Platinum and platinum–ruthenium nanoparticles supported on ordered mesoporous carbon and their electrocatalytic performance for fuel cell reactions

Platinum and platinum–ruthenium nanoparticles supported on ordered mesoporous carbon and their electrocatalytic performance for fuel cell reactions

Electrochimica Acta 50 (2005) 3131–3141 Platinum and platinum–ruthenium nanoparticles supported on ordered mesoporous carbon and their electrocatalyt...

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Electrochimica Acta 50 (2005) 3131–3141

Platinum and platinum–ruthenium nanoparticles supported on ordered mesoporous carbon and their electrocatalytic performance for fuel cell reactions Jie Ding a , Kwong-Yu Chan a,∗ , Jiawen Ren a , Feng-shou Xiao b a

b

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, Hong Kong, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry & College of Chemistry, Jilin University, Changchun 130023, PR China

Received 4 August 2004; received in revised form 8 November 2004; accepted 11 November 2004 Available online 14 April 2005

Abstract Highly ordered meso-porous carbon, denoted CMK-3 was synthesized by using mesoporous silicates, SBA-15 as the starting templating materials. The ordered mesoporous carbon was loaded with platinum and platinum–ruthenium nanoparticles using alternative synthesis techniques. The metal loaded ordered mesoporous carbon powders were characterized by transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX), X-ray diffraction, and nitrogen adsorption isotherm experiments. Micrometer-scale and centimeter-scale electrodes containing the mesocarbon/nanometal electrocatalysts were tested for some typical fuel cell reactions. While the nanometal/mesocarbon catalysts have well-defined and uniform properties in the nanometer scale, they have mixed electrocatalytic performance. A synthesized Pt/mesocarbon electrocatalyst outperformed a commercial electrocatalyst for oxygen reduction on a gas-diffusion electrode. The Pt–Ru/mesocarbon electrocatalyst synthesized, however, was not as effective for methanol oxidation. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ordered mesoporous carbon; CMK-3; SBA-15; Platinum nanoparticles; Platinum–ruthenium nanoparticles; Methanol oxidation; Oxygen reduction

1. Introduction High expectations of the commercialization of fuel cells and the accompanied environmental benefits has led to intensified studies on various fundamental and applied fronts. One area holding great promise is the control and optimization of electrode structures in the nanometer scale. The studies of platinum and mixed-metal nanoparticles have been very active with maturing synthetic and characterization techniques. Recent reviews of metal nanoparticles’ synthesis, catalysis [1–5] and performance in fuel cell electrodes [6] are available. While a lot of advances have been made in the synthesis of metal nanoparticles with good control of size and compo∗

Corresponding author. Fax: +852 2857 1586. E-mail address: [email protected] (K.-Y. Chan).

0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.11.064

sition, these nanoparticles are often supported on Vulcan 72 or other high surface area porous carbon that do not have uniform and well-defined structures in the nanoscale. Some studies of metal nanoparticles supported on graphite have been made [7]. But graphite is of very low surface area with poor affinity to ensure stable and dispersed metal nanoparticles. It is desirable to have relatively high surface area porous carbon support with uniform and controllable structures. The primary role of the carbon support is to disperse metal nanoparticles and to provide the electrical connection between them. As a porous and continuous solid phase, it also functions to a certain extent as a current collector. Common practical carbon supports have broad pore size distributions and irregular structures. Limited kinetics studies have been made on welldefined carbon structures supporting a high current density. Specific surface area of the carbon support has been the main

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characterization parameter to account for the catalytic performance. The effect of the ordering and regularity of the carbon structure on the performance of the catalyst has not yet been considered in detail, partly due to difficulty in preparing welldefined porous carbon structures. Platinum nanoparticles deposited on the outside of carbon nanotube bundles have been reported [8,9], but without good dispersion. The synthesis of carbon nanocoils and Pt–Ru loaded nanocoils for methanol oxidation has also been reported [10,11]. The recent discovery of ordered mesoporous silicas [12–16] provides suitable templates for the template synthesis of carbons with ordered mesoporous structures. Ordered mesoporous carbons such as CMK-1 and CMK-3 [17–21], SNU-1,2 [22,23] and CFDU-12 [24], have been synthesized by using the three-dimensional (3D) structured mesoporous silica as a hard template. These ordered mesostructured carbons have a regular array of uniform pores, large pore volume (1–2 cm3 g−1 ), and high specific surface area (1300–2000 m2 g−1 ). Limited reports have been made for ordered mesoporous carbon loaded with metal nanoparticles and their corresponding electrocatalytic performance. A notable example is the Pt-loaded CMK mesocarbon prepared by impregnation with chloroplatinic acid solution, followed by hydrogen reduction at 300 ◦ C. The Pt/CMK-3 catalyst was tested to have very good performance for oxygen reduction, but only for small area microelectrodes submerged in a solution [25]. It is desirable to test the performance on a more practical large area gas diffusion electrode. In this paper, we report some kinetics studies of platinum nanoparticles supported on ordered mesoporous carbon CMK-3 prepared by hard templating of SBA-15 mesoporous silica. The Pt-mesoporous carbon was tested for oxygen reduction on large area gas diffusion electrodes. An alternative synthetic route is also reported for loading the platinum nanoparticles into the mesoporous carbons. In addition to Pt/CMK-3, synthesis of Pt–Ru loaded ordered mesoporous carbon and the kinetics for methanol oxidation are reported. To our best knowledge, the synthesis and kinetics of Pt–Ru nanoparticles on CMK-3 carbon has not been reported before.

2. Materials synthesis SBA-15 mesoporous silicates were synthesized following a published hydrothermal procedure [16] using amphiphilic poly(alkylene oxide)-type triblock copolymers in an aqueous medium. The SBA-15 silicate material was then used as a template to synthesize ordered mesoporous carbon using sucrose as the carbon source according to a reported carbonization procedure [18]. The carbonized SBA-15 was denoted as CMK-3 in the literature. Fig. 1 shows the transmission electron microscopy (TEM) images of the starting SBA-15 silica template structures and the resulting CMK-3 material synthesized. The TEM was performed with a JEOL 2000FX microscope. From Fig. 1, it can be seen that the or-

der and the hexagonal structural can be retained to a good extent in the synthesized CMK-3 carbon material. In previous reports [22,23], platinum was added to CMK-3 carbon after complete carbonization. The platinization was achieved by impregnation of chloroplatinic acid and followed by hydrogen reduction at 300 ◦ C [22,23]. In this paper, the mesoporous carbon structures were synthesized similarly but platinization was carried out before carbonization. Platinization was started at elevated temperature with the addition of Pt(NH3 )4 (NO3 )2 to the SBA-15 template material. In a typical synthesis, one gram of dry SBA-15 was mixed with 0.17 g of Pt(NH3 )4 (NO3 )2 and water. The mixture was calcined at 400 ◦ C for 4 h. The amount of platinum introduced was 0.0856 mol g−1 of starting SBA-15 material. The Pt loaded SBA-15 silica was then added into a solution of 10 g water, 0.14 g of 98.0% H2 SO4 , and 1.25 g sucrose pre-dissolved. The mixture was placed in a drying oven at 100 ◦ C for 6 h, and then at 160 ◦ C for another 6 h. The sample turned dark brown during treatment in the oven and the sucrose in the silica pores partially polymerized and carbonized. With further addition of 0.8 g of sucrose and 0.09 g of 98.0% H2 SO4 , the sample was treated again at 100 and 160 ◦ C. The carbonization was eventually completed by pyrolysis at 900 ◦ C under nitrogen. The Pt–carbon–silica composite was washed twice with excess 1.0 M NaOH in 50% ethanol–water solution at 100 ◦ C to remove the silica template. The template-free Pt–carbon product (Pt/CMK-3) was filtered, washed with ethanol, and dried at 120 ◦ C. The concentration of Pt(NH3 )4 (NO3 )2 solution added affects the size of the platinum nanoparticles to be formed and the subsequent mesoporous carbon structure. We presented two samples prepared with different amounts of water added to Pt precursor and SBA-15 mixture. Sample A has 10 g of water, while sample B has 25 g of water added, respectively. To prepare Pt–Ru/CMK-3, we impregnate the synthesized mesoporous carbon with a colloidal solution of Pt–Ru nanoparticles prepared by the ethylene glycol method [26]. In a typical preparation, an EG solution of equal molar mixture of RuCl3 and H2 PtCl6 , containing 0.03 g Pt–Ru, was added dropwise into 3 mL of 0.2 M NaOH in ethylene glycol solution under vigorous stirring at room temperature. The resulting brown-yellow solution was heated at 439 K for 8 h with N2 passing through. When the reaction system cooled down to room temperature, 3.0 mL 1.6 M HCl and an EG solution of CMK-3* (0.12 g) were dropped into the system. After a further 8 h of stirring, the CMK-3* supported PtRu nanocomposite can be obtained by filtering, washing and drying at 353 K. Although the initial Pt:Ru in the precursor was 1:1, the different reduction rate resulted in a different composition, as will be determined by EDX and discussed later. CMK-3* was synthesized with a slight modification to the standard carbonization procedure [22,23]. Stirring dispersion has been used for mixing of sucrose and SBA-15 before carbonization and eventually leading to minor variations in the structural parameters.

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Fig. 1. Transmission electron micrographs of (a) SBA-15 silica showing the parallel mesopores, (b) SBA-15 showing the hexagonal order, (c) CMK-3 (carbonized SBA-15) showing the parallel mesopores, and (d) CMK-3 showing the hexagonal order. All scale bars (white in colour) are 20 nm in length.

3. Results and discussion 3.1. TEM and EDX characterization Transmission electron micrographs (TEM) obtained with a JEOL 2000FX microscope of the two Pt/carbon samples are shown in Fig. 2, together with that of a commercial Pt|C sample from Chempur. From the TEM picture, the size of the Pt particles in the commercial sample ranged from 2 to 4 nm. Fig. 2d shows the size distribution of Pt nanoparticles in the two synthesized samples. With a lower concentration of Pt precursor (3.42 mol) solution in sample B, smaller Pt particles are formed. The mesoporous carbon structures are also affected by the concentration of the Pt precursor. The Pt particles in sample A were much larger, but the carbon support retained more of the characteristic ordered structure of

CMK-3, as shown in Fig. 2a. On the other hand, the hexagonal structure almost disappeared in sample B (Fig. 2b). Thus, the size and density of the Pt particles and the concentration of the Pt precursor affects significantly the structure of the carbon support formed. Smaller particles and higher dispersion of Pt correlated strongly to the fading of the CMK-3 structure. The Pt loading on the carbon materials was quantified through energy Dispersive X-ray (EDX) analysis by a scanning electron microscope Model LEO 1530. The EDX results are shown in Table 1 with 8.7% and 10.42% by mass of Pt, for samples A and B, respectively. The carbonization process occurred with some possible losses of carbon. Platinization from the precursor may also not be 100% complete and the unreacted platinum precursor was washed away at later stages. According to the synthetic procedure described earlier, a complete platinization and carbonation of the starting carbon and plat-

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Fig. 2. Transmission electron micrographs of (a) Pt/CMK-3 sample A, (b) Pt/CMK-3 sample B, and (c) commercial Pt/C product. Each scale bar is 20 nm. (d) Histogram of size distribution (sample A clear and sample B darken).

inum sources should give a Pt mass percent of 10.5% in each of the Pt/CMK-3 sample. Deviations from this indicate that there is either a greater Pt loss or a greater carbon loss. The resulting Pt loading of 10.4% in sample B was basically consistent with the theoretical value. The lower Pt loading in sample A suggested more Pt losses in platinization of the more concentrated precursor. The Pt–Ru loaded CMK-3* was also examined by transmission electron microscopy (TEM) (Fig. 3a) and high-resolution transmission electron microscopy (HRTEM) (Fig. 3b and c). It can be clearly seen that metal particles distribute rather homogeneously and the particle size appears quite uniform (Fig. 3a). The HRTEM image shows patches of crystalline regions representing the locations of the metal nanoparticles. The extent of a crystalline region is not

sufficient to have definitive answers of the lattice parameters. The lattice plane, packing, and orientation of the crystalline region often vary slightly after a few lines of atoms within a nanoparticle, as seen from the single projection of the HRTEM. In Fig. 3c, the crystalline region of a nanoparticle have lines of high density atoms, which appears to be side view of (1 1 1) layers of the fcc packing. From average spacing between seven lines (layers) is determined to be ˚ This is slightly lower than the bulk platinum about 2.23 A. ˚ perhaps due to some alloying with ruthespacing of 2.26 A, nium atoms. The histogram of the particle size distribution is shown in Fig. 3d. The particle diameter average is 1.5 nm with a narrow standard deviation of ±0.6 nm. The energy dispersive X-ray analysis (EDX) of many micro analytical regions over an area of the PtRu/CMK-3* material indicates

Table 1 Properties of different Pt loaded carbon samples

Pt content (w/w, %) ˚ d value(A) N2 BET surface area (m2 /g) Pore volume (cm3 /g) ˚ Average pore diameter (A) Open circuit potential (V) Average Pt particle size (nm) Pt concentration of Precursor (mol)

Sampe A Pt/CMK-3

Sample B Pt/CMK-3

8.7 156 438 0.20 33 0.730 40 8.56

10.4 140 669 0.36 32 0.730 4 3.42

Commercial Pt/C

CMK-3 [16]

10 1000

0.730 2

84 1520 1.3 45

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Fig. 3. Transmission electron micrograph of PtRu|CMK-3 (a), corresponding high resolution TEM micrographs (b,c) and particle size distribution histogram.

a uniform composition. In the PtRu/CMK-3* synthesized, the atomic ratio of Pt:Ru was 3:1 and the metal loading was 15.30% by mass. The composition deviated from the original 1:1 atomic ratio of Pt:Ru precursors in the initial solution. Fig. 4 shows the EDX image mapping of carbon, Pt, and Ru and the micro-region element distribution exhibited the good and uniform dispersion of PtRu in the carbon support. 3.2. XRD and nitrogen sorption analyses Small angle powder X-ray diffraction (PXRD) spectra of Pt/CMK-3 recorded on a Siemens D5000 diffractometer using Cu K␣ radiation are shown in Fig. 5. Consistent with the TEM results, the Pt/CMK-3 sample B was composed of weakly ordered mesostructured carbon and did not show any peaks in the PXRD diffraction pattern. On the contrary, sample A exhibited a single sharp peak in its PXRD pattern. The corresponding d spacing values are shown in Table 1 together with other structural properties of the different Pt/carbon materials. The d value of samples A and B were higher than the value of 8.4 nm (d100 ) reported for the CMK-3 material without Pt [18]. The presence of Pt nanoparticles within the ordered carbon structures might have caused this larger d spacing. Large angle PXRD patterns performed with a PANalytical X-ray diffractometer X’pert Pro, are shown as insert in Fig. 5. All the samples have sharp peaks at 2θ = 39.8◦ , 46.28◦ , 67.50◦ , 81.29◦ , and 85.73◦ matching those of a pure platinum face centered cubic (fcc) lattice. The platinum nanoparticles

in the mesocarbon support are therefore crystalline, while the mesoporous carbon support is amorphous but with a longrange order in the nm scale. From the full widths at half mast (FWHM) of the XRD peaks of the samples, the averaged size of platinum can be deduced and are in agreement with those determined from TEM images. The XRD pattern of the Pt–Ru/CMK-3* is shown in Fig. 6. Also shown in Fig. 6 is the pattern for the original SBA15 template, with well-resolved peaks, assigned to (1 0 0), (1 1 0), (2 0 0), (2 1 0) and (3 0 0) reflections of the 2D hexagonal space group (p6mm). The XRD diffraction peaks of the carbonization material CMK-3* shift somewhat to higher angles with well-resolved XRD peaks assigned to (1 0 0), (1 1 0), and (2 0 0). The peaks of CMK-3* are still clear, but the width of peak (1 0 0) increased slightly. The peaks broadening should be due to structural defects created in the carbonization step and the silica removal step. The silica dissolution can result in a general structural shrinkage. In contrast with the XRD pattern of CMK-3* , the peak intensity of PtRu|CMK-3 decreases sharply, indicating that the adsorption of PtRu nanoparticles resulted in the decrease of the mesostructural ordering. It suggests that PtRu nanoparticles not only adsorbed on the surface of CMK-3* , but also ended up inside the mesoporous structure, while a good and homogeneous dispersion of PtRu is obtained. Large-angle XRD pattern for PtRu|CMK-3 was also obtained, as shown in the insert of Fig. 6. Three diffraction peaks can be resolved and located in the diffraction lines of 0.2269, 0.1388, and

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Fig. 5. PXRD patterns of Pt|CMK-3 sample A (solid line), and sample B (dot-dash line). Insert: large angle XRD patterns.

of alloying may be estimated by this decrease of lattice constant according to Vegard’s law, as reported by several authors [27–30]. According to the correlation of XRD determined lattice parameter to Ru content from Ref. [27], the composition is estimated to be about 93 at.% Pt or a Pt:Ru ratio of 13:1. From the HRTEM images (Fig. 3d), a much higher Ru content of 50% is indicated. The estimation of Pt–Ru composition from changes in lattice parameter cannot be very reliable for a number of factors. From the correlation of [27], a 1% error in the spatial resolution will lead to a difference of 50% in Pt content. The crystalline region in a nanoparticle is not large enough for very precise determination of lattice parameters from diffraction or TEM. While the Pt% content determined by XRD determined lattice correlation or HRTEM may not be accurate, there is a good possibility of additional amorphous Ru or Pt present on the surface of elsewhere of the nanoparticles. Peaks related to hexagonal close-packed (hcp)

Fig. 4. Images of EDX mapping for PtRu/CMK-3: (a) carbon, (b) platinum, and (c) ruthenium. Bright spots represent intensity of the element in a microregion. Scale bar is 500 nm.

0.1178 nm. These are consistent with those of unsupported pure metallic Pt nanoclusters reported by Wang et al. [26], but the peaks are broadened due to the small particle sizes. From the location of XRD peaks, the lattice parameter of the ˚ This value is less than fcc can be determined to be 3.911 A. ˚ the value 3.915 A of pure bulk platinum reported in [27]. This shrinkage is likely caused by ruthenium alloying. The extent

Fig. 6. Powder XRD patterns for (a) CMK-3, (b) PtRu/CMK-3, and (c) SBA-15. Insert: large angle XRD pattern of PtRu/CMK-3.

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Fig. 7. Nitrogen adsorption isotherms and pore size distribution (inserts) of Pt/CMK-3 for (a) sample A and (b) sample B.

Ru phase cannot be detected, suggesting the absence of Ru crystalline regions. We believe the EDX analyses of an overall 3:1 Pt:Ru ratio to be more reliable and both crystalline and non-crystalline regions of the nanoparticles are included in the EDX analyses. Nitrogen adsorption/desorption isotherms of the Pt/CMK3 materials were obtained with a Micromeritics ASAP 2010 analyzer and the results for the two Pt/CMK-3 samples are shown in Fig. 7. The adsorption branches are located between relative pressures of 0.2–0.6. Pore size distributions were constructed from the isotherms and are shown as inserts. Each sample material has a peak in a similar position of the pore size distribution curve. Brunauer–Emmett–Teller (BET) calculations of surface area and Barrett–Joyner–Halenda (BJH) model calculations of pore volume and pore size distributions were performed for the adsorption branch of the isotherm and

the results are shown in Table 1. Although the pore sizes are similar, the pore volume and BET specific surface area of sample B are higher. Nitrogen sorption isotherms for the SBA-15, CMK-3* , PtRu/CMK-3* , and PtRu/CMK-3* samples are shown in Fig. 8, whereas the relevant structural parameters determined on the basis of the isotherms are listed in Table 2. It can be seen that for SBA-15, the adsorption capacity, the BET specific surface area, the primary (ordered) mesopore volume, and the pore diameter are in agreement with the literature values [16]. However, nitrogen adsorption or desorption isotherms for the CMK-3* , and PtRu/CMK-3* samples have discrepancies with the literature data [18]. The difference should originate from the impregnation of silica with sucrose with different degrees of penetration to produce different carbon mesopores. In CMK-3* , the isotherms show two obvious cap-

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Table 2 Properties of Pt–Ru|CMK-3 synthesized d (nm)

SBET (m2 /g)

Vp (cm3 /g)

t-Plot results Vmi

SBA-15 CMK-3* PtRu/CMK-3*

10.38 9.29 8.74

820 1643 1284

1.35 1.86 1.32

illary condensation steps suggesting at least a bimodal porosity with pore sizes in the range of 2–4 and >5 nm, respectively. The increase in the adsorbed amount corresponds to pores of size beyond 10 nm (see Fig. 8). It can be speculated to be due to defects in the ordered structure of the material depending on the degree of infiltration of the carbon precursor solution in the carbonization process. This reflects the existence of unfilled silica pores according to that reported in [31]. The pore size distributions (PSDs) displayed in Fig. 8 show more clearly the presence of both kinds of pores for CMK-3* and PtRu/CMK-3* The size of the pores at 2.4 nm

(cm3 /g)

0.05 0.08 0.05

wBJH–BdB (nm) Smi 137 176 126

(m2 /g) 7.5 2.4, 7.3∼14.0 2.3, 7.0∼11.2

for CMK-3 is derived from the silica walls. The adsorption of PtRu nanoparticles does not produce any influence on the primary pores (see Table 2). The wider PSDs at the range of 5–20 nm, can be further separated into two kinds of pores: one is around 7.3 nm, the other at 14.0 nm for CMK-3* . The above regions are obviously affected by the adsorption of PtRu nanoparticles, which causes a decrease of the pore diameter (see Table 1). According to the data in Table 1, the adsorption of PtRu nanoparticles leads to a 22 and 29% decrease of the BET surface area and total pore volume, respectively, while the surface area of the micropore from t-plot dropped to 71%. This suggests that PtRu nanoparticles were adsorbed without selectivity between the mesopores and the micropores. Since the surface area of CMK-3* is not constituted from the surface of the carbon powders but mainly from the surface of the carbon rods spanning the mesoporous structure, PtRu nanoparticles should be along the carbon rods. The location and dispersion of the nanoparticles would affect the mass-transfer and ohmic effects during a fuel cell reaction. 3.3. Evaluation of electrocatalytic performance

Fig. 8. (a) Nitrogen adsorption isotherms for SBA-15 (), CMK-3* (䊉) and PtRu/CMK-3* (). The isotherm for PtRu/CMK-3 was offset vertically by 400 cm3/g STP. (b) Corresponding pore size distributions (PSDs) calculated from nitrogen adsorption for SBA-15 (), CMK-3* (䊉), and PtRu|CMK-3* (). To facilitate the comparison, the PSDs for CMK-3* and PtRu/CMK-3* were multiplied by 5.

To characterize the basic electrochemical properties of the synthesized Pt/CMK-3 materials, we performed cyclic voltammetry (CV) in sulfuric acid. The Pt/mesoporous carbons have high surface areas and generally do not produce clear cyclic voltammograms with well-defined peaks due to the large electrochemical capacitance. We attempted the powder microelectrodes method introduced by Cha et al. [32]. The resulting CV’s, however, indicated rather high resistance with the hydrogen–oxygen evolutions spanning over 1.8 V. Here, we reported instead CVs on larger area electrodes prepared with the ink electrode method. A 2 mm diameter gold disk electrode from CH Instruments was etched to create a cavity of approximately 20 ␮m deep. Each of the different Pt/CMK3 powders was applied into the cavity together with about 2 ␮l of 5% Nafion solution. The amount of carbon loaded was too small to have a consistent mass among the different powder samples. Therefore, we look for mainly a qualitative comparison of the CVs of the various materials. The CVs were performed in 0.5 M sulfuric acid at room temperature at a scan rate of 20 mV/s using a calomel reference electrode. The solution was left air saturated prior to the cyclic voltammetry experiments. The steady CVs after a few cycles are shown in Fig. 9 for the two Pt/CMK-3 materials, a 10% Pt/C powder from ChemPur, and a plain gold disk electrode without carbon catalysts. The CVs show hydrogen and oxygen evolution

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Fig. 9. Cyclic voltammograms of ink electrodes in 0.5 M H2 SO4 at 20 mV/s for sample A Pt|CMK-3 (dashed line), sample B Pt/CMK-3 (dotted line), a commercial Pt/C (solid line), and a blank gold electrode (thick solid line).

at the usual potentials uncorrected for ohmic drop. The gold disk electrode showed negligible currents. The two Pt/CMK3 materials have similar features compared to the ChemPur sample. Hydrogen adsorption and desorption peaks can be clearly seen. Since the amount of carbon loaded may not be consistent, we do not feel confident for a quantitative comparison of the hydrogen adsorption regions for platinum surface area and particle size determination based on loading. Nevertheless, the hydrogen peaks are more pronounced for sample B, showing good activity of the platinum nanoparticles. For practical applications, a more meaningful comparison can be made with centimeter scale gas diffusion electrodes. We prepared gas diffusion electrodes with the catalyst layers containing the various carbon samples listed in Table 1. To a 0.5 cm2 circular E-Tek carbon cloth electrode, the mesoporous Pt/carbon sample was mixed with Nafion and applied to the hydrophilic side of the electrode. An appropriate quantity of the different samples was used to give the same loading of platinum at 0.6 mg/cm2 . The hydrophobic side was exposed to ambient air for polarization measurement using an AutoLab model PGSTAT 30 potentiostat/galvanostat and a three-electrode test cell at room temperature. The counter electrode was a Pt plate and a saturated calomel reference electrode (SCE) was used in 0.5 M H2 SO4 . The electrodes were electrochemically reduced at potential <0.0 V for 5 min to ensure the elimination of reducible impurities. The electrochemical performance was similar before and after this reduction step. Fig. 10 shows the oxygen reduction polarization curves of different gas diffusion electrodes made of different carbon/Pt samples. The open circuit potentials, given in Table 1, were the same for the three samples. From Fig. 10, the electrode with sample B Pt|CMK-3 displayed the best performance, although the commercial Pt/C catalyst possessed smaller Pt particles and more than two times specific surface area. Comparing the structure of the carbon support, it is suspected that the size and uniformity of the pores may play a

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Fig. 10. Potential–current density plots for oxygen reduction in 0.5 M H2 SO4 at room temperature on gas diffusion electrodes made of sample A (down triangles and dash-dot line), sample B (up triangles and solid line) and commercial Pt/C product (filled circles and dotted line) all at loadings of 0.6 mg Pt/cm2 .

crucial role. The commercial Pt/C have a very high specific surface area but contributed mostly by micropores less than 1 nm and are therefore more difficult to be fully accessible. Sample B Pt|CMK-3 has uniform pores of 3.2 nm, which may be a better structure for accessible surface area. The performance of sample A, however, is not as good. Perhaps this is due to the lower surface area and rather large Pt particles. Sample A, having the highest structural ordering, did not give a better performance than sample B, suggesting a lesser role of ordering. The performance of the Pt–Ru/CMK-3* material for methanol oxidation was tested on microelectrode and a larger scale electrode. A 125 ␮m diameter powder microelectrode was packed with the Pt–Ru powder in the same method described by Cha et al. [32]. A cavity was chemically etched out from a 125 ␮m diameter gold microelectrode. The Pt–Ru/CMK-3 powder was packed into the microelectrode cavity. A commercial E-Tek catalyst with 1:1 Pt–Ru on Vulcan XC72 and a 20% total metal content was used for comparison. For the E-Tek catalyst, the Pt:Ru ratio is different and the total metal loading is higher, compared to the synthesized Pt–Ru/CMK-3* , but this is the closest commercial catalyst we can compared so far. Fig. 11 shows cyclic voltammograms at 30 mV/s obtained on the powder microelectrodes in 1.0 M methanol mixed with 0.5 M H2 SO4 solution at room temperature. The voltammograms shown are the stable and consistent cycles after a few initial scans. A Ag/AgCl reference electrode and a platinum counter electrode were used. Nitrogen bubbling was applied for 5–10 min to remove dissolved oxygen, before the CV recording. The performance of the Pt–Ru/CMK-3* is inferior to the E-Tek Pt–Ru/Vulcan XC72 electrode. A higher oxidation peak potential is observed and there is also a high reverse scan oxidation peak. The difference could be due to the better Pt:Ru ratio and slightly higher metal loading (20% versus 15.3%) in the E-

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Fig. 11. Cyclic voltammograms of powder microelectrodes in 1 M methanol in 0.5 M H2 SO4 at 30 mV/s for Pt–Ru|CMK-3* (dashed line) and a E-Tek Pt–Ru/Vulcan XC72 catalyst (solid line).

Tek electrode. But it is probable that the long and narrow channels in the CMK-3* materials lead to higher ionic resistance and mass-transfer resistance. It is, however, valuable that the Pt–Ru/CMK-3* catalyst is at least comparable to a commercial catalyst, given that it has lower surface area and long narrow pores. The well-defined porous structure will enable more detailed analyses of the kinetics, ohmic, and mass-transfer effects. Steady-state performance was also investigated by polarization curves obtained on larger area electrodes. A 1.0 M methanol in 0.5 M H2 SO4 solution was used. A controlled amount of ethanol solution with suspended Pt–Ru/CMK-3* powder was applied to a carbon cloth with an exposed working area of 0.646 cm2 . The total metal loading on the electrode was 0.65 mg/cm2 . The stable current over a 10 min interval for each potential set point was considered to be the corresponding steady-state current. The reference electrode and counter electrode was a Ag/AgCl electrode and a platinum electrode, respectively. The room temperature polarization curve of the Pt–Ru/CMK-3* electrocatalyst in Fig. 12 shows interesting behaviour. Starting at a relatively low potential, polarization increases quickly at moderate current with a steady linear region. This suggests a significant ohmic resistance, probably due to ionic transport in the long mesopores of CMK-3* . At still higher currents (>60 mA/cm2 ), however, the polarization levels off and even decreases at a higher current. The abnormal polarization is intriguing in the region where mass-transfer effect usually dominates. This phenomenon at high current was reproducible on several Pt–Ru/CMK-3* materials tested. The steady-state polarization curves of two other Pt–Ru/CMK-3* samples are shown in Fig. 12. The Pt–Ru compositions in the three samples are identical. The preparations of the three samples are identical except the SBA-15 calcinations time is slightly longer in the last sample. A speculative explanation is the unsteady trans-

Fig. 12. Potential–current density plots for methanol oxidation on three separate electrodes loaded with similar Pt–Ru/CMK-3* powder at a total metal loading of 0.65 mg/cm2 . The Pt–Ru compositions and loadings are identical but one sample (triangles) has a longer calcinations time in the SBA-15 synthesis.

port of reactants, intermediates, and ions in the long narrow pores of CMK-3* . The transport could also depend on local electric field and current. Further tests and ac impedance analyses would be needed to explore the details of polarization mechanism in these ordered mesocarbon materials. While detailed characterizations of the synthesized metal/mesocarbon materials have been reported, the electrochemical studies reported here are only preliminary. Among the various parameters of metal particle size, specific surface area, pore volume, pore diameter, and ordering, there is no clear indication of a single parameter that would determine the electrocatalytic performance. It is a combination of these structural parameters that will lead to the optimal performance. The variations in synthetic procedure proposed allow possible preparations of mesoporous Pt/carbon, Pt–Ru/carbon, or other metal/carbon with well-defined structural properties. Systematic investigations are called for to correlate the electrocatalytic performance with structural parameters. This will require careful and meticulous synthesis of materials with a sequential variation of individual structural parameters. These investigates will be valuable to further advance the fundamental knowledge of fuel cell electrode behaviour, particular with a scale of current density sufficiently high to be of practical interest. Further studies along the lines of controlled synthesis of structured carbon should be made to optimize the parameters of structure of the carbon support in the nanometer scale.

4. Conclusions Through minor variations in the synthetic procedure, CMK-3 mesoporous carbon with well-defined structures

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loaded with platinum nanoparticles and platinum–ruthenium can be made. The ordering of mesoporous carbon in the nanoscale will be weakened or destroyed depending on the concentration of the Pt precursor used before carbonization. A higher Pt precursor concentration will lead to larger particle sizes and higher ordering. From the electrochemical studies of oxygen reduction, on gas diffusion electrodes made of the synthesized materials, a better performance than that of a commercial catalyst can be obtained, probably due to higher accessible surface area provided by uniform mesopores of only a few nanometers in size. The Pt–Ru/CMK-3* synthesized is slightly inferior to a commercial Pt–Ru/carbon catalyst for methanol oxidation. The polarization curve, however, shows some unsteady behaviour in the mass-transfer region, probably due to transport in the long mesopores.

Acknowledgements This work was supported by the Research Grants Council of Hong Kong (HKU 7072/01P and HKU 7005/03P), HKU Foundation Seed Grant, and a Seed Grant for the Area of Excellence on Water Environment Engineering.

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