Platinum catalyst on ordered mesoporous carbon with controlled morphology for methanol electrochemical oxidation

Applied Surface Science 256 (2010) 6688–6693

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Platinum catalyst on ordered mesoporous carbon with controlled morphology for methanol electrochemical oxidation Ling-Bin Kong a,b,∗ , Heng Li a , Jing Zhang a , Yong-Chun Luo b , Long Kang b a b

State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, PR China School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China

a r t i c l e

i n f o

Article history: Received 8 January 2010 Received in revised form 3 April 2010 Accepted 19 April 2010 Available online 24 April 2010 Keywords: Methanol oxidation Ordered mesoporous carbon Platinum Morphology

a b s t r a c t Ordered mesoporous carbons CMK-3 with various morphologies are synthesized by using various mesoporous silica SBA-15 as template and then support to prepare Pt/CMK-3 catalyst. The obtained catalysts are compared in terms of the electrocatalytic activity for methanol oxidation in sulfuric acidic solutions. The structure characterizations and electrochemical analysis reveal that Pt catalysts with the CMK-3 support of large particle size and long channel lengths possess larger electrochemical active surface area (ECSA) and higher activity toward methanol oxidation than those with the other two supports. The better performance of Pt/CMK-3 catalyst may be due to the larger area of electrode/electrolyte interface and larger ECSA value of Pt catalyst, which will provide better structure in favor of the mass transport and the electron transport. © 2010 Elsevier B.V. All rights reserved.

1. Introduction As it has the advantage of environmental-friendliness, easy used liquid fuel, and relatively low requirement of operation condition, direct methanol fuel cell (DMFC) will be expected to be a new generation of power. The operation of DMFC involves methanol oxidation and oxygen reduction over precious metal catalysts dispersed on a carbon support with large surface area, such as carbon nanofibers [1–3], carbon nanotubes [4–8], carbon spheres [9,10] and ordered mesoporous carbons (OMC) [11–13]. Numerous research works have been focused on both catalyst dispersion and carbon-support design in order to enhance the catalytic activities. Among various carbon materials, the OMC has attracted considerable interest because of its high BET surface areas, uniform pore diameters, large adsorption capacities, high thermal, acid-base, and mechanical stabilities. The OMCs are typically synthesized by using ordered mesoporous silica (OMS) as sacrificing templates via nanotemplating route. The resulting mesoporous carbons with wide varieties of pore connectivities, pore sizes, and framework structures can be obtained depending on the pore structures and diameters of OMS templates and the carbon precursors [12,14]. The CMK-3 carbon replica is the first example of OMC retaining the structural symmetry of the SBA-15 silica template. In the SBA-

∗ Corresponding author at: State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, 287 Langongping Road, Lanzhou 730050, PR China. Tel.: +86 931 2976579; fax: +86 931 2976578. E-mail address: [email protected] (L.-B. Kong). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.071

15 silica, the large ordered mesopore channels are connected by smaller disordered pores located in the pore walls. The replica, CMK-3 carbon, showing a 2D hexagonal arrangement of mesorods, provides electrical conduction and an effective transport of fuels and by-products [15]. CMK-3 mesoporous carbon with controlled structure parameter, changeable electronic conductivity and desirable surface functional groups could be obtained. As a result, investigations on the CMK-3 carbon as fuel cell catalyst support have already been carried out in several aspects, such as nanocasting methods [16], selecting various precursors [17] and modifying surfaces [18,19]. As for fuel cell catalyst support, the structural parameters of CMK-3 carbons, such as pore structure, pore size, particle morphology, and surface area, is very important. Increasing the area of electrode/electrolyte interface will enhance the charge transport of Pt catalyst. The porous support materials with suitable pore structure, large surface area can offers an efficient way to increase the interfacial area as well as decrease the electrolyte ion diffusion distance [20,21]. Thus, CMK-3 could be the ideal candidate support material, because it possesses high chemical stability, mesoporous structure, accessible porosity, large surface area and three-dimensional conducting network [22]. The particles of CMK-3 carbon can be made into rods, thin films, and monoliths by nanocasting SBA-15 template with the same morphologies [12,14,23]. During the CMK-3 replication process from an SBA-15, in most cases, the external morphology of the primary particles is preserved, while the internal porous structure of the SBA-15 templates is replicated inversely. Thus, the CMK-3 particles with various morphologies can simply be prepared by nanocast-

L.-B. Kong et al. / Applied Surface Science 256 (2010) 6688–6693

ing SBA-15s with different external morphologies. In terms of materials, pore size and particle morphology of porous materials can have a strong effect on intramolecular diffusions, such as hydrogen absorption/desorption processes [24]. As for the cathode reaction of fuel cells, three factors, including catalyst particles, proton conducting materials and oxygen, are main causations to affect electrocatalytic activity [25]. Therefore, the diffusion and transport of these reaction components could be controlled by adjusting the structural parameters of the carbon supports. In this work, three different CMK-3 carbons are prepared from three different SBA-15 templates. The Pt/CMK-3 (loading ∼20 wt.%) catalysts have been prepared using formaldehyde as the reducing agent. The Pt nanoparticles with similar particle sizes range (1.6–1.9 nm) are uniformly and homogeneously distributed inside the mesopores of CMK-3 supports. The particle sizes, morphologies, surface area and agglomeration degree of the three CMK-3 support are systematically characterized. Experimental data reveal that Pt catalysts with the CMK-3 support of large particle size and long channel lengths possess larger electrochemical active surface area (ECSA) and higher activity toward methanol oxidation than those with the other two supports, suggesting its potential application in DMFC.

2. Experimental

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2.2. Preparation of the Pt/CMK-3 catalyst The CMK-3 supported Pt catalysts (Pt/CMK-3) with a 20 wt.% Pt loading were prepared by an HCHO reduction method, in which 80 mg of CMK-3 which was prepared in the previous description was suspended in 5 ml of deionized water, and 2.6 ml of 0.039 mol/l H2 PtCl6 was stirred with ultrasonic treatment for 20 min first, and then was mechanically stirred for 4 h, during which 10 ml of formaldehyde (37%) was added to the solution dropwise and 2 M NaOH was added to adjust the pH of the solution to about 11. Then the solution was heated up at 85 ◦ C for 1.5 h, and was passed through N2 to remove isolate oxygen as well as organic. Finally, the samples were filtered, cleaned with deionized water, and dried in a vacuum oven at 80 ◦ C for 20 h. 2.3. Structure characterization The obtained products were characterized by transmission electron microscope (JEOL, JEM-2010, Japan), field emission scanning electron microscope (JEOL, JSM-6701F, Japan), X-ray diffraction measurements (Rigaku, D/Max-2400, Japan), and Nitrogen absorption and desorption experiments (Micromeritics, ASAP 2020M, USA). The surface area was calculated by using the Brunauer–Emmett–Teller (BET) equation. Pore size distributions were calculated by the Barrett–Joyner–Halenda (BJH) method using the desorption branch of the isotherm.

2.1. Synthesis and characterization of mesoporous carbon CMK-3 2.4. Electrochemical measurements Mesoporous silica molecular sieve SBA-15 was employed as a hard template to prepare mesoporous carbon CMK-3. Rod-like SBA15 was synthesized, following the method reported by Zhao et al. [26]. 4.0 g of Pluronic P123 was dissolved in 30 g of water and 120 g of 2 M HCl solution. Then, 8.50 g of TEOS was added, and the resulting mixture was stirred for 5 min and then kept at 25 ◦ C for 24 h without stirring (sample S1) and stirring for 24 h (sample S2), respectively. Both of the white milky suspensions were transferred into an autoclave, and were preserved for 2 days at 100 ◦ C. The product was directly filtered off without washing, dried at room temperature, and then calcined in air at 550 ◦ C for 6 h. The platelike SBA-15 was synthesized by using TMOS as a silica source at 303 K under static condition [27]. In a typical synthesis, 2.0 g of P123 was dissolved in 60 g of 2.0 M HCl at 30 ◦ C. To this solution, 3.3 g of TMOS was added under vigorous stirring. After the mixture was stirred for 6 min, it was kept in static condition at the same temperature for 1 day, then transferred into an autoclave and heated at 100 ◦ C for another 24 h. The solid products were collected by filtration and dried at room temperature in air. The resulted powders were calcined at 550 ◦ C for 4 h to obtain SBA-15 (sample S3). The obtained hexagonally mesostructured SBA-15 with three different morphologies (samples S1, S2 and S3) were used as templates to prepare the CMK-3 mesoporous carbon, and they will be referred to as C1, C2 and C3, respectively. CMK-3 was synthesized according to a previous publication [28]. 1 g of SBA-15 was added to a solution obtained by dissolving 1 g of sucrose and 0.147 g of H2 SO4 in 5 g of H2 O. The mixture was placed in a drying oven for 6 h at 100 ◦ C, and then the oven temperature was increased to 160 ◦ C and maintained for 6 h. The silica sample, containing partially polymerized and carbonized sucrose at the present step, was treated again at 100 and 200 ◦ C using the same drying oven after the addition of 0.8 g of sucrose, 0.09 g of H2 SO4 and 5 g of H2 O. The carbonization was completed by pyrolysis with heating to typically 900 ◦ C under vacuum. The carbon–silica composite obtained after pyrolysis was washed with 5 wt.% hydrofluoric acid at room temperature, to remove the silica template. The template-free carbon product thus obtained was filtered, washed with ethanol, and dried at 100 ◦ C.

Electrochemical measurements were carried out using an electrochemical working station (CHI660C, Shanghai, China) and a conventional three-electrode electrochemical cell. A Pt foil served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The working electrode was prepared as follows: A Pt/CMK-3 catalyst of 8 mg and 3.25 ml solution consisting of 20% ethanol, 73.75% deionized water, and 6.25% Nafion (5 wt.%) were mixed and sonicated for 20 min. The slurry of 4 ␮l was coated on the surface of a cleaning glassy carbon electrode (0.071 cm2 in geometric surface area). After drying at 35 ◦ C for 30 min, the working electrode whose loading of Pt was ∼28 ␮g/cm2 was obtained. The ECSA values of the catalysts was determined by the cyclic voltammetry (CV) of the hydrogen absorption/desorption in 0.5 M H2 SO4 solution at room temperature. The cyclic voltammetry and chronoamperometric were collected in 1 M CH3 OH + 0.5 M H2 SO4 at 36 (±1) ◦ C. Cyclic voltammetric measurements of all the catalysts were carried out at a scan rate of 50 mV/s. 3. Results and discussion 3.1. Characterizations of CMK-3 SEM images of CMK-3 samples are presented in Fig. 1. As shown in Fig. 1a–c, the CMK-3 typically exhibits distinctively different morphology and primary particle sizes. The C1 samples shows the a rod-like morphology with the length of a long axis being around 10 ␮m, as confirmed by SEM image observed in large scales (Fig. 1a). Compared with C2 and C3 samples, the long channel lengths and large particle size of the independent C1 particles are not agglomerated or connected. The total surface area of C1 support can be fully utilized to disperse Pt nanoparticles as well as increasing the area of electrode/electrolyte interface. Conversely, the particles of the C2 and C3 are agglomerated, not independent (Fig. 1b and c), so the Pt nanoparticles can only dispersed on limited surface area of C2 or C3 support. Therefore, a large portion of C2 or C3 support surface area is not utilized to disperse Pt nanoparticles effectively. This would lead to a great loss of the inherent advantages of CMK-3

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Fig. 2. XRD patterns of CMK-3s.

XRD analysis is in good agreement with the results from the SEM observation. The nitrogen adsorption–desorption isotherms of the CMK-3 samples and corresponding pore size distributions calculated from the adsorption branches are shown in Fig. 3. The nitrogen adsorption isotherms for C1, C2 and C3 samples exhibit typical type IV isotherms with H1 hysteresis, similar to those for high-quality template SBA-15 [26]. A rapid increase in the adsorption amount occurs in the relative pressure range of 0.4–0.6 in the isotherms due to the

Fig. 1. SEM images of CMK-3s with different morphology: (a) C1, (b) C2 and (c) C3.

support and would eventually reduce its utilization of surface area seriously, suggesting its potential application in DMFC. The XRD patterns of the CMK-3 in Fig. 2 exhibit three wellresolved peaks, which can be assigned to (1 0 0), (1 1 0), (2 0 0) peaks of the 2-dhexagonal p6mm space group, as reported [29]. Compared with C1, the broadening and decreasing of the peaks of C2 should be due to its smallest particle size. The peaks of C2 are still clear, but the width of peak (100) has increased slightly and their intensity is lower than those of C1. The C3 shows only one broad peak, indicative of the structured of hexagonal. It is known that the peak width of XRD has a reciprocal relationship with the area of the coherent scattering domain. Hence, the change in the peak width of the CMK3 samples is consistent with the variation of the particle size. The

Fig. 3. Nitrogen adsorption–desorption isotherms (a) and pore size distribution curves (b) of CMK-3s.

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Table 1 Structural parameters for CMK-3 samples. Sample

Pore size (nm)

BET surface area (m2 /g)

Pore volume (ml/g)

C1 C2 C3

4.80 4.76 4.65

1206 877 916

1.50 1.18 1.15

capillary condensation (Fig. 3a). The mesopore sizes determined by the maximum in pore size distribution curves were 4.80, 4.76, and 4.65 nm for the C1, C2 and C3 samples, respectively (Fig. 3b). The BET surface areas and pore volumes of the three CMK-3 samples are summarized in Table 1. The pore sizes of the three CMK-3s are similar, but the sample of C1 yields the highest BET surface area of 1206 m2 /g, which may be due to its special rod-like morphology with the length of long axis. 3.2. Characterization of the Pt/CMK-3 catalysts The XRD pattern of the 20 wt.% Pt loaded CMK-3 catalysts are shown in Fig. 4. The XRD peaks for all three catalysts exhibit at around 2 = 39.8◦ , 46.3◦ , 67.5◦ and 81.3◦ , which can be indexed as platinum (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections, respectively,

Fig. 4. XRD patterns of the catalysts.

indicating that the catalysts is the face-centered cubic structure of Pt. There is no evidence of peaks related to Pt oxide or hydroxides indicating that the Pt precursor is completely reduced by formaldehyde. The average size of Pt particles is evaluated using

Fig. 5. TEM images of 20 wt.% Pt/CMK-3 samples: (a) Pt/C1, (b) Pt/C2, (c) Pt/C3 and (d) Pt/C3 (with higher magnification).

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L.-B. Kong et al. / Applied Surface Science 256 (2010) 6688–6693 Table 2 Structural and electrochemical characterizations for 20 wt.% Pt/CMK-3 samples. Sample

I (mA/cm2 )

D (nm)

SECSA (m2 /gPt−1 )

Pt/C1 Pt/C2 Pt/C3

3.32 2.87 3.11

1.9 1.6 1.7

208.2 156.4 163.8

I, current density of the hydrogen oxidation peak; D, particle size calculated by Scherrer equation; SECSA , ECSA estimated from CV.

Fig. 6. Cyclic voltammograms of the catalysts in 0.5 M H2 SO4 .

the Scherrer equation ˇ(2) = 0.94/dcos  [24], where d is the average size of the Pt particles,  is the X-ray wavelength (Cu K␣1, k␣1 = 0.15418 nm), ˇ(2) is the half-peak width for Pt(1 1 1) peak in radians, and  is the diffraction angle of the (1 1 1) reflection. Thus the calculated values are 1.9, 1.6 and 1.7 nm for Pt/C1, Pt/C2 and Pt/C3, respectively. When considering the pore size among C1, C2 and C3 and the identical loading amount (20 wt.%) of Pt on the three carbon materials, the size differences of Pt catalysts which are supported by the three carbon materials should be caused by the different particle size of samples, and the Pt nanoparticle support on C1 is in some sort larger than that on other samples. The excellent dispersion of the metal nanoparticles is crucial to high activity of these catalysts toward methanol oxidation. The shape and distribution of Pt nanoparticles form TEM images in Fig. 5 are also observed. It can be found that Pt nanoparticles are well dispersed on three catalysts with a diameter of 1.5–2 nm, and the TEM image of Pt/C1 in Fig. 5d clearly shows that the Pt nanoparticles are uniformly distributed inside the mesopores of CMK-3 support, which is in good agreement with the results from the XRD analysis. 3.3. Electrochemical performance A high surface-to-mass ratio, as provided by small metal nanoparticles, is desirable in order to achieve improved metal utilization [30]. However, small particles themselves are inadequate. The ECSA values matters more than the geometrical area (based on particle size and shape) in determining the intrinsic catalyst activity for carbon-supported electrocatalysts and, in this work, these are measured by hydrogen absorption/desorption. A three-electrode system is used to test the cyclic voltammograms in 0.5 M H2 SO4 at a scan rate of 50 mV/s between −0.2 and 1.0 V. Fig. 6 shows the curves of the Pt/C1, Pt/C2 and Pt/C3 catalysts. The Pt/C1 catalyst exhibits a relatively broader peak with a higher current in the absorption–desorption region than the other catalysts. The ECSA of Pt/CMK-3 is obtained by integrating the total charge corresponding to desorption peak of hydrogen and normalizing with scan rate, Pt loading, and the charge value of 210 ␮C/cm2 for Pt surface [31]. Thus the obtained active surface areas of Pt/CMK3 catalysts are 208.2, 156.4 and 163.8 m2 /g for Pt/C1, Pt/C2 and Pt/C3, respectively, as listed in Table 2. Obviously, the ECSA value of Pt/C1 is much higher than the other two samples. Considering the pore size among C1, C2 and C3, and the identical loading amount (20 wt.%) of Pt on the three carbon materials, the particle size difference of samples has a little effect [17,32]. Therefore, the highest ECSA values of Pt/C1 may be due to the larger available BET surface area of the C1, which could help to increase the proton to the Pt

surfaces bared in the interface of the Pt crystallites of C1 supports when compared with the other two CMK-3 supports. Fig. 7 shows the typical cyclic voltammograms of methanol oxidation under acidic conditions (0.5 M H2 SO4 + 1.0 M CH3 OH) catalyzed by the Pt/CMK-3 catalysts. It can be observed that along the direction of positive scan, the potentials of the oxidation peaks of CH3 OH at three Pt catalyst electrodes are almost the same, located at 0.60–0.62 V. In order to understand the CVs in methanol solution, the conventional reactions are considered [24,33]: Pt + CH3 OH—Pt-CO + 4H+ + 4e− +

Pt + H2 O—Pt-OH + H + e

(1)

−

(2) +

Pt-CO + Pt-OH—2Pt + CO2 + H + e

−

(3)

Thus, the complete electrooxidation of methanol is expressed as CH3 OH + H2 O—CO2 + 6H+ + 6e−

(4)

When the potential arrives at ∼0.30 V in the anodic sweep, methanol molecules will adsorb the Pt with the formation of PtCO (Eq. (1)). At the same time, the H2 O oxidation generates Pt-OH (Eq. (2)). The adsorption and oxidation will be enhanced gradually with the increased potential until the potential reaches the highest value of ∼0.62 V, then the reaction of Eq. (3) will occur around this potential. As the potential increases, more and more reaction products will prevent the deeper oxidation. As a result, the peak will then decrease as shown in Fig. 7. When the potential arrives at ∼0.60 V in cathodes sweep, methanol reoxidation is generated again and the peak climbs to a new high point of ∼0.40 V. The calculated parameters of the electrochemical performance according to Fig. 7 are listed in Table 3. The current density of the methanol oxidation for Pt/C1 reaches a peak value of 37.3 mA/cm2 , which is higher than that of Pt/C2 (33.0 mA/cm2 ) and Pt/C3 (32.6 mA/cm2 ), indicating that Pt/C1 exhibits higher electrochemical activity in methanol oxidation than the other two catalysts do. The ratio of the forward anodic peak current (Ia ) to the reverse anodic peak current (Ib ) can be used to gauge the toler-

Fig. 7. Cyclic voltammograms of the catalysts in 0.5 M H2 SO4 + 1.0 M CH3 OH.

L.-B. Kong et al. / Applied Surface Science 256 (2010) 6688–6693 Table 3 Electrochemical parameters of the catalysts in 0.5 M H2 SO4 + 1.0 M CH3 OH. Sample

Ua (V)

Ia (mA/cm2 )

Ia /Ib

Pt/C1 Pt/C2 Pt/C3

0.60 0.59 0.61

37.3 33.0 32.6

1.09 1.03 0.93

Ua , potential of the methanol oxidation peak; Ia , current density of the methanol oxidation peak; Ib , the reverse anodic peak current density.

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larger electrochemical active surface area (ECSA) and higher activity toward methanol oxidation in sulfuric acid solution than those with the other two supports. The better performance of Pt/C1 catalyst may be due to the independent C1 particles which are not agglomerated or connected. The total surface area of C1 support can be fully utilized to disperse Pt nanoparticles as well as increasing the area of electrode/electrolyte interface. In addition, the higher BET surface area of independent C1 support particles and larger ECSA value of Pt catalyst will provide better structure in favor of the mass transport and the electron transport. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50602020), the Natural Science Foundation of Gansu Province (No. 0803RJZA002) and the Program for Outstanding Young Teachers in Lanzhou University of Technology (No. Q200803). References

Fig. 8. Chronoamperometric curves of the catalysts in 0.5 M H2 SO4 + 1.0 M CH3 OH.

ance of the catalyst to CO. A higher Ia /Ib suggests better oxidation of methanol to carbon dioxide during the anodic scan and less accumulation of carbonaceous residues on the catalyst surface. The ratio of the Pt/C1 (1.07) is much higher than that of the Pt/C2 (1.03) and Pt/C3 (0.93), so the Pt/C1 catalyst has better electrocatatytic performance than the other two catalysts do, suggesting its potential application in DMFC. The better performance of Pt/C1 catalyst may be due to the independent C1 particles which are not agglomerated or connected. The total surface area of C1 support can be fully utilized to disperse Pt nanoparticles as well as increasing the area of electrode/electrolyte interface. In addition, the higher BET surface area of independent C1 support particles and larger ECSA value of Pt catalyst will provide better structure in favor of the mass transport and the electron transport. The chronoamperometric curves of 1.0 M CH3 OH in 0.5 M H2 SO4 solution at the Pt catalyst electrodes at 0.60 V are shown in Fig. 8. The current density at the Pt/C1 catalyst-coated electrode is much higher after 1200 s compared with that at the Pt/C2 and Pt/C3 catalyst-coated electrodes. This further demonstrates that the electrocatalytic activity of the Pt/C1 catalyst for the methanol oxidation is higher than that of the Pt/C2 and Pt/C3 catalysts. 4. Conclusions In summary, three types of the CMK-3 carbon (C1, C2, and C3) supported Pt catalyst were synthesized and their electrocatalytic activities were investigated. The XRD, SEM, TEM and BET specific surface area studies show different structural parameters for three samples. Experimental data reveal that Pt catalysts with the CMK3 support of large particle size and long channel lengths possess

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