Electrochemical activity and durability of platinum nanoparticles supported on ordered mesoporous carbons for oxygen reduction reaction

Electrochemical activity and durability of platinum nanoparticles supported on ordered mesoporous carbons for oxygen reduction reaction

international journal of hydrogen energy 35 (2010) 8149–8154 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Electr...

1MB Sizes 3 Downloads 42 Views

international journal of hydrogen energy 35 (2010) 8149–8154

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Electrochemical activity and durability of platinum nanoparticles supported on ordered mesoporous carbons for oxygen reduction reaction Shou-Heng Liu a, Chien-Chang Chiang a,b, Min-Tsung Wu a,b, Shang-Bin Liu a,b,* a b

Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 10617, Taiwan Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan

article info


Article history:

A facile procedure for synthesizing platinum nanoparticles (NPs) studded in ordered mes-

Received 1 September 2009

oporous carbons (Pt–OMCs) based on the organic–organic self-assembly (one-pot) approach

Received in revised form

is reported. These Pt–OMCs, which can be easily fabricated with controllable Pt loading, were

25 December 2009

found to possess high surface areas, highly accessible and stable active sites and superior

Accepted 30 December 2009

electrocatalytic properties pertinent as cathode catalysts for hydrogen–oxygen fuel cells. The

Available online 2 February 2010

enhanced catalytic activity and durability observed for the Pt–OMC electrocatalysts are attributed to the strengthened interactions between the Pt catalyst and the mesoporous


carbon that effectively precludes migration and/or agglomeration of Pt NPs on the carbon

Pt nanocatalyst


Mesoporous carbon

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Electrocatalytic properties Fuel cell Oxygen reduction reaction



Highly dispersed noble metal (Pt, Ru) nanoparticles (NPs) supported on conductive materials with high surface areas, such as carbon blacks [1,2], carbon nanotubes [3–5], and mesoporous carbons [6–9] are pertinent anodic/cathodic electrocatalysts for direct methanol fuel cells (DMFCs) and proton-exchange membrane fuel cells (PEMFCs). Nonetheless, the durability of the Pt on carbon catalysts, especially those for oxygen reduction reaction (ORR) at cathode, remains as one of the most critical issues to be resolved for practical commercialization of DMFCs/PEMFCs [10–12]. Alloying of Pt with a second metal is one of the most common strategies invoked for the improvement of catalyst stability [13–17]. However, these bifunctional

catalysts were still handicapped by severe dissolution and diffusion of alloy NPs into the PEMFC membrane, leading to a decrease in proton conductivity [18]. As such, strengthening of Pt–carbon support interaction may be a more convenient, cost-down effective, and hence favorable way to enhance the catalytic properties and stabilities of the electrocatalysts [10]. Previously, we developed a novel method to synthesize mono- (Pt) [19] and bifunctional (PtRu) [20] NPs supported on ordered mesoporous carbons (OMCs) based on the pyrolysis of co-fed carbon sources and Pt/Ru precursors in a mesoporous silica template. Subsequent polymerization, carbonization, silica template removal, and proper washing and drying, thus leading to the formation of well-dispersed and highly stable Pt/PtRu NPs (ca. 2–3 nm) on OMCs. The Pt- and PtRu–OMC

* Corresponding author. Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 10617, Taiwan. Tel.: þ886 2 23668230; fax: þ886 2 23620200. E-mail address: [email protected] (S.-B. Liu). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.183


international journal of hydrogen energy 35 (2010) 8149–8154

catalysts were found to possess superior electrocatalytic performances and long-term durabilities compared to most commercially available anodic electrocatalysts (Pt/PtRu on activated carbons). Nevertheless, the synthesis routes invoked in those electrocatalysts were still circumscribed by the ineffectiveness in material cost and preparation time, which further limit their practical industrial applications. Recently, fabrication of OMCs by cross-linking phenolic resins in the presence of various self-assembled block-copolymer templates, followed by pyrolysis at moderate temperature, have been reported [21,22]. The resultant OMCs were found to possess large surface areas with ordered mesopores and structure matrices abundant with hydroxyl groups that facilitate further surface functionalization by dispersion/ loading of catalysts in a controllable fashion. We report herein a facial procedure to synthesize Pt NPs studded in the pore walls of OMC (denoted as Pt–OMC-x, where x represents the Pt loading) by organic–organic self-assembly (one-pot) approach. These Pt–OMC-x so fabricated were found to possess not only highly stable, well-dispersed Pt NPs but also superior catalytic activities and durabilities during ORR, rendering potential applications as cathodic electrocatalysts for DMFCs and PEMFCs.




Catalysts preparation

The OMC materials were synthesized by dissolving 1.6 g of resorcinol (98%, Acros) and 2.5 g of F127 tri-block copolymer (Sigma) in 18.0 g of ethanol and water mixture (1:1 vol%). After the complete dissolution of the solid ingredients under stirring at room temperature (298 K), 0.2 g of HCl (37 wt.%) was added into the solution as a catalyst, then, the mixture solution was further stirred for 2 h. Subsequently, 2.5 g of formaldehyde (37 wt.%) was slowly introduced drop-wise into the above solution. The resultant solution was kept for 4 days after which two separate layers were readily observed. After discarding the upper solution layer, the lower polymer-rich layer was cured at 373 K for 2 d, followed by a gradual carbonization treatment (1 K/min) under vacuum to 1123 K and maintained at the same temperature for additional 3 h. Similar procedures were adopted for the syntheses of Pt–OMC-x, except that various amounts of Pt precursor (H2PtCl6; 39%, Acros) were added after the addition of HCl. To activate the Pt–OMC-x electrocatalysts, a thermal pre-treatment of Pt–OMC-4.5 sample was performed at a temperature of 753 K under air atmosphere, followed by reduction with H2 at 673 K to yield the Pt–OMC-4.5T. The Pt/OMC-8.7I sample was synthesized by using a conventional post-synthesis impregnation method described elsewhere [23].


Characterization methods

The amounts of platinum in various samples were analyzed by thermal gravimetric analyzer (TGA, Netzsch TG209). All powdered X-ray diffraction (PXRD) patterns were recorded on a PANalytical (X’Pert PRO) diffractometer using CuKa radiation (l ¼ 0.1541 nm). Nitrogen adsorption/desorption isotherm

measurements were carried out at 77 K on a Quantachrome Autosorb-1 volumetric adsorption analyzer. Transmission electron microscopy (TEM) images were obtained at room temperature using an electron microscope (JEOL JEM-2100F) that has a field-emission gun at an acceleration voltage of 200 kV. The dispersions of platinum on various samples were measured by hydrogen chemisorption at 323 K on a chemisorption analyzer (Micromeritics, AutoChem II).


Evaluation of electrocatalytic performance

Electrocatalytic tests were performed in a single compartment glass cell with a standard three-electrode configuration. A glassy carbon electrode with a diameter of 5 mm was used as a working electrode and a saturated Ag/AgCl electrode and a platinum wire were used as reference and counter electrodes, respectively. The glossy carbon thin-film electrode was prepared by the following steps: first, ca. 5 mg of Pt-loaded carbon sample was added into 2.5 mL deionized water, followed by ultrasonication treatment for 0.5 h. Then, ca. 20 mL of the resultant suspension mixture was withdrawn and injected onto the glassy carbon electrode, followed by drying in air at 333 K for 1 h. Finally, 20 mL of 1% Nafion (DuPont) solution was added as a binder under N2 environment. Electrocatalytic activity measurements of various Pt–OMC-x and Pt/OMC-8.7I samples were performed on a potentiostat (Autolab, PGSTAT30). Oxygen reduction reaction was evaluated by the linear sweep voltammetry (LSV) technique. The 0.1 M H2SO4 electrolyte was saturated with ultrahigh purity oxygen for at least 0.5 h. For rotating disk electrode (RDE) voltammetric experiments, the polarization curves were obtained between 0 and 1 V and varied rotating speeds from 400 to 3000 rpm with a scanning rate of 5 mV/s at room temperature. All potentials in this study are referred to the reversible hydrogen electrode (RHE).


Results and discussion

The influence of Pt loading on pore properties and structures of various samples was studied by powdered XRD. As shown in Fig. 1a, the OMC sample exhibits a (100) diffraction peak at  2q ¼ 0.83 , indicating the existence of long-range structural ordering with 2D hexagonal symmetry. Upon introducing the Pt precursor during synthesis, a notable decrease in (100) peak 2q values with increasing Pt loading was observed, revealing that the incorporation of Pt led to distortion of mesostructures of the carbon support [24]. The large-angle XRD patterns (Fig. 1b) observed for Pt–OMC-x and Pt/OMC-8.7I (prepared by conventional post-synthesis impregnation method) samples     showed pronounced peaks at 2q ¼ 39.8 , 46.2 , 67.8 , and 81.3 (whose intensities increase in accordance with increasing Pt loading), revealing the presence of crystalline platinum metal Pt(0) with face centered cubic (fcc) lattice. Based on the Scherrer formula, the average size of Pt deduced from the (220) peak of the XRD profiles in Pt–OMC-4.5, Pt–OMC-6.4 and Pt/ OMC-8.7I was found to be 3.6, 5.2 and 5.0 nm, respectively, coinciding with those determined from TEM images (see Fig. 2). All N2 adsorption/desorption isotherms displayed in Fig. 3a obtained from OMC, Pt–OMC-x and Pt/OMC-8.7I samples showed typical type-IV isotherms with well-defined

international journal of hydrogen energy 35 (2010) 8149–8154



(111) (200)


(220) (311)

Pt-OMC-6.4 Pt-OMC-4.5T Pt-OMC-4.5 Pt-OMC-1.6 OMC 1



4 5 6 2θ (degree)



20 30 40 50 60 70 80 90 2θ (degree)

Fig. 1 – (a) Small- and (b) large-angle powdered XRD patterns of OMC, various Pt–OMC-x and Pt/OMC-8.7I samples.

hysteresis loops, which are typical mesoporous structure. The pore size distributions of OMC and Pt–OMC-x samples are narrow, which center at ca. 5 nm (see Fig. 3b). Accordingly, their structural parameters are summarized in Table 1 Upon increasing Pt loading, gradual decreases in surface area (S ) and pore volume (V) were evident for Pt–OMC-x, indicating a progressive increase in carbon wall thickness. This


observation is in accordance with the gradual increase in unit cell parameter (a) with Pt loading, suggesting that progressive incorporation of Pt metal onto to carbon pore walls tend to suppress the framework shrinkage during the carbonization process at high temperature [24]. As shown in Fig. 2, the TEM image of the Pt–OMC-4.5 sample exhibits a uniform mesopores of 5 nm with long-range ordering and a pore wall thickness of ca. 7 nm. However, upon further increasing Pt loading led to x ¼ 6.4, notable perturbation of mesoporous framework was evident, as also evidenced by the XRD results (Fig. 1a). In comparison, the Pt/OMC-8.7I sample prepared by impregnation method led to the formation of a much broader distribution of Pt particle size (Fig. 2) and hence is less desirable in terms of the amount of noble metals required to obtain the same catalytic activity per unit mass. To evaluate the electrocatalytic activities of Pt–OMC-4.5, Pt/ OMC-8.7I, and the commercially available JM-Pt/C (John-Matthey; 10 wt.% Pt on Vulcan XC-72) catalysts during ORR, additional LSV test in O2 saturated 0.1 M H2SO4 solution were performed. However, Pt–OMC-4.5 catalyst was found to be inactive for ORR, as shown in Fig. 4. This is ascribed due to the low Pt dispersions observed for Pt–OMC-x samples (Table 1). Most likely, the Pt NPs may be buried in the carbon framework during the self-assembly processes. To unravel this problem, additional thermal treatment of the Pt–OMC-4.5 sample was performed based on an additional thermal gravimetric analysis (Fig. 5). Accordingly, an optimal thermal pre-treatment temperature of 753 K (corresponding to a carbon burn-off

Fig. 2 – TEM images of OMC, various Pt–OMC-x and Pt/OMC-8.7I samples.


international journal of hydrogen energy 35 (2010) 8149–8154



-40 I (A g-1 Pt)

Volume Adsorbed (cm3/g, STP)




OMC Pt-OMC-1.6 Pt-OMC-4.5 Pt-OMC-6.4 Pt-OMC-4.5T Pt/OMC-8.7I









0.0 0.0






Relative pressure (P/P0)



0.4 0.6 E (V) vs. RHE



Fig. 4 – Polarization curves for various Pt–OMC-x, Pt/OMC, and commercial JM-Pt/C catalysts.


0.012 OMC Pt-OMC-1.6 Pt-OMC-4.5 Pt-OMC-6.4 Pt-OMC-4.5T Pt/OMC-8.7I



0.000 0







Pore diameter (nm)

Fig. 3 – (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution of OMC, Pt–OMC-x and Pt/OMC samples.

weight loss of ca. 23 wt.%) were chosen for Pt–OMC-4.5 sample. Subsequently, the thermally treated sample was subjected to reduction with H2 at 673 K to yield the Pt–OMC4.5T catalyst. As can be seen in Fig. 1a, the ordered structures of Pt–OMC-4.5T catalyst remain practically intact after the

aforementioned thermal and reduction treatments. As expected, a notable increase in Pt dispersion of the Pt–OMC4.5T catalyst was also observed after the partial carbon decomposition treatment (Table 1), leading to a remarkable enhancement in their ORR activities (Fig. 4). That the Pt–OMC4.5T catalyst exhibits an electrocatalytic activity surpassing that of the Pt/OMC-8.7I, indicating that the former possesses more well-dispersed Pt NPs in the mesoporous channels. Moreover, the ORR activity of Pt–OMC-4.5T is comparable to that of commercial JM-Pt/C. The electrocatalytic stabilities of the Pt–OMC-4.5T and Pt/OMC-8.7I catalysts were also evaluated by repeated LSV tests performed using O2 saturated 0.1 M H2SO4 solution at room temperature. As shown in Fig. 6, ca. 30% of current density (measured at 0.8 V vs. RHE) was lost for Pt/OMC-8.7I, however, a nearly constant current density was observed for Pt–OMC-4.5T sample over the total period of ca. 10 h, revealing the enhanced stability of catalysts prepared by one-pot synthesis during ORR. It is noted that the Pt NPs in theses Pt–OMCs are also thermally stable and sustainable


Table 1 – Physical Properties of OMC, Pt–OMC-x, and Pt/ OMC Samples sample

Pt (wt%)

aa (nm)

Sb (m2g-1)

dc (nm)

Vd (cm3g-1)

De (%)

OMC Pt–OMC-1.6 Pt–OMC-4.5 Pt–OMC-6.4 Pt–OMC-4.5Tf Pt/OMC-8.7Ig

– 1.6 4.5 6.4 4.5 8.7

12.1 12.4 14.9 – 14.9 –

700 634 616 338 619 407

5.6 4.7 4.6 5.4 4.4 5.0

0.66 0.54 0.47 0.36 0.50 0.37

– 2 5 4 25 16

a b c d e f g

Unit cell parameter. BET surface area. BJH pore diameter. Total pore volume. Pt dispersion measured by H2 chemisorption at 323 K. Obtained after mild thermal and reduction treatments (see text). Prepared by conventional post-synthesis impregnation method.

Weight Loss (wt%)





0 0





Temperature (oC ) Fig. 5 – TGA curves (solid lines) and the corresponding profiles for the first derivative of weight loss with respective to temperature (dashed lines) of Pt–OMC-4.5 electrocatalysts.

international journal of hydrogen energy 35 (2010) 8149–8154


peroxide radicals whose presence prone to attack the carbon support and the proton-exchange membrane, resulting undesirable degradation of the fuel cell [25].

20 60 Pt-OMC-4.5T

40 Pt/OMC-8.7I




0 0





I (A cm-2 g-1 Pt)

I (A cm-2 g-1 Pt)



Cycle number

Fig. 6 – Durability tests observed during ORR for the Pt–OMC-4.5T and Pt/OMC-8.7I catalysts.

to repeated oxidation and reduction cycles at elevated temperatures. The ORR performance of the Pt–OMC-4.5T catalyst was further studied by RDE voltammetry (see Fig. 7). Accordingly, the number of electron (n) involved during ORR was deduced to be 3.7, which is close to the theoretical value for fourelectron reduction, excluding the existence of hydrogen



In summary, the physical/chemical properties and electrocatalytic performance of ordered mesoporous carbon supported Pt electrocatalyst synthesized by the self-assembly method are reported. It is found that the Pt–OMCs so fabricated possess not only well-dispersed and highly stable Pt nanoparticles but also superior catalytic activities and durabilities, which may be attributed to the strengthened interactions between the Pt catalyst and the mesoporous carbon that effectively precludes migration and/or agglomeration of Pt NPs on the carbon support. These Pt–OMCs should render practical cost-down effective commercial applications in hydrogenenergy related areas, for examples, as adsorbents for hydrogen fuel storage and as supported electrocatalysts for PEMFCs and DMFCs. Further efforts have been undertaken in fabricating an MEA (membrane electrode assembly) for possible single cell testing.



The supports of this work by the National Science Council, Taiwan (NSC95-2113-M-001-040-MY3 and NSC98-2113-M-001017-MY3) are gratefully acknowledged.


j (mA cm-2)


rpm -2



800 1200 1600 2000 2500 3000


-4 0.0


0.4 0.6 E (V) vs. RHE



0.7 0.6

i-1 (mA-1 cm2)


Slope = 2.37 n = 3.7

0.5 0.4 V 0.3 V 0.2 V

0.4 0.3 0.2 0.04









-1/2 1/2


s )

Fig. 7 – (a) RDE voltammograms and (b) their corresponding Koutecky-Levich plots of the Pt–OMC-4.5T electrocatalyst.

[1] Shanahan PV, Xu LB, Liang CD, Waje M, Dai S, Yan YS. Graphitic mesoporous carbon as a durable fuel cell catalyst support. J Power Sources 2008;185:423–7. [2] Nores-Pondal FJ, Vilella IMJ, Troiani H, Granada M, de Miguel SR, Scelza OA, et al. Catalytic activity vs. size correlation in platinum catalysts of PEM fuel cells prepared on carbon black by different methods. Int J Hydrogen Energy 2009;34:8193–203. [3] Wang X, Li WZ, Chen ZW, Waje M, Yan YS. Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell. J Power Sources 2006; 158:154–9. [4] Guo SJ, Dong SJ, Wang E. Gold/platinum hybrid nanoparticles supported on multiwalled carbon nanotube/silica coaxial nanocables: Preparation and application as electrocatalysts for oxygen reduction. J Phys Chem C 2008;112:2389–93. [5] Jafri RI, Sujatha N, Rajalakshmi N, Ramaprabhu S. Au-MnO2/ MWNT and Au-ZnO/MWNT as oxygen reduction reaction electrocatalyst for polymer electrolyte membrane fuel cell. Int J Hydrogen Energy 2009;34:6371–6. [6] Chang H, Joo SH, Pak CH. Synthesis and characterization of mesoporous carbon for fuel cell applications. J Mater Chem 2007;17:3078–88. [7] Kim HT, You DJ, Yoon HK, Joo SH, Pak CH, Chang H, Song IS. Cathode catalyst layer using supported Pt catalyst on ordered mesoporous carbon for direct methanol fuel cell. J Power Sources 2008;180:724–32.


international journal of hydrogen energy 35 (2010) 8149–8154

[8] Fang B, Kim JH, Lee C, Yu JS. Hollow macroporous core/ mesoporous shell carbon with a tailored structure as a cathode electrocatalyst support for proton exchange membrane fuel cells. J Phys Chem C 2008;112:639–45. [9] Ambrosio EP, Francia C, Manzoli M, Penazzi N, Spinelli P. Platinum catalyst supported on mesoporous carbon for PEMFC. Int J Hydrogen Energy 2008;33:3142–5. [10] Shao YY, Yin GP, Gao YZ. Understanding and approaches for the durability issues of Pt-based catalysts for PEM fuel cell. J Power Sources 2007;171:558–66. [11] Yu XW, Ye SY. Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC-Part I. Physico-chemical and electronic interaction between Pt and carbon support, and activity enhancement of Pt/C catalyst. J Power Sources 2007;172:133–44. [12] Chung CG, Kim L, Sung YW, Lee J, Chung JS. Degradation mechanism of electrocatalyst during long-term operation of PEMFC. Int J Hydrogen Energy 2009;34:8974–81. [13] Colo´n-Mercado HR, Popov BN. Stability of platinum based alloy cathode catalysts in PEM fuel cells. J Power Sources 2006;155:253–63. [14] Wang JJ, Yin GP, Wang GJ, Wang ZB, Gao YZ. A novel Pt/Au/C cathode catalyst for direct methanol fuel cells with simultaneous methanol tolerance and oxygen promotion. Electrochem Commun 2008;10:831–4. [15] Garcı´a-Contreras MA, Ferna´ndez-Valverde SM, Vargas´ ngelesGarcı´a JR, Corte´s-Ja´come MA, Toledo-Antonio JA, A Chavez C. Pt, PtCo and PtNi electrocatalysts prepared by mechanical alloying for the oxygen reduction reaction in 0.5 M H2SO4. Int J Hydrogen Energy 2008;33:6672–80. [16] Chen S, Gasteiger HA, Hayakawa K, Tada T, Shao-Horn Y. Platinum-alloy cathode catalyst degradation in proton exchange membrane fuel cells: Nanometer-scale compositional and morphological changes. J Electrochem. Soc 2010;157:A82–97.

[17] Gupta G, Slanac DA, Kumar P, Wiggins-Camacho JD, Wang XQ, Swinnea S, et al. Highly stable and active Pt-Cu oxygen reduction electrocatalysts based on mesoporous graphitic carbon supports. Chem Mater 2009;21:4515–26. [18] Okada T, Ayato Y, Satou H, Yuasa M, Sekine I. The effect of impurity cations on the oxygen reduction kinetics at platinum electrodes covered with perfluorinated ionomer. J Phys Chem B 2001;105:6980–6. [19] Liu SH, Lu RF, Huang SJ, Lo AY, Chien SH, Liu SB. Controlled synthesis of highly dispersed platinum nanoparticles in ordered mesoporous carbon. Chem Commun 2006:3435–7. [20] Liu SH, Yu WY, Chen CH, Lo AY, Hwang BJ, Chien SH, Liu SB. Fabrication and characterization of well-dispersed and highly stable PtRu nanoparticles on carbon mesoporous material for applications in direct methanol fuel cell. Chem Mater 2008;20:1622–8. [21] Zhang FQ, Meng Y, Gu D, Yan Y, Chen ZX, Tu B, et al. An aqueous cooperative assembly route to synthesize ordered mesoporous carbons with controlled structures and morphology. Chem Mater 2006;18:5279–88. [22] Liu CY, Li LX, Song HH, Chen XH. Facile synthesis of ordered mesoporous carbons from F108/resorcinol-formaldehyde composites obtained in basic media. Chem Commun 2007: 757–9. [23] Joo SH, Choi SJ, Oh I, Kwak J, Liu Z, Terasaki O, Ryoo R. Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles. Nature 2001;412: 169–72. [24] Wang H, Wang AQ, Wang XD, Zhang T. One-pot synthesized MoC imbedded in ordered mesoporous carbon as a catalyst for N2H4 decomposition. Chem Commun 2008:2565–7. [25] Sarapuu A, Kasikov A, Laaksonen T, Kontturi K, Tammeveski K. Electrochemical reduction of oxygen on thinfilm Pt electrodes in acid solutions. Electrochim Acta 2008;53: 5873–80.