Uniformly dispersed pt nanoparticles as fuel-cell catalyst supported onto ordered mesoporous carbon–silica composites

Electrochimica Acta 63 (2012) 318–322

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Uniformly dispersed pt nanoparticles as fuel-cell catalyst supported onto ordered mesoporous carbon–silica composites Yiou Ma, Long Cui, Jianping He ∗ , Tao Wang, Yunxia Guo, Jing Tang, Guoxian Li, Yuan Hu, Hairong Xue, Mingzhu Liu, Xin Sun College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China

a r t i c l e

i n f o

Article history: Received 13 August 2011 Received in revised form 17 November 2011 Accepted 26 December 2011 Available online 2 January 2012 Keywords: Ordered Carbon–silica composites Direct-templating method Electrocatalytic activity

a b s t r a c t Ordered mesoporous carbon–silica composites, were synthesized via direct-templating method using soluble phenolic resins as carbon sources, tetrabutyl orthosilicate as SiO2 precursors and triblock copolymer F127 as the structure-directing agent. The structure and the corresponding electrocatalytic activity of carbon–silica composites with various silica contents are systematically investigated. Textual characterization results show that the ordered mesoporous structure of carbon–silica composite with trace SiO2 is well preserved. The electrochemical properties of the supported catalysts were studied by cyclic voltammogram, revealing that ordered mesoporous carbon–silica composites as the support possess excellent electrochemical performance for hydrogen oxidation. The electrochemical active surface area of carbon–silica supported Pt nanoparticles can achieve 63.8 m2 /g, which is about 7-fold that of non-silica supported Pt catalyst. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to the high surface area, large uniformly distributed pores and pore volumes, tunable pore size, and high porosity, ordered mesoporous carbon (OMC) have been proposed as one of the most attractive candidates for various applications, including catalyst supports [1], energy storage/conversion systems [2], proximal probes [3], optical components [4], absorption, separation, hydrogen absorption system and so on [5–9]. OMC has been successfully synthesized using facile and producible soft-templating methods which are cost-effective and simplify the experimental procedure [10–14]. The mesoporous carbon–silica composites (CS) have been prepared by original tri-constituent co-assembly approach using soluble phenolic resins, silicate oligomer and triblock copolymer F127 as carbon sources, inorganic precursors and structuredirecting agent, respectively [15]. The mesoporous carbon can be obtained with cross-linked frameworks in high surface area, large pore cavities and homogenous pore diameter, followed by etching silica in HF solution. The existence of reinforced silica prevents from structural shrinkage and preserves the ordered mesoporous fabric. The OMC drawn from the CS in Xia and co-workers [16] delivered the specific capacity of 110 F/g as electrode material in supercapacitor and 3083 mAh/g as anode material in lithium ion

batteries. However, this two-step method is complicated and timeconsuming in term of removing the mesoporous silica frameworks. Furthermore, the silica has not been sufficiently utilized in practical applications. It has been reported that appropriate SiO2 additives in the mesoporous carbon can contribute to improving the electrocatalytic performance of carbon materials supported by Pt catalyst [17,18]. This work systematically presents the relationship between the pore structural properties and the corresponding electrocatalytic performance of CS-(x). Here, the carbon–silica composites have been synthesized via the combination of direct-templating and tri-constituent co-assembly methods. The reagent TEOS was used as silica precursor, which is utilized to reinforce the mesoporous carbon framework. The silica template was applied to reinforce the mesoporous carbon framework by its self-assembly with, the low molecular weight resol, followed by calcinations. The Pt nanoparticles were then homogenously loaded on CS composites by microwave techniques, and the electrocatalytic performance and properties were evaluated using cyclic voltammograms. In our work, the synthesis procedure was simplified, and the outstanding electrocatalytic performance was achieved. 2. Experimental 2.1. Synthesis of ordered mesoporous carbon–silica composites

∗ Corresponding author. Tel.: +86 25 52112626; fax: +86 25 52112626. E-mail address: [email protected] (J. He). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.106

The phenolic resins with low molecular weight (<500) as carbon precursor were prefabricated via the method reported by

Y. Ma et al. / Electrochimica Acta 63 (2012) 318–322

OMC

Intensity (a.u.)

the Zhao and co-workers [19]. 1.0 g F127 was dissolved and continuously stirred in 20 g ethanol at 40 ◦ C for 1 h. 1.0 g HCl (0.2 M), 2.5 g phenolic resins and x g TEOS were added into the obtained transparent solution. After subsequent stirring for 2 h, the homogenous solution was evaporated in 5–8 h at ambient and transferred to a vacuum oven for thermpolymerization for 24 h at 100 ◦ C. The carbonizations of as-prepared soft film were carried out in tubular furnace with nitrogen flowing of 200 mL/min at 350 ◦ C for 3 h and further 900 ◦ C for 2 h, with the heating rate of 1 ◦ C/min. The resulting sample was denoted as CS-(x), wherein x designates the adding amount of TEOS in the process of preparation.

319

CS-(0.25)

CS-(0.5)

2.2. Preparation of catalyst The microwave method was adopted to load Pt nanoparticles on the CS-(x) using the LG MG-5021MW1 microwave oven.1.4 ml of 0.038 mol/L hexachloroplatinic acid was dissolved in ethylene glycol, followed by adjusting its pH to 9 with 2.5 mol/L NaOH. 40 mg of the prepared CS-(x) was dispersed into the above solution by ultrasonic vibrating for 30 min. The suspension was obtained when the precursors were heated in the microwave oven in 60 s with the heating power of 700 W. After cooling down to room temperature, the deposition of Pt nanoparticles was accelerated by adjusting the pH to 3 with 0.2 mol/L HCl. The black mixtures were achieved after centrifugation and washing with acetone and distilled water several times. The resulting Pt/CS-(x) catalyst was gained followed by being heated in vacuum oven for 12 h at 80 ◦ C, and the weight ratio of Pt in Pt/CS-(x) is 20%. 2.3. Characterization The ordered pore property of carbon framework and crystalline characteristics of Pt particles in CS-(x) were measured by small-angle and large angle X-ray diffraction on th Bruker D8 ADVANCE diffractometer, respectively. Morphologies, and particle sizes of the samples were observed on a FEI Tecnai G2 transmission electron microscope (TEM) at an acceleration voltage of 200 kV. The properties of pore system in CS-(x) were estimated by the nitrogen adsorption–desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 system. The specific surface area was calculated using the Bruauer–Emmett–Teller (BET) equation, and the mesopore size distribution was determined by the Barret–Hoyner–Halenda (BJH) method using the adsorption branch. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the contents of Pt on the catalysts on Jarrell-Ash 1100 analyzer. 2.4. Electrochemical measurement The working electrode was composed of 5.0 mg Pt/CS-(x), 1.0 ml ethanol and 50 ␮L Nafion solution with the weight ratio of 5 wt%, followed by ultrasonic-vibrating for 30 min. The slurry was coated onto the polished glassy carbon substrate to form a thin layer of ca. 0.1256 cm2 in geometrical area and then dried at 80 ◦ C in 2 h. A conventional three-electrode system was performed in electrocatalytic measurements using a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The cyclic voltammograms were carried out in 0.5 M H2 SO4 between −0.22 and 0.98 V at a scan rate of 20 mV/s on a Solartron 1287 electrochemical measurer. The linear sweep voltammograms (LSVs) for oxygen reduction reaction (ORR) is measured with a glassy carbon rotating disk electrode test system in O2 -saturated 0.5 M H2 SO4 solution (5 mV/s, 2600 rpm).

CS-(1) 0

2

4

6

8

ο

2θ ( ) Fig. 1. Small-angle XRD patterns of OMC and CS-(x) composites.

3. Results and discussion 3.1. Physicochemical characterization of the carbon support The well-resolved small-angle XRD patterns in Fig. 1 reveals that the as-prepared CS-(x) can possess ordered structure with the assistance of TEOS as comparison to OMC. The weak diffraction peak of CS-(1) is probably attributed to the excessive adding amount of TEOS. Fig. 2 displays the N2 sorption isotherms and BJH pore size distribution of CS-(x) composites, and the corresponding parameters are listed in Table 1. The isotherms in Fig. 2(a) present typical Langmuir IV curves and H1 hysteresis with a sharp capillary condensation step at middle relative pressures, indicating the outstanding mesoporous characteristics. The pore size distributions are tunable in CS-(x) by modulating the adding mount of TEOS during preparation. The surface area and mesoporosity are both declined with the regularly increasing weight ratio of SiO2 in CS-(x), which can be attributed to the established mesoporous SiO2 framework in composite structure. The specific surface areas, total pore volume and porosity of CS-(0.5) composites are 485.6 m2 /g, 0.33 cm3 g−1 and 54%, respectively. The carbon precursor (phenol/formaldehyde) was introduced into the synthesis system, comprising hard template silica and soft template F127, and further self-assemblage on the basis of mixing fully. Wherein phenol/formaldehyde possess hydroxyl groups, which can interact with that of soft template F127. Then the soft template F127 was slowly and thoroughly removed by heating at 350 ◦ C under nitrogen. Upon the calcination at 900 ◦ C in N2 flow, polymers were transformed into carbon. During this procedure, a phase of intergradation existed. Without the effect of the silica template, carbon may be subsequently arrayed as the intergradation. The polymer shrank continuously along with the carbonization. The presence of silica template inhibited the shrinkage of the polymer attached to its surface due to the strong interaction between it and

Table 1 Textural characteristics for OMC and CS-(x) composites. Sample

SBET (m2 /g)

Vtotal (cm3 g−1 )

OMC CS-(0. 25) CS-(0.5) CS-(1)

603.4 713.7 656.9 381.6

0.43 0.60 0.57 0.35

Rmeso (%) 85% 76% 75% 85%

D (nm) 2.80 3.39 3.49 3.63

SBET : the specific surface areas; Vtotal : total pore volume; Rmeso : porosity; D: pore diameter.

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Volumn absorbed (a.u.)

a

Pt(111)

Intensity (a.u.)

OMC

CS-0.25 CS-0.5 CS-1

0.0

0.2

0.4

0.6

0.8

1.0

Pt(200) Pt(220)

C(002)

Pt/OMC

Pt/CS-(0.5)

Relative pressure P/P0

20

40

0.10 b

3 -1 -1 dV/dD cm g nm

Pt(311)

60

2

80

o

Fig. 4. Wide-angle XRD patterns of Pt/OMC and Pt/CS-(0.5) composites.

0.08

3.2. Physicochemical characterization of the catalysts

0.06 OMC

0.04

CS-0.25

0.02 CS-0.5 CS-1

0.00 0

2

4

6

8

10

12

14

16

18

20

pore diameter (nm) Fig. 2. Nitrogen adsorption–desorption isotherms (a) and the BJT pore diameter distribution curves (b) of OMC and CS-(x) composites.

the polymer, as can be clearly seen from the contrastingly TEM images. Then one part of the polymer was adhered to silica and the other part was ruptured to form the core of the framework. The TEM images of OMC and CS-(0.5) composites were captured in Fig. 3. The high ordered mesoporous pore arrangements can also be viewed in CS-(0.5) with the pore size of 4 nm. The shrinkage of mesoporous framework in CS-(0.5) is assigned to the structural occupation of SiO2 in this nanocomposites.

The CS-(x) composites were used to support Pt nanoparticles as catalyst for hydrogen electro-oxidation fuel cell. The XRD patterns of Pt/OMC and Pt/CS-(0.5) in Fig. 4 illustrate the good crystallinity and pure phase of Pt catalyst. All intense peaks are indexed to the face-centered cubic structure [20,21]. The broad diffraction peaks located at 2Â of 23◦ of Pt/CS-(x) are the amorphous carbon peaks. It has been reported that the distribution and particle size of the Pt catalysts dispersed on carriers are crucial to the outstanding electrocatalytical performance and properties of hydrogen electrooxidation [22,23]. There are few metal particles settled on the carbon surface without adding SiO2 from TEM images in Fig. 5(a). Active atoms were lost a lot in this way, and thus the subsequent performances in hydrogen electro-oxidation were dissatisfactory. As shown in Fig. 5(b), Pt/CS-(0.5) exhibits uniform dispersion of Pt nanoparticles on the framework of CS-(0.5) with no agglomerations, and the Pt particles possess an average diameter of ∼4 nm. 3.3. Electrochemical performances of the catalyst The three-electrode system was used to evaluate the practical electrocatalytical performance of Pt/CS-(x) by cyclic voltammograms in the voltage range of −0.22 to 0.98 V at a scan rate

Fig. 3. TEM images of samples: (a) OMC and (b) CS-(0.5) calcined at 900 ◦ C.

Y. Ma et al. / Electrochimica Acta 63 (2012) 318–322

321

Fig. 5. TEM images of Pt/OMC Pt/CS-(0.5) composites.

Table 2 The analytical parameters of Pt/CS-(x) in sulfuric acid. Samples

LPt (mg)

I (mA/cm2 )

QH (mC)

SEA (m2 /g)

Pt/CS-(0) Pt/CS-(0.25) Pt/CS-(0.5) Pt/CS-(1)

0.0238 0.0238 0.0238 0.0238

0.96 1.34 2.90 1.46

1.13 1.54 3.19 1.043

22.5 30.76 63.85 20.86

LPt : the actual Pt loading on glassy carbon substrate; I: current density of the hydrogen oxidation peak; QH : total charge; SEA : electrochemical active surface area.

of 20 mV/s in 0.5 M H2 SO4 under ambient conditions. The electrochemically active surface of Pt particles is considered to reflect the intrinsic electrocatalytic activity [24]. Calculated by the formula [25], S = Q/LPt QH , where Q is the total charge, LPt is the actual Pt loading on glassy carbon substrate, and QH is assumed to be 0.21 mC/cm2 corresponding to a surface density of 1.3 × 105 atom/cm2 of Pt. The calculated electrochemical parameters are enumerated in Table 2. The CV curves of the Pt/CS-(x) in Fig. 6 indicate that Pt/CS-(0.5) shows the highest electrochemical activity. Both the reduction and oxidation current peaks of the electro-oxidation to hydrogen are larger than that of other samples. Its electrochemical active area is 63.8 m2 /g, which is about 7 times that of Pt/CS-(0), and its current density of oxidation peak to hydrogen is as high as 2.9 mA/cm2 . The improved electrocatalytical activity of CS-(0.5) can be attributed to three reasons as

0.6

follows: (1) SiO2 and Pt has cooperative catalytic activity [26–29]; (2) a large quantity of Pt nanoparticles and high metal dispersion contribute to the structure sensitivity of the catalysis process (Fig. 6); (3) the parallel arranged 1-dimentional pore channels, as observed in TEM images, facilitate the mass and electron transport during the reaction, thus producing high active performance for hydrogen electro-oxidation. Maybe silicon can improve the catalytic graphitization of carbons [30], thus it can improve the catalytic graphitization of carbons. The catalytic activity of Pt/CS(0.25) is similar to Pt/CS-(0), and Pt/CS-(0.5) exhibits the optimum catalytic performance. The three-electrode system was used to evaluate the practical electrocatalytical performance of Pt/CS-(x) by linear sweep voltammetry for oxygen reduction reaction in the voltage range of 0.85–0 V at a scan rate of 5 mV/s in 0.5 M H2 SO4 under ambient conditions. As shown in Fig. 7, with SiO2 increases, the diffusion-limited current density gradually increased. When the content of SiO2 reached 0.5, the diffusion-limited current density reaches the maximum, which is 1.2 mA. While continuing to increase the ratio, the diffusionlimited current density has a sharp decline, less than 1 mA. So the optimal dosage of SiO2 is 0.5. Because the silicon in Pt/CS-(x) prevents the dissolution of Pt metal particles as well as the migration and agglomeration of Pt metal particles on the supports, thereby increasing the effective surface area of catalyst, the reaction rate increases. Due to the conductivity of silicon is less than C, when

Pt/CS-(0.5) Pt/CS-(1)

0.0

Pt/CS-(0.25)

0.4

Pt/OMC Pt/CS-(0.25) Pt/CS-(0.5) Pt/CS-(1)

Current (mA)

Current (mA)

0.2

0.0 Pt/OMC

-0.2

-0.4

-0.5

-1.0

-0.6

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs. SCE) Fig. 6. Cyclic voltammograms of and Pt/OMC and Pt/CS-(x) in sulfuric acid.

-1.5 0.0

0.2

0.4

0.6

Potential (V vs. SCE) Fig. 7. Polarization curves of oxygen reduction in sulfuric acid.

0.8

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the content of silicon is too high, the entire material conductivity decreased, thus greatly affecting the performance of oxygen reduction of Pt. In addition, Pt/CS-(x) has the maximum diffusion-limited current density, but compared to Pt/OMC, the initial oxygen reduction reaction occurs at lower potential, which may be due to the formation of Pt nanoparticles on the carrier. Therefore, in order to make the material to achieve optimal performance of oxygen reduction, the reaction onset potential should be further improved. 4. Summary This work presents a novel preparation of ordered mesoporous carbon–silica composites with 1-dimentional pore channel via direct-templating method, followed by Pt nanoparticles loaded as catalyst for hydrogen electro-oxidation in fuel cells by microwave method. The highly ordered parallel 1-dimentional pore channel structure can be preserved in carbon–silica composites. The Pt/CS-(0.5) exhibits considerably improved electrocatalytical performance in sulfuric acid solution, which can be attributed to the excellent dispersion of Pt nanoparticles, and optimum structure favoring the mass and electron transport. The electrochemical active areas of Pt/CS-(0.5) can reach as high as 63.8 m2 /g. Overall, ordered mesoporous carbon–silica composites have potential applications as catalysts support for fuel cell technology. Acknowledgement The authors appreciate the financial support of the National Natural Science Foundation (50871053). References [1] L.S. Schadler, S.C. Giannaris, P.M. Ajayan, Appl. Phys. Lett. 73 (1998) 3842. [2] M.B. Shiflett, H.C. Foley, Science 285 (1999) 1902.

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