Figure 1. Nitrogen adsorption-desorption isotherms of (a) OMC and (b) BP2000, together with their PSD curves (insets).
A commercial E-TEK Pt/C catalyst with a nominal Pt loading of 40 wt % was employed for a comparison. The working electrode was fabricated by casting Nafion-impregnated catalyst ink onto a 5 mm diameter vitreous glassy carbon disk electrode. A total of 10.0 mg of carbon-supported Pt catalyst was ultrasonically dispersed into 2.0 mL of 2-propanol containing Nafion solution (5 wt %, DuPont) for 30 min to form a catalyst ink. A total of 10 μL of the catalyst ink was coated on the disk and dried at 80 °C for 30 min. A platinum foil and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. MOR was carried out in the 0.5 M H2SO4 and 0.5 M CH3OH aqueous electrolyte that was purged with high-purity nitrogen gas to remove the oxygen in the electrolyte. The ORR measurement was conducted in a 0.5 M H2SO4 aqueous electrolyte saturated with oxygen at a scan rate of 5 mV/s and a disk rotation rate of 2000 rpm. The catalysts were electrochemically cleaned by continuous cycling until a stable response was obtained before recording the CV curve. The electrochemically active surface areas of the Pt catalysts were estimated from the charges associated with hydrogen adsorption on Pt in the potential range from -0.2 to 0.1 V. The baseline of the measurement was extended from the double-layer region of each CV curve. Electrochemical impedance spectroscopy (EIS) of the samples was measured in 0.5 M H2SO4 and 0.5 M CH3OH solution over the frequency range from 105 to 1 Hz at around 0.4 V. All measurements were carried out at room temperature.
Figure 3. XRD patterns: (a) carbon supports OMC and BP2000 (the inset is the SAXS of OMC) and (b) catalysts Pt/OMC, Pt/BP2000, and E-TEK.
that the pore size of OMC is centered at around 4.1 nm. The surface area of OMC is 1210 m2/g. Figure 1b shows the isotherm and PSD of BP2000. The type-I isotherm curve indicates the microporous structure of BP2000 with a pore size of less than 2 nm (see the inset of Figure 1b). The remarkable nitrogen uptake above the relative pressure of 0.90 is due to the capillary condensation of nitrogen in interparticulate porosity that exists among agglomerate nanospheres forming a mesoporous texture. The surface area of BP2000 is 1280 m2/g. Figure 2 shows the TG curves of catalysts Pt/OMC, Pt/ BP2000, and E-TEK, together with carbon supports OMC and BP2000. It is seen that the weight loss for carbon supports OMC and BP2000 occurs in the temperature range of 500700 °C. For OMC, the negligible residual weight suggests that the inorganic template compounds were removed completely. A 2.0 wt % residue within BP2000 was obtained. For catalysts, the weight loss of Pt/OMC takes place in the temperature range of 300-400 °C, slightly lower than that of Pt/ BP2000 and E-TEK (350-430 °C). The much lower temperature range of weight loss for Pt catalysts compared to their carbon supports is because of the introduction of Pt metal nanoparticles, which could lead to the catalytic combustion of
3. Results and Discussion Figure 1 shows the N2 adsorption-desorption isotherms and PSD curves of carbon supports OMC and BP2000. It can be seen that the adsorption isotherm of OMC in Figure 1a is of type IV according to the International Union of Pure and Applied Chemistry (IUPAC) classification, indicative of mesoporous material. The remarkable nitrogen uptake above the relative pressure of 0.40 is due to the capillary condensation of nitrogen in the mesoporous texture. The inset of Figure 1a shows the PSD curve derived from the BJH method, indicating 3729
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Figure 5. XPS spectra of Pt/OMC and Pt/BP2000 catalysts: (a) wide scan and (b) Pt4f.
Pt/OMC, Pt/BP2000, and E-TEK. The diffraction peaks at 2θ values of about 39.8°, 46.2°, 67.5°, and 81.4° are ascribed to the facets (111), (200), (220), and (311), characteristic of facecentered cubic (fcc) crystalline Pt, suggesting that Pt species were reduced to the metallic state by hydrogen reduction in this work. The average size of Pt nanoparticles was calculated from the Pt(220) peak using the Debye-Scherrer equation to be ∼4.8, 5.3, and 4.4 nm for Pt/OMC, Pt/BP2000, and E-TEK, respectively. Figure 4 shows the TEM images of samples Pt/OMC and Pt/BP2000 as well as the commercial E-TEK catalyst. The images of the Pt/OMC catalyst in Figure 4a show that Pt nanoparticles are homogenously distributed within the pore channels of OMC supports, although the Pt nanoparticle size distribution looks broad. Figure 4b presents the crystal lattice of Pt particles with a size range of 4-5 nm. Panels c and d of Figure 4 exhibit the TEM images of the Pt/BP2000 catalyst, in which Pt nanoparticles are also highly dispersed on the carbon particle surface and the Pt particle size looks a little bigger than that of Pt/OMC, consistent with XRD analysis in Figure 3b. It is observed that there is no agglomeration of Pt nanoparticles occurring for catalysts Pt/OMC and Pt/ BP2000. It should be mentioned that most Pt nanoparticles on Pt/OMC were located in the pore channels because of the use of the impregnation method and the average Pt particle size is comparable to that of pore channels of OMC. On the contrary, most Pt nanoparticles on catalyst Pt/BP2000 were dispersed on the external surface of BP2000 carbon particles because Pt nanoparticles are too large to fit into most of the micropores. For the E-TEK catalyst in panels e and f of Figure 4, many aggregate Pt nanoparticles can be found possibly because of the low surface area of the support. The high dispersion of Pt nanoparticles on OMC and BP2000, in particular at this high Pt loading, should be because of the high surface area of supports and developed pore structure,
Figure 4. TEM images of Pt catalysts: (a and b) Pt/OMC, (c and d) Pt/BP2000, and (e and f) E-TEK.
support in air. The weight loss for these samples below 200 °C should result from the desorption of water vapor. Thus, the Pt metal content in the catalysts can be obtained from their TG curves after subtracting the weight of adsorbed water and residue. When the weight of the anhydrous material is set to 100 wt %, the content of Pt in Pt/OMC, Pt/BP2000, and E-TEK is 38.3, 35.4, and 40.2 wt %, respectively. Additionally, the profile of TG curves for both Pt/BP2000 and E-TEK is very similar, possibly because of the effects of carbon support (both are carbon black) and Pt nanoparticle chemistry properties, but the detail for this coincidence is unclear. Figure 3a shows the XRD patterns of carbon supports OMC and BP2000. Two broad diffraction peaks located at around 25.5° and 43.5° 2θ can be seen, corresponding to (002) and (101) diffractions of graphitic carbon, respectively. No peaks at 53° and 78° corresponding to the (004) and (110) diffractions can be observed, suggesting that both carbon samples have a turbostratic structure with a large curvature of the graphene layers. The intensity of the (002) peak for BP2000 is much less than that for OMC, indicating a much poorer graphitic crystallinity of BP2000. However, the reason why the intensity of the (002) peak for BP2000 is much lower than its (101) peak is unclear. The inset of Figure 3a shows the SAXS pattern of OMC, displaying well-resolved (100), (110), and (200) peaks and demonstrating the presence of ordered two-dimensional (2D) hexagonal pore arrays with mesoporous structure.11-13 Figure 3b presents XRD curves of catalysts 3730
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Figure 6. (a) CV curves of catalysts measured in the electrolyte of 0.5 M H2SO4 at a scan rate of 50 mV/s, (b) MOR curves of catalysts measured at a scan rate of 50 mV/s, (c) EIC curves of catalysts in 0.5 M H2SO4 þ 0.5 M CH3OH solution at 0.4 V, and (d) ORR curves of Pt catalysts in electrolyte of 0.5 M H2SO4 saturated with O2 at a rotation rate of 2000 rpm and scan rate of 5 mV/s.
-0.20 and 0.10 V originate from the adsorption and desorption of atomic hydrogen on the Pt surface in acidic media, respectively. Thus, from the area of H adsorption,22 electrochemical active area (EAA) of Pt/OMC, Pt/BP2000, and E-TEK catalysts was calculated to be 270, 244, 191 cm2/mg, respectively. High EAA of Pt/OMC and Pt/BP2000 may originate from the more exposed active Pt atoms on the Pt nanoparticles and would be partially responsible for the good electrochemical catalytic performance. The electrocatalytic activities of the catalysts toward MOR are shown in Figure 6b, in which the methanol oxidation activity can be recorded by the anodic peak current in the forward scan. The peak in the reverse scan is due to the reactivation of Pt associated with the removal of the residual carbon species.24 In the forward scan, although a similar onset potential at which the methanol oxidation initiates is observed for Pt/OMC, Pt/BP2000, and E-TEK, their methanol oxidation peaks are located at 0.67, 0.69, and 0.68 V and their mass catalytic activities (peak current density, which have been normalized to the Pt loading) are 250, 217, and 190 mA/mg, respectively. Pt/OMC shows the highest mass activity and the lowest oxidation potential, which are desired for MOR. This may be because of the presence of mesoporous channels within OMC, which could effectively facilitate the transportation of reactants and products in liquid electrochemical reactions.11-13 The EIS technique has been used to probe the interfacial processes and kinetics of electrode reactions in electrochemical systems. The methanol electro-oxidation on different catalysts at different potentials shows different impedance patterns for DMFC.27-30 The Nyquist plots of the three catalysts are shown in Figure 6c. It can be seen that a typical pseudo-inductive
which could prevent Pt nanoparticles from aggregation incurred during the high-temperature treatment. The XPS survey spectra of samples Pt/OMC and Pt/ BP2000 are depicted in Figure 5. The presence of C, O, and Pt elements can be clearly seen in the wide spectra for all catalysts in Figure 5a. The relatively low intensity of the Pt peak for Pt/OMC may result from the incorporation of Pt nanoparticles within pore channels of OMC. Figure 5b shows the Pt4f spectra, which can be deconvoluted into three pairs of doublets labeled with I, II, and III. The most intense doublet with BE of 71.2 eV (Pt4f7/2) and 74.5 eV (Pt4f5/2) was attributed to metallic Pt, but the peaks at around 72.2 and 75.5 eV could be assigned to the Pt2þ chemical state in either PtO or Pt(OH)2.23 The third pair peaks found at around 74.2 and 77.5 eV are most likely ascribed to Pt4þ species on the surface, such as PtO2.24,25 These Pt oxide species may be due to oxygen chemisorption at step and kink sites present on the Pt surface.26 The integration of peak areas indicates that most Pt species exist as metallic Pt for both Pt/OMC and Pt/BP2000 and the metallic Pt content on Pt/OMC is higher than that on Pt/BP2000, although Pt nanoparticles on both supports seem very similar in the chemistry state based on the XPS analysis. The electrochemical active surface areas of catalysts Pt/ OMC, Pt/BP2000, and E-TEK were determined by CV measurements performed in 0.5 M H2SO4 aqueous solution, as shown in Figure 6a. It can be seen that well-defined CV curves were obtained for three carbon-supported catalysts. In general, the cathodic and anodic peaks appearing between (23) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18 (10), 4054–4060. (24) Liu, Z.; Lee, J. Y.; Chen, W.; Han, M.; Gan, L. M. Langmuir 2004, 20 (1), 181–187. (25) Liang, Y.; Zhang, H.; Zhong, H.; Zhu, X.; Tian, Z.; Xu, D.; Yi, B. J. Catal. 2006, 238 (2), 468–476. (26) Aric o, A. S.; Shukla, A. K.; Kim, H.; Park, S.; Min, M.; Antonucci, V. Appl. Surf. Sci. 2001, 172 (1-2), 33–40.
(27) Hsing, I.-M.; Wang, X.; Leng, Y.-J. J. Electrochem. Soc. 2002, 149 (5), A615–A621. (28) Chakraborty, D.; Chorkendorff, I.; Johannessen, T. J. Power Sources 2006, 162 (2), 1010–1022.
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behavior with a large arc at high frequency and a small arc in the fourth quadrant at low frequency was observed. The smaller diameter of arc for Pt/OMC at high frequency indicates the small charge-transfer resistance for MOR. The smaller intersect of impedance with real axis (Z0 ) (designated as the polarization resistance) at low frequency for Pt/OMC compared to Pt/BP2000 and E-TEK suggests the faster overall MOR rate, namely, dehydrogenation of methanol molecules and oxidation of intermediate COads species.27-30 EIS observation is consistent with the results in Figure 6b, further confirming the enhanced electrocatalytic activity of the Pt/ OMC catalyst. Because the methanol electro-oxidation at different potentials and temperatures also shows different impedance behaviors, more EIS measurement would be needed for a detailed mechanism study. Figure 6d shows typical ORR polarization curves of Pt catalysts obtained at room temperature in O2-saturated 0.5 M H2SO4 using a rotating disk electrode (RDE) at 2000 rpm. The polarization curves of all catalysts exhibit two distinguishable potential regions, well-defined diffusion limiting currents (0.0-0.3 V), followed by a mixed kinetic-diffusion control region in the potential window of 0.4-0.7 V, with the onset potential of around 0.80 V. The limiting current (mass activity) for Pt/OMC is 83 mA/mg, much lower than that of Pt/BP2000 (101 mA/mg), indicating that the micropore structure may be more suitable for the gas reaction in fuel cells. In addition, the interparticulate voids among carbon nanoparticles may form a three-dimensional mesoporous structure, providing a rapid molecule transport within electrodes. Although the activity of E-TEK is 99 mA/mg, higher than that of Pt/OMC, it may hardly be compared to both catalysts synthesized in this work because the preparation methods are different for the deposition of Pt nanoparticles. However, it can be seen that the ORR curve profile of Pt/BP2000 is very similar to that of E-TEK, suggesting that both surface-deposited catalysts have
a comparable ORR performance to their similar TG curves shown in Figure 2 and approximate Pt nanoparticle size shown in Figure 4. For the high performance in electrocatalysis, not only facile mass transport of molecules within the carbon support but also its surface reactivity, electronic conductivity, ionic conductivity, and separation of electron-hole pairs are critical to enhance the fuel molecular conversion. Although our present investigation is still insufficient to identify the exact role of the nanoporous structure of carbon supports on the formation of Pt nanoparticles and their electrochemical performance for MOR and ORR, the work may generate some fundamental insights into the understanding of electrocatalyst architectures for fuel cell electrodes. 4. Conclusions In summary, we comparatively investigated the electrochemical performance of Pt nanoparticles supported on the OMC and microporous carbon (BP2000) compared to commercial Pt catalysts E-TEK (40 wt % Pt loading) for MOR and ORR in fuel cells. For MOR, the Pt/OMC catalyst possesses a better mass activity than that of Pt/BP2000 and E-TEK because of its smaller charge-transfer resistance and faster overall MOR rate. For ORR, the mass activity of Pt/BP2000 is higher than that of Pt/OMC. The results may indicate that the abundant mesoporous structure of a carbon support plays an important role in the liquid-phase electrochemical reaction, such as MOR, and the micropore structure may be more suitable for the gas reaction (here as ORR) at electrodes of fuel cells considering mass transportation. The work exhibited that the porous nanostructure of the electrocatalyst support is of importance for the molecular transportation of reactants and products in electrochemical reactions. Acknowledgment. We thank the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences (CAS) in China and the Agency for Science, Technology and Research (A*STAR) in Singapore for financial support.
(29) Hsu, N.-Y.; Yen, S.-C.; Jeng, K.-T.; Chien, C.-C. J. Power Sources 2006, 161 (1), 232–239. (30) Wang, Z.-B.; Yin, G.-P.; Shao, Y.-Y.; Yang, B.-Q.; Shi, P.-F.; Feng, P.-X. J. Power Sources 2007, 165 (1), 9–15.
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