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

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

a r t i c l e

i n f o

Article history:

Received 1 September 2009 Received in revised form 25 December 2009

Accepted 30 December 2009 Available online 2 February 2010 Keywords:

Pt nanocatalyst Mesoporous carbon Electrocatalytic properties Fuel cell

Oxygen reduction reaction

a b s t r a c t

A facile procedure for synthesizing platinum nanoparticles (NPs) studded in ordered mes-oporous carbons (Pt–OMCs) based on the organic–organic self-assembly (one-pot) approach is reported. These Pt–OMCs, which can be easily fabricated with controllable Pt loading, were found to possess high surface areas, highly accessible and stable active sites and superior electrocatalytic properties pertinent as cathode catalysts for hydrogen–oxygen fuel cells. The 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 support.

1. Introduction

Highly dispersed noble metal (Pt, Ru) nanoparticles (NPs) sup-ported 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 reduc-tion reacreduc-tion (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 improve-ment 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).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 8 1 4 9 – 8 1 5 4

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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 (seeFig. 7). Accordingly, the number of electron (n) involved during ORR was deduced to be 3.7, which is close to the theoretical value for four-electron reduction, excluding the existence of hydrogen

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

4. Conclusions

In summary, the physical/chemical properties and electro-catalytic performance of ordered mesoporous carbon sup-ported 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 nano-particles 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 hydrogen-energy 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.

Acknowledgments

The supports of this work by the National Science Council, Taiwan (NSC95-2113-M-001-040-MY3 and NSC98-2113-M-001-017-MY3) are gratefully acknowledged.

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

0.0 0.2 0.4 0.6 0.8 1.0 -4 -3 -2 -1 0 j( mc A m 2- ) E (V) vs. RHE 400 800 1200 1600 2000 2500 3000 rpm 0.2 V 0.3 V 0.4 V 0.2 0.3 0.4 0.5 0.6 0.7 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Slope = 2.37 n = 3.7 -1/2_(rad-1/2_s1/2₎ i 1-A m( 1- mc 2 )

a

b

ω

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

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 8 1 4 9 – 8 1 5 4

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