Surfactant-Free, Stable Noble Metal–Graphene Nanocomposite as

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Surfactant-free, Stable Noble Metal-Graphene Nanocomposite as High Performance Electrocatalyst Avijit Mondal, and Nikhil R Jana ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs401032p • Publication Date (Web): 10 Jan 2014 Downloaded from http://pubs.acs.org on January 13, 2014

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ACS Catalysis

Surfactant-free, Stable Noble Metal-Graphene Nanocomposite as High Performance Electrocatalyst Avijit Mondal and Nikhil R. Jana*

Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata-700032, India *Corresponding author. E-mail: [email protected]. Telephone: +91-33-24734971. Fax: +91-33-24732805

Abstract: Preparation of noble metal-graphene nanocomposite (MGN) based clean and stable catalyst with uniform distribution of ultrafine nanoparticles can greatly improve the performance of fuel cell. Here we show that surfactant free MGN for different noble metals can be prepared for high-performance fuel cell catalysis by reaction of respective ultra-small colloidal metal oxide/hydroxide with partially reduced colloidal graphene oxide. Resultant MGN is composed of highly dispersed M-M+n based nanoparticle of ultra-small size and produces strong and stable catalytic current for ethanol and formic acid oxidation.

Key words: graphene, nanoparticle, nanocomposite, catalyst, fuel cell

Introduction: Development of high performance fuel cell catalyst has attracted extensive attention to explore the alternative green energy sources.1 Noble metal based nanoparticles are used as most efficient fuel cell catalyst and it is now well established that small particle size with high surface area and accessible metal surface can greatly improve the catalytic performance.2 However, nanoparticles have high surface energy with inherent tendency to minimize their energy via aggregation.3 Thus nanoparticles are generally capped with various stabilizers4 (e.g. surfactant,4a polymer,4c dendrimers,4d ligands4b) and these stabilizers severely limit the catalytic performance by lowering the accessibility of surface metal atoms.5 To overcome this issue noble metal

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nanoparticles are dispersed on various solid supports that can stabilize nanoparticles as well as increase the accessibility of nanoparticle surface.6 Recently graphene has been explored as promising 2D catalyst support due to its enormous surface area (2000-3000 m2g-1), high conductivity (105–106 Sm-1), good mechanical strength and good thermal stability.7 Metal and semiconductor nanoparticle based composites with graphene have been synthesized and shown to improve electrocatalysis, photocatalysis, catalytic organic transformation and efficient solar energy conversion.7 Similarly, graphene based composites with Pt, Pd and Au have been explored as fuel cell electrocatalyst8 for efficient methanol oxidation,8a,d,g formic acid oxidation8b,c and oxygen reduction reaction.6c,8e,f,h These results show that graphene can be a better fuel cell catalyst support than conventionally used carbon black. However, poor solubility of graphene and weak interaction with nanoparticle contribute major challenge in preparing homogeneous loading of nanoparticle and in stabilizing the nanocomposite.8b,9 In common approach chemically synthesized colloidal graphene oxide is physically or chemically linked with nanoparticle via hydrophobic interaction, electrostatic interaction or covalent bonding. Although these approach offer homogeneous loading and attachment of nanoparticle on graphene surface, an additional linker molecule/surfactant/polymer is essential to stabilize the nanocomposite.2a,b,8b,d For example, currently available Pt-graphene based composites are composed of surfactant stabilizers and these stabilizer molecules partially block the catalytic sites, lower the catalytic efficiency and unable to stabilize the nanocomposite for repeated catalytic cycle.8c,d Moreover Pt nanoparticles are prepared separately prior to produce graphene based composite and thus size of Pt nanoparticle is limited by the colloid-chemical roots and are larger than 2.5 nm along with broad size distribution.2b,6b,8a Although there is a recent report on atomic layer deposition based synthesis of Pt-graphene composite having single atom and subnanometer Pt clusters, the catalytic current for methanol oxidation is shown to decrease after repeated cycle.8g Thus it would be ideal to prepare graphene based composites with Pt, Pd and Au with uniformly distributed ultrafine nanoparticle that are free from capping/stabilizing molecules and produces strong and stable catalytic current under repeated cycles. Here we show that surfactant free Pt/Pd/Au/Ag-graphene nanocomposite can be prepared for high-performance fuel cell catalysis by the simple one step reaction of

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respective colloidal metal oxide/hydroxide with partially reduced colloidal graphene oxide. Resultant PtGN/PdGN produces strong and stable catalytic current for ethanol and formic acid oxidation beyond 100 cycles. Our results show that colloidal form of ultrafine metal oxide/hydroxide offers their increased adsorption on partially reduced graphene oxide surface and reduced form of graphene oxide offers partial reduction of metal oxide/hydroxide in absence of any external reducing agent. Such an in situ reduction of platinum oxide produces graphene based stable nanocomposite with highly dispersed Pt-PtII-PtIV based nanoparticle of 2.2 nm size. Similarly, PdGN is composed of 3.4 nm size Pd based nanoparticle, AgGN is composed of 9 nm Ag based nanoparticle and AuGN is composed of 26 nm Au based nanoparticle. Developed PtGN and PdGN have all the three advantages required for fuel cell catalysts: First, catalyst produces strong and stable current for repeated cycles. Second, catalyst can be used for ethanol and formic acid oxidation with significant tolerance from carbon monoxide poisoning effect. Third, synthetic method is simple and catalyst can be preserved in solid form without any significant loss of activity. The comprehensive study of reported literature on graphene or graphene oxide based noble metal nanocomposite clearly shows the advantages of our method. (Table S1)

Experimental Section: Materials and reagents: Graphite powder ( 2.5 nm size or formation of aggregated Pt particles that decreases the active catalytic surface area. Advantage of using colloidal Pt oxide is that it restrict the Pt nanoparticle size < 2.5 nm and provides a thin protective oxide layer that stabilize both the Pt nanoparticle and graphene based composite.

Conclusion: In summary, we have synthesized metal nanoparticle and graphene based nanocomposite which is clean, free from surfactant/stabilizer and acts as high performance fuel cell catalyst. Preparation method involves redox reaction between ultrasmall colloidal metal oxide/hydroxide and partially reduced graphene oxide and formation of nanocomposite with small size metal-metal oxide based nanoparticle. The presented Pt and Pd based nanocomposites offer all the three advantages that are required for fuel cell catalyst i.e. produces high catalytic current, stable current for repeated catalytic cycles and catalyst is easy to prepare. Currently we are trying to

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improve the results with Au/Ag-graphene based nanocomposite via alloying with Pt/Pd.

Acknowledgement: This work is financially supported by DST and CSIR, Government of India. AM acknowledges CSIR India for research fellowship. Authors are thankful for support from the XPS facility of DST Unit of Nanoscience, IACS.

Supporting Information: Experimental procedure and characterization details of MGN and control samples, results on stable electrocatalytic current upto 100 cycles and other control electrochemical experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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redox reaction

partially reduced graphene oxide metal oxide/hydroxide nanoparticle

oxygen functionality/redox centre metal-metal oxide nanoparticle

Scheme 1: Schematic representation of synthesis approach for noble metal-graphene nanocomposite.

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a

b

5 nm

50 nm

c

d

5 nm

e

f

100 nm

200 nm

Figure 1: TEM image of graphene based composite with Pt (a,b), Pd (c,d), Ag (e) and Au(f).

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Figure 2: a) XRD spectra of colloidal PtIV oxide (i), partially reduced graphene oxide (ii) and PtGN (iii). b) Raman spectra of PtGN (red line) in comparison to partially reduced graphene oxide (black line). c) XPS spectra of Pt 4f in PtGN showing that platinum nanoparticles consist of Pt0, PtII and PtIV. Deconvoluted Pt 4f spectrum displays

five

fitted

signals

at

70.7 eV (Pt 4f7/2) and 74.7 eV (Pt 4f5/2)

0

corresponding to Pt , 73.1 eV (Pt 4f7/2) and 76.3 eV (Pt 4f5/2) corresponding to PtII and 77.0 eV attributed to PtIV and d) XPS spectra of C 1s of PtGN showing different signature of carbon atoms in deconvoluted nature.

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Figure 3: Electrocatalytic oxidation of formic acid (a,c,e) and ethanol (b,d,f) by PtGN in comparison to three different control nanocomposites. (current density is based on geometrical surface area) Control 1 nanocomposite is prepared by in situ reduction of H2PtCl6 by NaBH4 in presence of partially reduced graphene oxide. Control 2 is commercially available 20 wt % Pt/C nanocomposite. In each case glassy carbon electrode is modified with nanocomposites and then oxidation of 0.25 M formic acid and 0.5 M ethanol is performed in 0.5 M H2SO4 and 1M KOH at a scan rate of 50 mV/s.

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Figure 4: Electrocatalytic oxidation of ethanol by PdGN and AuGN in comparison to control 4 and control 5. (current density is based on geometrical surface area) Control 4 nanocomposite is prepared by in situ reduction of PdCl2 by NaBH4 in presence of partially reduced graphene oxide. Control 5 is synthesized by galvanic reaction between HAuCl4 and partially reduced graphene oxide. Glassy carbon electrode is modified with nanocomposites and then oxidation of 0.5 M ethanol is performed in 1 M KOH at a scan rate of 50 mV/s.

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Table of Contents

Surfactant-free, Stable Noble Metal-Graphene Nanocomposite as High Performance Electrocatalyst Avijit Mondal and Nikhil R. Jana*

PtGN

50 nm

PdGN

100 nm

Current Intensity (mA/cm2)

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1st cycle 100th cycle

PtGN Control PdGN Control (Pt/G) (Pd/G)

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